2 Plate Tectonics

Learning Objectives

By the end of this chapter, students should be able to:

  • Describe how the ideas behind started with Alfred Wegener’s hypothesis of continental drift.
  • Describe the physical and chemical layers of the Earth and how they affect movement.
  • Explain how movement at the three types of boundaries causes earthquakes, , and mountain building.
  • Identify boundaries, including and collisions, as places where come together.
  • Identify boundaries, including and , as places where separate.
  • Explain boundaries as places where adjacent past each other.
  • Describe the , beginning with , ocean creation, , and ending with ocean closure.
  • Explain how the tracks of , places that have continually rising , is used to calculate motion.
The map shows many plates.
Figure 2.1: Detailed map of all known plates, their boundaries, and movements.

Revolution is a word usually reserved for significant political or social changes. Several of these idea revolutions forced scientists to re-examine their entire field, triggering a paradigm shift that shook up their conventionally held knowledge. Charles Darwin’s book on evolution, On the Origin of Species, published in 1859; Gregor Mendel’s discovery of the genetic principles of inheritance in 1866; and James Watson, Francis Crick, and Rosalind Franklin’s model for the structure of DNA in 1953 did that for biology. Albert Einstein’s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics.

The concept of was just as revolutionary for geology. The of attributes the movement of sections of the Earth’s outer layers with creating earthquakes, mountains, and . Many earth processes make more sense when viewed through the lens of . Because it is so important in understanding how the world works, is the first topic of discussion in this textbook.

2.1 Alfred Wegener’s Continental Drift Hypothesis

He is a male in a suit.
Figure 2.2: Wegener later in his life, ca. 1924-1930.

Alfred Wegener (1880-1930) was a German scientist who specialized in meteorology and climatology. His knack for questioning accepted ideas started in 1910 when he disagreed with the explanation that the Bering Land Bridge was formed by , and that similar land bridges once connected the continents. After reviewing the scientific literature, he published a stating the continents were originally connected, and then drifted apart. While he did not have the precise mechanism worked out, his was backed up by a long list of evidence.

2.1.1 Early Evidence for Continental Drift Hypothesis

It shows South America and Africa connected, then apart.
Figure 2.3: Snider-Pellegrini’s map showing the continental fit and separation, 1858.

Wegener’s first piece of evidence was that the coastlines of some continents fit together like pieces of a jigsaw puzzle. People noticed the similarities in the coastlines of South America and Africa on the first world maps, and some suggested the continents had been ripped apart. Antonio Snider-Pellegrini did preliminary work on separation and matching in 1858.

The shape of the continents is different than what is seen by just coastlines.
Figure 2.4: Map of world elevations. Note the light blue, which are continental shelves flooded by shallow ocean water. These show the true shapes of the continents.

What Wegener did differently was synthesize a large amount of data in one place. He used true edges of the continents, based on the shapes of the shelves. This resulted in a better fit than previous efforts that traced the existing coastlines.

There are four different fossil organisms that connect South America, Africa, India, Antartica, and Australia.
Figure 2.5: Image showing fossils that connect the continents of Gondwana (the southern continents of Pangea).

Wegener also compiled evidence by comparing similar rocks, mountains, , and across oceans. For example, the of the primitive aquatic reptile Mesosaurus were found on the separate coastlines of Africa and South America. of another reptile, Lystrosaurus, were found on Africa, India, and Antarctica. He pointed out these were land-dwelling creatures could not have swum across an entire ocean.

Opponents of drift insisted trans- land bridges allowed animals and plants to move between continents. The land bridges eventually eroded away, leaving the continents permanently separated. The problem with this is the improbability of a land bridge being tall and long enough to stretch across a broad, deep ocean.

More support for drift came from the puzzling evidence that once existed in normally very warm areas in southern Africa, India, Australia, and Arabia. These could not be explained by land bridges. Wegener found similar evidence when he discovered tropical plant in the frozen region of the Arctic Circle. As Wegener collected more data, he realized the explanation that best fit all the , rock, and observations involved moving continents.

2.1.2 Proposed Mechanism for Continental Drift

The rising material is drawn red. The cool material is blue.
Figure 2.6: Animation of the basic idea of convection: an uneven heat source in a fluid causes rising material next to the heat and sinking material far from the heat.

Wegener’s work was considered a fringe science for his entire life. One of the biggest flaws in his was an inability to provide a mechanism for how the continents moved. Obviously, the continents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that supported a credible mechanism. Other pro-continental drift followers used expansion, contraction, or even the moon’s origin to explain how the continents moved. Wegener used centrifugal forces and , but this model was proven wrong. He also speculated about seafloor spreading, with hints of , but could not substantiate these proposals. As it turns out, current scientific knowledge reveals is one the major forces in driving movements, along with gravity and density.

2.1.3 Development of Plate Tectonic Theory

The map shows many data points all over the world.
Figure 2.7: GPS measurements of plate motions.

Wegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science. However, in the 1950s, evidence started to trickle in that made drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism—namely, seafloor spreading—for Wegener’s of drift to be accepted as the of . Ongoing GPS and earthquake data analyses continue to support this . The next section provides the pieces of evidence that helped one man’s wild notion into a scientific .

Mapping of the Ocean Floors

The diagram shows water going into the ground and coming out, with many different reactions.
Figure 2.8: The complex chemistry around mid-ocean ridges.

In 1947 researchers started using an adaptation of to map a region in the middle of the Atlantic Ocean with poorly-understood topographic and thermal properties. Using this information, Bruce Heezen and Marie Tharp created the first detailed map of the to reveal the Mid-Atlantic Ridge, a basaltic mountain range that spanned the length of the Atlantic Ocean, with rock chemistry and dimensions unlike the mountains found on the continents. Initially scientists thought the ridge was part of a mechanism that explained the expanding Earth or ocean- growth . In 1959, Harry Hess proposed the of seafloor spreading—that the represented factories, where new was issuing from these long ridges. Scientists later included perpendicular to the ridges to better account for varying rates of movement between the newly formed . When earthquake were discovered along the ridges, the idea that earthquakes were linked to movement took hold.


Video 2.1: Uncovering the secrets of the ocean floor.

If you are using an offline version of this text, access this YouTube video via the QR code.

Seafloor , measured by dredging and drilling, provided another clue. Scientists once believed accumulated on the ocean floors over a very long time in a static environment. When some studies showed less than expected, these results were initially used to argue against movement. With more time, researchers discovered these thinner layers were located close to , indicating the ridges were younger than the surrounding . This finding supported the idea that the sea floor was not fixed in one place.

Paleomagnetism

The north end of the magnet is south topographically, and vice versa.
Figure 2.9: The magnetic field of Earth, simplified as a bar magnet.

The seafloor was also mapped magnetically. Scientists had long known of strange magnetic that formed a striped pattern of symmetrical rows on both sides of mid-oceanic ridges. What made these features unusual was the north and south magnetic poles within each stripe was reversed in alternating rows. By 1963, Harry Hess and other scientists used these magnetic reversal patterns to support their model for seafloor spreading (see also Lawrence W. Morley).

The poles shift slightly every year.
Figure 2.10: This animation shows how the magnetic poles have moved over 400 years.

is the study of magnetic fields frozen within rocks, basically a fossilized compass. In fact, the first hard evidence to support motion came from .

rocks containing magnetic like magnetite typically provide the most useful data. In their liquid state as or , the magnetic poles of the align themselves with the Earth’s magnetic field. When the rock cools and solidifies, this alignment is frozen into place, creating a permanent paleomagnetic record that includes magnetic inclination related to global , and declination related to magnetic north.

Animated gif depicting a mid-ocean ridge with two oceanic plates moving away from the center of the ridge. As the movement progresses, symettrical magnetic stripes appear on each side of the ridge.
Figure 2.11: The iron in the solidifying rock preserves the current magnetic polarity as new oceanic plates form at mid ocean ridges.

Scientists had noticed for some time the alignment of magnetic north in many rocks was nowhere close to the earth’s current magnetic north. Some explained this away are part of the normal movement of earth’s magnetic north pole. Eventually, scientists realized adding the idea of movement explained the data better than pole movement alone.

Wadati–Benioff Zones

The earthquakes descend at an angle into the Earth.
Figure 2.12: The Wadati-Benioff zone, showing earthquakes following the subducting slab down.

Around the same time were being investigated, other scientists linked the creation of ocean trenches and island arcs to activity and movement. Several independent research groups recognized earthquake traced the shapes of sinking into the . These deep earthquake zones congregated in planes that started near the surface around ocean trenches and angled beneath the continents and island arcs. Today these earthquake zones called Wadati-Benioff zones.

He is an older man in this 1992 image.
Figure 2.13: J. Tuzo Wilson.

Based on the mounting evidence, the of continued to take shape. J. Tuzo Wilson was the first scientist to put the entire picture together by proposing that the opening and closing of the ocean basins. Before long, scientists proposed other models showing moving with respect to each other, with clear boundaries between them. Others started piecing together complicated histories of movement. The revolution had taken hold.


Complete this interactive activity to check your understanding.

If you are using an offline version of this text, access this interactive activity via the QR code.

 


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 2.1 via the QR code.

 

2.2 Layers of the Earth

The crust and lithosphere are on the outside of the Earth and are thin. Below the crust is the mantle and core. Below the lithosphere is the asthenosphere.
Figure 2.14: The layers of the Earth. Physical layers include lithosphere and asthenosphere; chemical layers are crust, mantle, and core.

In order to understand the details of , it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing data and materials. In general, the Earth can be divided into layers based on chemical and physical characteristics.

2.2.1 Chemical Layers

Certainly the earth is composed of a countless combination of . Regardless of what are involved two major factors— and pressure—are responsible for creating three distinct chemical layers.

Crust

The outermost chemical layer and the one we currently reside on, is the . There are two types of . has a relatively low density and similar to . has a relatively high density, especially when cold and old, and similar to . The surface levels of are relatively . The deeper parts of the are subjected to higher temperatures and pressure, which makes them more . materials are like soft plastics or putty, they move under force. materials are like solid glass or pottery, they break under force, especially when it is applied quickly. Earthquakes, generally occur in the upper and are caused by the rapid movement of relatively materials.

Places with mountain building have a deeper moho.
Figure 2.15: The global map of the depth of the Moho.

The base of the is characterized by a large increase in velocity, which measures how fast earthquake waves travel through solid matter. Called the Mohorovičić Discontinuity, or for short, this zone was discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee-cheech; audio pronunciation) in 1909 after studying earthquake wave paths in his native Croatia. The change in wave direction and speed is caused by dramatic chemical differences of the and . Underneath the oceans, the is found roughly 5 km below the . Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events known as orogenies, the depth is doubled.

Mantle

The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.
Figure 2.16: This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.

The sits below the and above the . It is the largest chemical layer by volume, extending from the base of the to a depth of about 2900 km. Most of what we know about the comes from analysis, though information is gathered by studying and . are pieces of that have risen through the until they are exposed as part of the . are carried within and brought to the Earth’s surface by volcanic eruptions. Most are made of , an class of (see section 4.2 for explanation). Because of this, scientists hypothesize most of the is made of .

Core

The meteorite is polished showing the Widmanstätten Pattern.
Figure 2.17: A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanstätten pattern.

The of the Earth, which has both liquid and solid layers, and consists mostly of iron, nickel, and possibly some oxygen. Scientists looking at data first discovered this innermost chemical layer in 1906. Through a union of hypothetical modeling, astronomical insight, and hard data, they concluded the is mostly iron. Scientists studying , which typically contain more iron than surface rocks, have proposed the earth was formed from meteoric material. They believe the liquid component of the was created as the iron and nickel sank into the center of the planet, where it was liquefied by intense pressure.

2.2.2 Physical Layers

The Earth can also be broken down into five distinct physical layers based on how each layer responds to . While there is some overlap in the chemical and physical designations of layers, specifically the boundary, there are significant differences between the two systems.

Lithosphere

There are about 10 major plates
Figure 2.18: Map of the major plates and their motions along boundaries.

Lithos is Greek for stone, and the is the outermost physical layer of the Earth. It is grouped into two types: and . is thin and relatively rigid. It ranges in thickness from nearly zero in new found around , to an average of 140 km in most other locations. is generally thicker and considerably more plastic, especially at the deeper levels. Its thickness ranges from 40 to 280 km. The is not continuous. It is broken into segments called . A is where two meet and move relative to each other. boundaries are where we see in action—mountain building, triggering earthquakes, and generating activity.

Asthenosphere

It is thin at a mid-ocean ridge, thick under collisions
Figure 2.19: The lithosphere–asthenosphere boundary changes with certain tectonic situations.

The is the layer below the . Astheno- means lacking strength, and the most distinctive property of the is movement. Because it is mechanically weak, this layer moves and flows due to currents created by heat coming from the earth’s cause. Unlike the that consists of multiple , the is relatively unbroken. Scientists have determined this by analyzing waves that pass through the layer. The depth of at which the is found is -dependent. It tends to lie closer to the earth’s surface around and much deeper underneath mountains and the centers of lithospheric .

Mesosphere

The atoms are arranged.
Figure 2.20: General perovskite structure. Perovskite silicates (i.e., Bridgmenite, (Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth.

The , sometimes known as the lower , is more rigid and immobile than the . Located at a depth of approximately 410 and 660 km below the earth’s surface, the is subjected to very high pressures and temperatures. These extreme conditions create a transition zone in the upper where continuously change into various forms, or pseudomorphs. Scientists identify this zone by changes in velocity and sometimes physical barriers to movement. Below this transitional zone, the is relatively uniform until it reaches the .

Inner and Outer Core

Antique photo of young woman
Figure 2.21: Lehmann in 1932.

The is the only entirely liquid layer within the Earth. It starts at a depth of 2,890 km and extends to 5,150 km, making it about 2,300 km thick. In 1936, the Danish geophysicist Inge Lehmann analyzed data and was the first to prove a solid existed within a liquid . The solid is about 1,220 km thick, and the is about 2,300 km thick.

The Earth is cut out with the core being shown.
Figure 2.22: The outer core’s spin causes our protective magnetic field.

It seems like a contradiction that the hottest part of the Earth is solid, as the making up the should be liquified or vaporized at this . Immense pressure keeps the of the in a solid phase. The grows slowly from the lower solidifying as heat escapes the interior of the Earth and is dispersed to the outer layers.

The earth’s liquid is critically important in maintaining a breathable and other environmental conditions favorable for life. Scientists believe the earth’s magnetic field is generated by the circulation of molten iron and nickel within the . If the were to stop circulating or become solid, the loss of the magnetic field would result in Earth getting stripped of life-supporting gases and water. This is what happened, and continues to happen, on Mars.


Complete this interactive activity to check your understanding.

If you are using an offline version of this text, access this interactive activity via the QR code.

 

2.2.3 Plate Tectonic Boundaries

The plate thins from continent to ocean
Figure 2.23: Passive margin.

At passive margins the don’t move—the transitions into oceanic lithosphere and forms made of both types. A may be made of both and connected by a . North and South America’s eastern coastlines are examples of passive margins. Active margins are places where the and lithospheric meet and move relative to each other, such as the western coasts of North and South America. This movement is caused by frictional drag created between the and differences in densities. The majority of mountain-building events, earthquake activity and active on the Earth’s surface can be attributed to movement at active margins.

It shows all the types
Figure 2.24: Schematic of plate boundary types.

In a simplified model, there are three categories of boundaries. boundaries are places where move toward each other. At boundaries, the move apart. At boundaries, the slide past each other.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 2.2 via the QR code.

 

2.3 Convergent Boundaries

The legend shows shields, platforms, orogens, basins, large igneous provinces, and extended crust.
Figure 2.25: Geologic provinces with the Shield (orange) and Platform (pink) comprising the craton, the stable interior of continents.

boundaries, also called destructive boundaries, are places where two or more move toward each other. boundary movement is divided into two types, and , depending on the density of the involved . is of lower density and thus more buoyant than the underlying . is more dense than , and, when old and cold, may even be more dense than .

When of different densities converge, the higher density plate is pushed beneath the more buoyant plate in a process called . When converge without occurring, this process is called .

2.3.1. Subduction

occurs when a dense meets a more buoyant , like a or warmer/younger , and descends into the . The worldwide average rate of is 25 miles per million years, about a half-inch per year. As an descends, it pulls the down into a . These trenches can be more than twice as deep as the average depth of the adjacent ocean , which is usually three to four km. The Mariana Trench, for example, approaches a staggering 11 km.

Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental
Figure 2.26: Diagram of ocean–continent subduction.
Subducting oceanic lithosphere moved under a magma chamber
Figure 2.27: Microcontinents can become part of the accretionary prism of a subduction zone.

Within the , are scraped together and compressed between the and overriding . This feature is called the , mélange, or accretionary prism. Fragments of material, including microcontinents, riding atop the may become sutured to the and accumulate into a large area of land called a . Vast portions of California are comprised of accreted terranes.

Map showing large areas of the western North American continent that are accreted.
Figure 2.28: Accreted terranes of western North America. Everything that is not the “Ancient continental interior (craton)” has been smeared onto the side of the continent by accretion from subduction.

When the , or , sinks into the , the immense heat and pressure pushes volatile materials like water and carbon dioxide into an area below the and above the descending called the . The are released mostly by hydrated that revert to non-hydrated in these higher and pressure conditions. When mixed with asthenospheric material above the plate, the volatile lower the melting point of the , and through a process called it becomes liquid . The molten is more buoyant than the lithospheric plate above it and migrates to the Earth’s surface where it emerges as . The resulting frequently appear as curved mountain chains, volcanic arcs, due to the curvature of the earth. Both and plates can contain volcanic arcs.

It is large and offshore.
Figure 2.29: Location of the large (Mw 8.5-9.0) 1755 Lisbon earthquake.

How is initiated is still a matter of scientific debate. It is generally accepted that zones start as passive margins, where and come together, and then gravity initiates and converts the into an active one. One is gravity pulls the denser oceanic plate down or the plate can start to flow ductility at a low angle. Scientists seeking to answer this question have collected evidence that suggests a new zone is forming off the of Portugal. Some scientists have proposed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a zone, although the evidence is not definitive. Another proposes happens at boundaries involving of different densities.

Some plate boundaries look like they should be active, but show no evidence of . The lithospheric on either side of the Atlantic Ocean for example, are denser than the underlying and are not beneath the . One is the holding the and together is stronger than the downwards force created by the difference in densities.

The earthquakes follow the slab down.
Figure 2.30: Earthquakes along the Sunda megathrust subduction zone, along the island of Sumatra, showing the 2006 Mw 9.1-9.3 Indian Ocean earthquake as a star.

zones are known for having the largest earthquakes and ; they are the only places with surfaces large enough to create -9 earthquakes. These -zone earthquakes not only are very large, but also are very deep. When a becomes stuck and cannot descend, a massive amount of energy builds up between the stuck . If this energy is not gradually dispersed, it may force the to suddenly release along several hundred kilometers of the zone. Because -zone are located on the , this massive amount of movement can generate giant such as those that followed the 2004 Indian Ocean Earthquake and 2011 Tōhoku Earthquake in Japan.

It shows backarc, forearc, and arc.
Figure 2.31: Various parts of a subduction zone. This subduction zone is ocean–ocean subduction, though the same features can apply to continent–ocean subduction.

All zones have a , a feature of the overriding plate found between the and . The experiences a lot of and activity, particularly within the .

In some zones, forces working on the create a backarc on the interior side of the . Some scientists have proposed a mechanism called rollback creates in the overriding plates. In this model, the descending does not slide directly under the overriding plate but instead rolls back, pulling the overlying plate seaward. The behind the gets stretched like pizza dough until the surface cracks and collapses to form a backarc . If the activity is extensive and deep enough, a backarc can develop into a zone. These boundaries may be less symmetrical than their counterparts.

In places where numerous young buoyant are converging and at a relatively high velocity, they may force the overlying to buckle and crack. This is called . pull rocks and chunks of plates apart. , also known as , push them together.

The dual spines of the Andes Mountain range include a example of thrust . The western spine is part of a . Thrust faults have deformed the non-volcanic eastern spine, pushing rocks and pieces of on top of each other.

There are two styles of : that occur in superficial rocks lying on top of the and that reach deeper into the . The Sevier in the western U.S. is a notable type of created during the . The Laramide , a type of , occurred near the end of and slightly after the Sevier in the same region.

The subducting plate goes right under the overriding plate
Figure 2.32: Shallow subduction during the Laramide orogeny.

Flat-, or shallow, caused the Laramide . When the descending at a low angle, there is more contact between the and the overlying than in a typical zone. The shallowly- pushes against the overriding plate and creates an area of on the overriding plate many kilometers away from the zone.

Oceanic–Continental Subduction

The thinner ocean plate is going under the thicker continental plate.
Figure 2.33: Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc.

occurs when an dives below a . This boundary has a and and frequently, a . Well-known examples of are the Cascade Mountains in the Pacific Northwest and western Andes Mountains in South America.

Oceanic–Oceanic Subduction

The ocean plate subducts beneath a different ocean plate.
Figure 2.34: Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc.

The boundaries of zones show very different activity from those involving . Since both are made of , it is usually the older that because it is colder and denser. The on the overlying may remain hidden underwater.. If the rise high enough the reach the ocean surface, the chain of forms an . Examples of these island arcs include the Aleutian Islands in the northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean.

2.3.2. Collisions

The two continental plates stay up.
Figure 2.35: Two continental plates colliding.

When converge, during the closing of an ocean for example, is not possible between the equally buoyant . Instead of one descending beneath another, the two masses of slam together in a process known as . Without , there is no and no . zones are characterized by tall, non- mountains; a broad zone of frequent, large earthquakes; and very little .

When connected by a to completely beneath a , an ocean closes, and begins. Eventually, as ocean basins close, continents join together to form a accumulation of continents called a , a process that has taken place in ~500 million year old cycles over earth’s history.

Pangaea has a crescent shape.
Figure 2.36: A reconstruction of Pangaea, showing approximate positions of modern continents.

The process of created , the supercontinent envisioned by Wegener as the key component of his drift . Geologists now have evidence that have been continuously converging into and splitting into smaller -separated continents throughout Earth’s existence, calling this process the cycle, a process that takes place in approximately 500 million years. For example, they estimate began separating 200 million years ago. was preceded by an earlier , one of which being , which existed 1.1 billion years ago and started breaking apart 800 million to 600 million years ago.

The mountains are loading the crust down, leading to a depressed basin, which is the Persian Gulf
Figure 2.37: The tectonics of the Zagros Mountains. Note the Persian Gulf foreland basin.

A foreland is a feature that develops near mountain belts, as the combined mass of the mountains forms a depression in the lithospheric . While foreland basins may occur at zones, they are most commonly found at boundaries. The Persian Gulf is possibly the best modern example, created entirely by the weight of the nearby Zagros Mountains.

The rock is cray with many circles inside
Figure 2.38: Pillow lavas, which only form under water, from an ophiolite in the Apennine Mountains of central Italy.

If and are fused on the same , it can partially subduct but its buoyancy prevents it from fully descending. In very rare cases, part of a may become trapped beneath a descending in a process called . When a portion of the is driven down into the zone, due to its buoyancy it returns to the surface relatively quickly.

As pieces of the break loose and migrate upward through the zone, they bring along bits of the and and amend them on top of the . Rocks composed of this and ocean-floor material are called and they provide valuable information about the of the .

Figure 2.39: Animation of India crashing into Asia.

The area of -zone and activity usually covers a broader area because is plastic and malleable. Unlike -zone earthquakes, which tend to be located along a narrow swath near the boundary, -zone earthquakes may occur hundreds of kilometers from the boundary between the .

The Eurasian has many examples of -zone deformations covering vast areas. The Pyrenees mountains begin in the Iberian Peninsula and cross into France. Also, there are the Alps stretching from Italy to central Europe; the Zagros mountains from Arabia to Iran; and Himalaya mountains from the Indian subcontinent to central Asia.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 2.3 via the QR code.

 

2.4 Divergent Boundaries

At boundaries, sometimes called constructive boundaries, lithospheric move away from each other. There are two types of boundaries, categorized by where they occur: zones and . zones occur in weak spots in the lithospheric . A usually originates in a as a zone that expands to the point of splitting the apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents. This process is known as rift-to-drift.

2.4.1. Continental Rifting

While the area extends, individual grabens drop down relative to the horsts.
Figure 2.40: Faulting that occurs in divergent boundaries.

In places where the are very thick, they reflect so much heat back into the it develops strong currents that push super-heated material up against the overlying , softening it. forces created by this convective upwelling begin to pull the weakened apart. As it stretches, it becomes thinner and develops deep cracks called or normal . Eventually plate sections located between large faults drop into deep depressions known as valleys, which often contain keystone-shaped blocks of down-dropped known as . The shoulders of these are called . If only one side of a section drops, it is called a . Depending on the conditions, can grow into very large lakes and even oceans.

The branches of the plate boundaries are 120 degrees apart.
Figure 2.41: The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120° apart.

While seemingly occurring at random, is dictated by two factors. does not occur in continents with older and more stable interiors, known as . When does occur, the break-up pattern resembles the seams of a soccer ball, also called a truncated icosahedron. This is the most common surface- pattern to develop on an evenly expanding sphere because it uses the least amount of energy.

Using the soccer ball model, tends to lengthen and expand along a particular seam while fizzling out in the other directions. These seams with little or no activity are called failed rift arms. A is still a weak spot in the ; even without the presence of active , it may develop into a called an . One example of a is the Mississippi Valley Embayment, a depression through which the upper end of the Mississippi River flows. Occasionally connected arms do develop concurrently, creating multiple boundaries of active . In places where the arms do not fail, for example the Afar Triangle, three boundaries can develop near each other forming a .

There is a series of mountains and valleys
Figure 2.42: NASA image of the Basin and Range horsts and grabens across central Nevada.

come in two types: narrow and broad. Narrow are characterized by a high density of highly active boundaries. The East African Rift Zone, where the of Africa is pulling away from the mainland, is an excellent example of an active narrow . Lake Baikal in Russia is another. Broad also have numerous zones, but they are distributed over wide areas of . The region located in the western United States is a type of broad . The Wasatch Fault, which also created the Wasatch Mountain Range in the state of Utah, forms the eastern boundary of this broad (animation 1 and animation 2).

"India" land mass moves closer and closer to Eurasian plate until it collides, creating the himalayan mountains.
Figure 2.43: India colliding into Eurasia to create the modern day Himalayas.

have earthquakes, although not of the and frequency of other boundaries. They may also exhibit . Unlike the flux-melted found in zones, -zone is created by . As the are pulled apart, they create a region of low pressure that melts the and draws it upwards. When this molten reaches the weakened and -riddled zone, it migrates to surface by breaking through the or escaping via an open . Examples of young dot the region in the United States. -zone activity is responsible for generating some unique , such as the Ol Doinyo Lengai in Tanzania. This erupts consisting largely of , a relatively cold, liquid .

2.4.2. Mid-ocean Ridges

The ocean starts as a valley and then gets wider and wider.
Figure 2.44: Progression from rift to mid-ocean ridge.

As and activity progress, the becomes more (see chapter 4) and thinner, with the eventual result transforming the under the area into . This is the process that gives birth to a new ocean, much like the narrow Red Sea emerged with the movement of Arabia away from Africa. As the continues to diverge, a is formed.

, also known as , have several distinctive features. They are the only places on earth that create new . in the zone changes material into new , which oozes up through cracks in . The amount of new being created at is highly significant. These undersea produce more than all other types of combined. Despite this, most mid-oceanic ridge remains unmapped because the volcanoes are located deep on the .

In rare cases, such as a few locations in Iceland, zones display the type of , spreading, and ridge found on the .

The map shoes colors that represent different ages.
Figure 2.45: Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.

The ridge feature is created by the accumulation of hot material, which is lighter than the dense underlying . This chunk of isostatically buoyant sits partially submerged and partially exposed on the , like an ice cube floating in a glass of water.

As the ridge continues to spread, the material is pulled away from the area of and becomes colder and denser. As it continues to spread and cool, the settles into wide swathes of relatively featureless topography called plains with lower topography.

This model of ridge suggests the sections of furthest away from the will be the oldest. Scientists have tested this idea by comparing the age of rocks located in various locations on the . Rocks found near ridges are younger than those found far away from any ridges. accumulation patterns also confirm the idea of sea-floor spreading. layers tend to be thinner near , indicating it has had less time to build up.

The older stripes are farther from the ridge.
Figure 2.46: A time progression (with “a” being youngest and “c” being oldest) showing a spreading center getting wider while recording changes in the magnetic field of the Earth.

As mentioned in the section on and the development of , scientists noticed contained unique magnetic that show up as symmetrical striping on both sides of the ridge. The Vine-Matthews-Morley hypothesis proposes these alternating reversals are created by the earth’s magnetic field being imprinted into after it emerges from the ridge. Very hot has no magnetic field. As the get pulled apart, the cools below the Curie point, the below which a magnetic field gets locked into magnetic . The alternating magnetic reversals in the rocks reflects the periodic swapping of earth’s magnetic north and south poles. This paleomagnetic pattern provides a great historical record of ocean-floor movement, and is used to reconstruct past activity and determine rates of ridge spreading.


Video 2.2: Pangea breakup and formation of the northern Atlantic Ocean.

If you are using an offline version of this text, access this YouTube video via the QR code.

There is a large build up of minerals around the vent
Figure 2.47: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms.

Thanks to their distinctive geology, are home to some of the most unique ecosystems ever discovered. The ridges are often studded with vents, deep fissures that allow seawater to circulate through the upper portions of the and interact with hot rock. The super-heated seawater rises back up to the surface of the , carrying gasses and , and small particulates. The resulting emitted water looks like black underwater smoke.

Scientists had known about these geothermal areas on the for some time. However, it was not until 1977, when scientists piloting a deep submergence vehicle, the Alvin, discovered a thriving community of organisms clustered around these vents. These unique organisms, which include 10-foot-long tube worms taller than people, live in the complete darkness of the deprived of oxygen and sunlight. They use geothermal energy provided by the vents and a process called bacterial to feed on sulfur compounds. Before this discovery, scientists believed life on earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest this type of environment could have been the origin of life on Earth, and perhaps even extraterrestrial life elsewhere in the galaxy, such as on Jupiter’s moon Europa.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 2.4 via the QR code.

 

2.5 Transform Boundaries

Sinistral moves to the left, dextral moves to the right.
Figure 2.48: The two types of transform/strike-slip faults.

A boundary, sometimes called a or conservative boundary, is where the lithospheric plates slide past each other in the horizontal plane. This movement is described based on the perspective of an observer standing on one of the plates, looking across the boundary at the opposing plate. , also known as , movement describes the opposing plate moving to the right. , also known as , movement describe the opposing plate moving to the left.

Most boundaries are found on the , around . These boundaries form zones, filled with earthquake-free , to accommodate different rates of spreading occurring at the ridge.

The fault runs through California.
Figure 2.49: Map of the San Andreas fault, showing relative motion.

Some boundaries produce significant activity, primarily as earthquakes, with very little mountain-building or . This type of boundary may contain a single or series of , which develop in places where are transferred to the surface. As with other types of active boundaries, if the are unable to past each other the forces will continue to build up. If the built up energy between the is suddenly released, the result is an earthquake.

In the eyes of humanity, the most significant occur within , and have a motion that frequently produces moderate-to-large earthquakes. Notable examples include the San Andreas Fault in California, Northern and Eastern Anatolian Faults in Turkey, Altyn Tagh Fault in central Asia, and Alpine Fault in New Zealand.

2.5.1 Transpression and Transtension

The fault is dextral, and has a leftward bend, causing uplift.
Figure 2.50: A transpressional strike-slip fault, causing uplift called a restraining bend.

Bends along may create or forces that cause secondary zones. occurs where there is a component of in addition to the motion. These forces build up around the area of the bend, where the opposing plates are restricted from sliding past each other. As the forces continue to build up, they create mountains in the restraining bend around the . The Big Bend area, located in the southern part of the San Andreas Fault includes a large area of where many mountains have been built, moved, and even rotated.

The fault is dextral, and has a rightward bend, causing a valley.
Figure 2.51: A transtensional strike-slip fault, causing a restraining bend. In the center of the fault, a depression with extension would be found.

zones require a that includes a releasing bend, where the plates are being pulled apart by forces. Depressions and sometimes develop in the releasing bend, along the fault. The Dead Sea found between Israel and Jordan, and the Salton Sea of California are examples of basins formed by transtensional forces.

2.5.2 Piercing Points

The offset is to the left.
Figure 2.52: Wallace (dry) Creek on the Cariso Plain, California. Note as the creek flows from the northern mountainous part of the image, it takes a sharp right (as viewed from the flow of water), then a sharp left. This is caused by the San Andreas Fault cutting roughly perpendicular to the creek, and shifting the location of the creek over time. The fault can be seen about halfway down, trending left to right, as a change in the topography.

When a geological feature is cut by a , it is called a . Piercing points are very useful for recreating past fault movement, especially along boundaries. faults are unique because their horizontal motion keeps a geological feature relatively intact, preserving the record of what happened. Other types of faults—normal and reverse —tend to be more destructive, obscuring or destroying these features. The best type of includes unique patterns that are used to match the parts of a geological feature separated by fault movement. Detailed studies of piercing points show the San Andreas Fault has experienced over 225 km of movement in the last 20 million years, and this movement occurred at three different fault traces.


Video 2.3: Video of the origin of the San Andreas fault. As the mid-ocean ridge subducts, the relative motion between the remaining plates become transform, forming the fault system. Note that because the motion of the plates is not exactly parallel to the fault, it causes divergent motion in the interior of North America.

If you are using an offline version of this text, access this YouTube video via the QR code.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 2.5 via the QR code.

 

2.6 The Wilson Cycle

The diagram shows the last 1000 million years.
Figure 2.53: Diagram of the Wilson Cycle, showing rifting and collision phases.

The is named for J. Tuzo Wilson who first described it in 1966, and it outlines the ongoing origin and breakup of , such as and . Scientists have determined this cycle has been operating for at least three billion years and possibly earlier.

There are a number of about how the works. One mechanism proposes that happens because reflect the heat much better than . When continents congregate together, they reflect more of the Earth’s heat back into the , generating more vigorous currents that then start the process. Some geologists believe plumes are remnants of these of increased and upwelling, and study them for clues about the origin of .

The mechanism behind how are created is still largely a mystery. There are three schools of thought about what continues to drive the continents further apart and eventually bring them together. The ridge-push suggests after the initial event, continue to be pushed apart by mid-ocean and their underlying currents. Slab-pull proposes the are pulled apart by descending slabs in the zones of the margins. A third idea, gravitational sliding, attributes the movement to gravitational forces pulling the lithospheric down from the elevated and across the underlying . Current evidence seems to support pull more than ridge push or gravitational sliding.

2.7 Hotspots

The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.
Figure 2.54: Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.

The provides a broad overview of movement. To analyze movement more precisely, scientists study . First postulated by J. Tuzo Wilson in 1963, a is an area in the lithospheric where molten breaks through and creates a center, islands in the ocean and mountains on land. As the moves across the , the center becomes because it is no longer over an active source. Instead, the emerges through another area in the to create a new active . Over time, the combination of moving and stationary creates a chain of islands or mountains. The classic definition of states they do not move, although recent evidence suggests that there may be exceptions.

Hotspots are scattered around the world.
Figure 2.55: Map of world hotspots. Larger circles indicate more active hotspots.

are the only types of not associated with or zones at boundaries; they seem totally disconnected from any processes, such as earthquakes. However, there are relationships between and . There are several , current and former, that are believed to have begun at the time of . Also, scientists use the age of eruptions and shape of the chain to quantify the rate and direction of movement relative to the .

Scientists are divided over how is generated in . Some suggest that originate from super-heated material from as deep as the that reaches the Earth’s as a . Others argue the molten material that feeds is sourced from the . Of course, it is difficult to collect data from these deep-Earth features due to the extremely high pressure and .

How are initiated is another highly debated subject. The prevailing mechanism has starting in boundaries during . Scientists have identified a number of current and past believed to have begun this way. slabs have also been named as causing plumes and hot-spot . Some geologists have suggested another geological process not involving may be involved, such as a large space objects crashing into Earth. Regardless of how they are formed, dozens are on the Earth. Some well-known examples include the Tahiti Islands, Afar Triangle, Easter Island, Iceland, Galapagos Islands, and Samoan Islands. The United States is home to two of the largest and best-studied : Hawaii and Yellowstone.

2.7.1 Hawaiian Hotspot

There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.
Figure 2.56: The Hawaii–Emperor seamount and island chain.

The active in Hawaii represent one of the most active sites on earth. Scientific evidence indicates the Hawaiian is at least 80 million years old. Geologists believe it is actually much older; however any rocks with proof of this have been under the . The big island of Hawaii sits atop a large that marks the active . The Kilauea is the main for this and has been actively erupting since 1983.

This enormous island chain, much of which is underwater, stretches across the Pacific for almost 6,000 km. The chain’s most striking feature is a sharp 60-degree bend located at the midpoint, which marks a significant change in movement direction that occurred 50 million years ago. The change in direction has been more often linked to a reconfiguration, but also to other things like plume migration.

The islands get older to the left.
Figure 2.57: Diagram of the Hawaiian hotspot and islands that it formed.

In an attempt to map the Hawaiian as far down as the lower , scientists have used , a type of three-dimensional imaging. This information—along with other evidence gathered from rock ages, vegetation types, and island size—indicate the oldest islands in the chain are located the furthest away from the active .

2.7.2 Yellowstone Hotspot

The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.
Figure 2.58: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.

Like the Hawaiian version, the Yellowstone is formed by rising through the . What makes it different is this is located under a thick, . Hawaii sits on a thin  , which is easily breached by coming to the surface. At Yellowstone, the thick presents a much more difficult barrier for to penetrate. When it does emerge, the eruptions are generally much more violent. Thankfully they are also less frequent.

Over 15 million years of eruptions by this have carved a curved path across the western United States. It has been suggested the Yellowstone is connected to the much older Columbia River and even to 70 million-year-old found in the Yukon region of Canada.

The eruptions trend eastward due to prevailing winds.
Figure 2.59: Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).

The most recent major eruption of this created the Yellowstone Caldera and Lava Creek approximately 631,000 years ago. The eruption threw 1,000 cubic kilometers of and into the , some of which was found as far away as Mississippi. Should the erupt again, scientists predict it will be another massive event. This would be a calamity reaching far beyond the western United States. These super eruptions fill the earth’s with so much gas and , they block sunlight from reaching the earth. Not only would this drastically alter climates and environments around the globe, it could affect worldwide food production.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for sections 2.6 and 2.7 via the QR code.

Summary


Video 2.4: Plate tectonics.

If you are using an offline version of this text, access this YouTube video via the QR code.

is a unifying ; it explains nearly all of the major geologic processes on Earth. Since its early inception in the 1950s and 1960s, geologists have been guided by this revolutionary perception of the world. The of states the surface layer of the Earth is broken into a network of solid, relatively . Underneath the is a much hotter and more layer that contains zones of convective upwelling generated by the interior heat of Earth. These currents move the surface around—bringing them together, pulling them apart, and them side-by-side. Earthquakes and form at the boundaries where the interact, with the exception of , which are not caused by movement.


Take this quiz to check your comprehension of this chapter.

If you are using an offline version of this text, access the quiz for chapter 2 via the QR code.

 

URLs Linked Within This Chapter

Audio pronunciation: https://www.merriam-webster.com/dictionary/Mohorovicic%20discontinuity

Animation 1: Basin and Range Structures. How do they form? [Video: 0:52] https://youtu.be/TvvWqAdNV84

Animation 2: Basin & Range: Extension, Erosion, Sedimentation. [Video: 0:31] https://www.youtube.com/watch?v=7DxcAMmNeZk

Text References

  1. Aitta, A., 2006, Iron melting curve with a tricritical point: J. Stat. Mech., v. 2006, no. 12, p. P12015.
  2. Alfe, D., Gillan, M.J., and Price, G.D., 2002, Composition and temperature of the Earth’s core constrained by combining ab initio calculations and seismic data: Earth Planet. Sci. Lett., v. 195, no. 1, p. 91–98.
  3. Atwater, T., 1970, Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America: Geol. Soc. Am. Bull., v. 81, no. 12, p. 3513–3536., doi: 10.1130/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2.
  4. Bacon, F., and Montagu, B., 1848, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author: The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author, Parry & McMillan, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author.
  5. Benioff, H., 1949, Seismic evidence for the fault origin of oceanic deeps: Geological Society of America Bulletin, v. 60, no. 12, p. 1837–1856., doi: 10.1130/0016-7606(1949)60[1837:SEFTFO]2.0.CO;2.
  6. Birch, F., 1952, Elasticity and constitution of the Earth’s interior: J. Geophys. Res., v. 57, no. 2, p. 227–286., doi: 10.1029/JZ057i002p00227.
  7. Birch, F., 1964, Density and composition of mantle and core: J. Geophys. Res., v. 69, no. 20, p. 4377–4388.
  8. Bott, M.H.P., 1993, Modelling the plate-driving mechanism: Journal of the Geological Society, v. 150, no. 5, p. 941–951., doi: 10.1144/gsjgs.150.5.0941.
  9. Coats, R.R., 1962, Magma type and crustal structure in the Aleutian Arc, in The Crust of the Pacific Basin: American Geophysical Union, p. 92–109., doi: 10.1029/GM006p0092.
  10. Conrad, C.P., and Lithgow-Bertelloni, C., 2002, How mantle slabs drive plate tectonics: Science (New York, N.Y.), v. 298, no. 5591, p. 207–209., doi: 10.1126/science.1074161.
  11. Corliss, J.B., Dymond, J.G., Gordon, L.I., Edmond, J.M., von Heezen, R.P., Ballard, R.D., Green, K., Williams, D.L., Bainbridge, A., Crane, K., and van Andel, T.H., 1979, Submarine thermal springs on the Galapagos Rift: Science, v. 203, p. 107321083.
  12. Davis, E.E., and Lister, C.R.B., 1974, Fundamentals of ridge crest topography: Earth Planet. Sci. Lett., v. 21, no. 4, p. 405–413.
  13. Dawson, J.B., Pinkerton, H., Norton, G.E., and Pyle, D.M., 1990, Physicochemical properties of alkali carbonatite lavas: Data from the 1988 eruption of Oldoinyo Lengai, Tanzania: Geology, v. 18, no. 3, p. 260–263.
  14. Drake, E.T., 1976, Alfred Wegener’s reconstruction of Pangea: Geology, v. 4, no. 1, p. 41–44., DOI: 10.1130/0091-7613(1976)4<41:AWROP>2.0.CO;2
  15. Engdahl, E.R., Flynn, E.A., and Masse, R.P., 1974, Differential PkiKP travel times and the radius of the core: Geophysical J Royal Astro Soc, v. 40, p. 457–463.
  16. Ewing, M., Ewing, J.I., and Talwani, M., 1964, Sediment distribution in the oceans: The Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 75, no. 1, p. 17–36., doi: 10.1130/0016-7606(1964)75[17:SDITOT]2.0.CO;2.
  17. Ewing, M., Houtz, R., and Ewing, J., 1969, South Pacific sediment distribution: J. Geophys. Res., v. 74, no. 10, p. 2477–2493., doi: 10.1029/JB074i010p02477.
  18. Fernandez, L.M., and Careaga, J., 1968, The thickness of the crust in central United States and La Paz, Bolivia, from the spectrum of longitudinal seismic waves: Bull. Seismol. Soc. Am., v. 58, no. 2, p. 711–741.
  19. Fluegel, von H.W., 1980, Wegener-Ampferer-Schwinner. Ein Beitrag zur Geschichte der Geologie in Österreich: Mitt. Oesterr. Geol. Ges., v. 73, p. 237–254.
  20. Forsyth, D.W., 1975, The Early Structural Evolution and Anisotropy of the Oceanic Upper Mantle: Geophys. J. Int., v. 43, no. 1, p. 103–162., doi: 10.1111/j.1365-246X.1975.tb00630.x.
  21. Frankel, H., 1982, The Development, Reception, and Acceptance of the Vine-Matthews-Morley Hypothesis: Hist. Stud. Phys. Biol. Sci., v. 13, no. 1, p. 1–39.
  22. Fukao, Y., and Obayashi, M., 2013, Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity: J. Geophys. Res. [Solid Earth], v. 118, no. 11, p. 2013JB010466.
  23. Hagstrum, J.T., 2005, Antipodal hotspots and bipolar catastrophes: Were oceanic large-body impacts the cause? Earth Planet. Sci. Lett., v. 236, no. 1–2, p. 13–27.
  24. Hanks, T.C., and Anderson, D.L., 1969, The early thermal history of the earth: Phys. Earth Planet. Inter., v. 2, no. 1, p. 19–29.
  25. Heezen, B.C., and Tharp, M., 1965, Tectonic Fabric of the Atlantic and Indian Oceans and Continental Drift: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 90–106., doi: 10.1098/rsta.1965.0024.
  26. Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, E.S., Shuster, M.W., Winslow, N.S., and Lawton, T.F., 1986, Time of initial thrusting in the Sevier orogenic belt, Idaho-Wyoming and Utah: Geology, v. 14, no. 5, p. 388–391.
  27. Herak, D., and Herak, M., 2007, Andrija Mohorovičić (1857-1936)—On the occasion of the 150th anniversary of his birth: Seismol. Res. Lett., v. 78, no. 6, p. 671–674.
  28. Hess, H.H., 1962, History of ocean basins: Petrologic studies, v. 4, p. 599–620.
  29. Hutson, P., Middleton, J., and Miller, D., 2003, Collision Zones: Online, http://www.geosci.usyd.edu.au/users/prey/ACSGT/EReports/eR.2003/GroupD/Report1/web%20pages/contents.html, accessed June 2017.
  30. Isacks, B., Oliver, J., and Sykes, L.R., 1968, Seismology and the new global tectonics: J. Geophys. Res., v. 73, no. 18, p. 5855–5899.
  31. Ito, E., and Takahashi, E., 1989, Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications: J. Geophys. Res. [Solid Earth], v. 94, no. B8, p. 10637–10646.
  32. Jacoby, W.R., 1981, Modern concepts of Earth dynamics anticipated by Alfred Wegener in 1912: Geology, v. 9, no. 1, p. 25–27., doi: <a href=”https://doi.org/10.1130/0091-7613(1981)92.0.CO;2″>10.1130/0091-7613(1981)9<25:MCOEDA>2.0.CO;2.
  33. Jakosky, B.M., Grebowsky, J.M., Luhmann, J.G., Connerney, J., Eparvier, F., Ergun, R., Halekas, J., Larson, D., Mahaffy, P., McFadden, J., Mitchell, D.F., Schneider, N., Zurek, R., Bougher, S., and others, 2015, MAVEN observations of the response of Mars to an interplanetary coronal mass ejection: Science, v. 350, no. 6261, p. aad0210.
  34. James, D.E., Fouch, M.J., Carlson, R.W., and Roth, J.B., 2011, Slab fragmentation, edge flow and the origin of the Yellowstone hotspot track: Earth Planet. Sci. Lett., v. 311, no. 1–2, p. 124–135.
  35. Ji, Y., and Nataf, H.-C., 1998, Detection of mantle plumes in the lower mantle by diffraction tomography: Hawaii: Earth Planet. Sci. Lett., v. 159, no. 3–4, p. 99–115.
  36. Johnston, S.T., Jane Wynne, P., Francis, D., Hart, C.J.R., Enkin, R.J., and Engebretson, D.C., 1996, Yellowstone in Yukon: The Late Cretaceous Carmacks Group: Geology, v. 24, no. 11, p. 997–1000.
  37. Kearey, P., Klepeis, K.A., and Vine, F.J., 2009, Global Tectonics: Oxford ; Chichester, West Sussex ; Hoboken, NJ, Wiley-Blackwell, 496 p.
  38. Le Pichon, X., 1968, Sea-floor spreading and continental drift: J. Geophys. Res., v. 73, no. 12, p. 3661–3697.
  39. Lehmann, I., 1936, P’, Publ: Bur. Centr. Seism. Internat. Serie A, v. 14, p. 87–115.
  40. Mantovani, R., 1889, Les fractures de l’écorce terrestre et la théorie de Laplace: Bull. Soc. Sc. et Arts Réunion, p. 41–53.
  41. Mason, R.G., 1958, A magnetic survey off the west coast of the United-States between latitudes 32-degrees-N and 36-degrees-N longitudes 121-degrees-W and 128-degrees-W: Geophysical Journal of the Royal Astronomical Society, v. 1, no. 4, p. 320.
  42. Mason, R.G., and Raff, A.D., 1961, Magnetic Survey Off the West Coast of North America, 32° N. Latitude to 42° N. Latitude: Geological Society of America Bulletin, v. 72, no. 8, p. 1259–1265., doi: 10.1130/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2.
  43. McCollom, T.M., 1999, Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa: J. Geophys. Res., v. 104, no. E12, p. 30729–30742., doi: 10.1029/1999JE001126.
  44. McKenzie, D.P., and Parker, R.L., 1967, The North Pacific: an Example of Tectonics on a Sphere: Nature, v. 216, p. 1276–1280., doi: 10.1038/2161276a0.
  45. Miller, A.R., Densmore, C.D., Degens, E.T., Hathaway, J.C., Manheim, F.T., McFarlin, P.F., Pocklington, R., and Jokela, A., 1966, Hot brines and recent iron deposits in deeps of the Red Sea: Geochimica et Cosmochimica Acta, v. 30, no. 3, p. 341–359., doi: 10.1016/0016-7037(66)90007-X.
  46. Morgan, W.J., 1968, Rises, trenches, great faults, and crustal blocks: J. Geophys. Res., v. 73, no. 6, p. 1959–1982., doi: 10.1029/JB073i006p01959.
  47. Mueller, S., and Phillips, R.J., 1991, On the initiation of subduction: J. Geophys. Res. [Solid Earth], v. 96, no. B1, p. 651–665.
  48. Oldham, R.D., 1906, The constitution of the interior of the Earth, as revealed by earthquakes: Q. J. Geol. Soc. London, v. 62, no. 1–4, p. 456–475.
  49. Pasyanos, M.E., 2010, Lithospheric thickness modeled from long-period surface wave dispersion: Tectonophysics, v. 481, no. 1–4, p. 38–50.
  50. Powell, R.E., and Weldon, R.J., 1992, Evolution of the San Andreas fault: Annu. Rev. Earth Planet. Sci., v. 20, p. 431.
  51. Raff, A.D., and Mason, R.G., 1961, Magnetic Survey Off the West Coast of North America, 40 N. Latitude to 52 N. Latitude: Geological Society of America Bulletin, v. 72, no. 8, p. 1267–1270., doi: 10.1130/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2.
  52. Runcorn, S.K., 1965, Palaeomagnetic comparisons between Europe and North America: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 1–11.
  53. Saito, T., Ewing, M., and Burckle, L.H., 1966, Tertiary sediment from the mid-atlantic ridge: Science, v. 151, no. 3714, p. 1075–1079., doi: 10.1126/science.151.3714.1075.
  54. Satake, K., and Atwater, B.F., 2007, Long-term perspectives on giant earthquakes and tsunamis at subduction zones*: Annu. Rev. Earth Planet. Sci., v. 35, p. 349–374.
  55. Scheidegger, A.E., 1953, Examination of the physics of theories of orogenesis: Geol. Soc. Am. Bull., v. 64, no. 2, p. 127–150., doi: 10.1130/0016-7606(1953)64[127:EOTPOT]2.0.CO;2.
  56. Simpson, G.G., 1943, Mammals and the nature of continents: Am. J. Sci., v. 241, no. 1, p. 1–31.
  57. Starr, A.M., 2015, Ambient resonance of rock arches: Salt Lake City, Utah, University of Utah, 134 p.
  58. Stern, R.J., 1998, A subduction primer for instructors of introductory geology courses and authors of introductory-geology textbooks: J. Geosci. Educ., v. 46, p. 221.
  59. Stern, R.J., 2004, Subduction initiation: spontaneous and induced: Earth Planet. Sci. Lett., v. 226, no. 3–4, p. 275–292.
  60. Stich, D., Mancilla, F. de L., Pondrelli, S., and Morales, J., 2007, Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake: Geophys. Res. Lett., v. 34, no. 12, p. L12308.
  61. Tatsumi, Y., 2005, The subduction factory: how it operates in the evolving Earth: GSA Today, v. 15, no. 7, p. 4.
  62. Todo, Y., Kitazato, H., Hashimoto, J., and Gooday, A.J., 2005, Simple foraminifera flourish at the ocean’s deepest point: Science, v. 307, no. 5710, p. 689., doi: 10.1126/science.1105407.
  63. Tolstoy, I., and Ewing, M., 1949, North Atlantic hydrography and the Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 60, no. 10, p. 1527–1540., doi: 10.1130/0016-7606(1949)60[1527:NAHATM]2.0.CO;2.
  64. Vine, F.J., and Matthews, D.H., 1963, Magnetic anomalies over oceanic ridges: Nature, v. 199, no. 4897, p. 947–949.
  65. Wächtershäuser, G., 1990, Evolution of the first metabolic cycles: Proc. Natl. Acad. Sci. U. S. A., v. 87, no. 1, p. 200–204.
  66. Wadati, K., 1935, On the activity of deep-focus earthquakes in the Japan Islands and neighbourhoods: Geophys. Mag., v. 8, no. 3–4, p. 305–325.
  67. Waszek, L., Irving, J., and Deuss, A., 2011, Reconciling the hemispherical structure of Earth’s inner core with its super-rotation: Nat. Geosci., v. 4, no. 4, p. 264–267., doi: 10.1038/ngeo1083.
  68. Wegener, A., 1912, Die Entstehung der Kontinente: Geol. Rundsch., v. 3, no. 4, p. 276–292., doi: 10.1007/BF02202896.
  69. Wegener, A., 1920, Die entstehung der kontinente und ozeane: Рипол Классик.
  70. Wells, H.G., Huxley, J., and Wells, G.P., 1931, The Science of Life: Philosophy, v. 6, no. 24, p. 506–507.
  71. White, I.C., and Moreira, C., 1908, Commissão de estudos das minas de Carvão de Pedra do Brazil.
  72. de Wijs, G.A., Kresse, G., Vočadlo, L., Dobson, D., Alfè, D., Gillan, M.J., and Price, G.D., 1998, The viscosity of liquid iron at the physical conditions of the Earth’s core: Nature, v. 392, no. 6678, p. 805–807., doi: 10.1038/33905.
  73. Wilson, J.T., 1966, Did the Atlantic close and then re-open? Nature.
  74. Wilson, M., 1993, Plate-moving mechanisms: constraints and controversies: Journal of the Geological Society, v. 150, no. 5, p. 923–926., doi: 10.1144/gsjgs.150.5.0923.
  75. Wyllie, P.J., 1970, Ultramafic rocks and the upper mantle, in Morgan, B.A., editor, Fiftieth anniversary symposia: Mineralogy and petrology of the Upper Mantle; Sulfides; Mineralogy and geochemistry of non-marine evaporites: Washington, DC, Mineralogical Society of America, p. 3–32.
  76. Zhou, Z., 2004, The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence: Naturwissenschaften, v. 91, no. 10, p. 455–471.

Figure References

Figure 2.1: Detailed map of all known plates, their boundaries, and movements. Eric Gaba. 2006-10, updated 2015-09. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Tectonic_plates_boundaries_detailed-en.svg

Figure 2.2: Wegener later in his life, ca. 1924-1930. Author unknown. ca. 1924 and 1930. Public domain. https://commons.wikimedia.org/wiki/File:Alfred_Wegener_ca.1924-30.jpg

Figure 2.3: Snider-Pellegrini’s map showing the continental fit and separation, 1858. Antonio Snider-Pellegrini. 1858. Public domain. https://commons.wikimedia.org/wiki/File:Antonio_Snider-Pellegrini_Opening_of_the_Atlantic.jpg

Figure 2.4: Map of world elevations. Tahaisik. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Tahacik.jpg

Figure 2.5: Image showing fossils that connect the continents of Gondwana (the southern continents of Pangea). Osvaldocangaspadilla. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Snider-Pellegrini_Wegener_fossil_map.svg

Figure 2.6: Animation of the basic idea of convection: an uneven heat source in a fluid causes rising material next to the heat and sinking material far from the heat. Oni Lukos. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Convection.gif

Figure 2.7: GPS measurements of plate motions. NASA. Public domain. https://cddis.nasa.gov/docs/2009/HTS_0910.pdf

Figure 2.8: The complex chemistry around mid-ocean ridges. NOAA. https://oceanexplorer.noaa.gov/explorations/04fire/background/chemistry/media/chemistry_600.html

Figure 2.9: The magnetic field of Earth, simplified as a bar magnet. Zureks. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Earth%27s_magnetic_field,_schematic.png

Figure 2.10: This animation shows how the magnetic poles have moved over 400 years. USGS. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Earth_Magnetic_Field_Declination_from_1590_to_1990.gif

Figure 2.11: The iron in the solidifying rock preserves the current magnetic polarity as new oceanic plates form at mid ocean ridges. USGS. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Mid-ocean_ridge_topography.gif

Figure 2.12: The Wadati-Benioff zone, showing earthquakes following the subducting slab down. USGS. 2013. Public domain. https://commons.wikimedia.org/wiki/File:Benioff_zone_earthquake_focus.jpg

Figure 2.13: J. Tuzo Wilson. Stephen Morris. 1992. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:John_Tuzo_Wilson_in_1992.jpg

Figure 2.14: The layers of the Earth. Drlauraguertin. 2015. CC BY-SA 3.0. https://wiki.seg.org/wiki/File:Earthlayers.png#file

Figure 2.15: The global map of the depth of the Moho. AllenMcC. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Mohomap.png

Figure 2.16: This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. Matt Affolter. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Iddingsite.JPG

Figure 2.17: A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanstätten pattern. H. Raab. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:TolucaMeteorite.jpg

Figure 2.18: Map of the major plates and their motions along boundaries. Scott Nash via USGS. 1996. Public domain. https://commons.wikimedia.org/wiki/File:Plates_tect2_en.svg

Figure 2.19: The lithosphere–asthenosphere boundary changes with certain tectonic situations. Nealey Sims. 2015. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Earth%27s_Inner_Layers_denoting_the_LAB.png

Figure 2.20: General perovskite structure. Perovskite silicates (i.e., Bridgmenite, (Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth. Cadmium. 2006. Public domain. https://en.wikipedia.org/wiki/File:Perovskite.jpg

Figure 2.21: Lehmann in 1932. Even Neuhaus. 1932. Public domain. https://commons.wikimedia.org/wiki/File:Inge_Lehman.jpg

Figure 2.22: The outer core’s spin causes our protective magnetic field. NASA. 2017. Public domain. https://www.nasa.gov/mission_pages/sunearth/news/gallery/earths-dynamiccore.html

Figure 2.23: Passive margin. Joshua Doubek. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Passive_Contiental_Margin.jpg

Figure 2.24: Schematic of plate boundary types. NOAA via USGS. Public domain. https://oceanexplorer.noaa.gov/facts/plate-boundaries.html

Figure 2.25: Geologic provinces with the Shield (orange) and Platform (pink) comprising the craton, the stable interior of continents. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:World_geologic_provinces.jpg

Figure 2.26: Diagram of ocean–continent subduction. K. D. Schroeder. 2016. CC-BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Subduction-en.svg

Figure 2.27: Microcontinents can become part of the accretionary prism of a subduction zone. MagentaGreen. 2014. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Volcanic_Arc_System_SVG_en.svg

Figure 2.28: Accreted terranes of western North America. Modified from illustration provided by Oceanus Magazine; original figure by Jack Cook, Woods Hole Oceanographic Institution; adapted by USGS. Used under fair use.

Figure 2.29: Location of the large (Mw 8.5-9.0) 1755 Lisbon earthquake. USGS. 2014. Public domain. https://commons.wikimedia.org/wiki/File:1755_Lisbon_Earthquake_Location.png

Figure 2.30: Earthquakes along the Sunda megathrust subduction zone, along the island of Sumatra, showing the 2006 Mw 9.1-9.3 Indian Ocean earthquake as a star. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/File:SundaMegathrustSeismicity.PNG

Figure 2.31: Various parts of a subduction zone. USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Volcanic_Arc_System.png

Figure 2.32: Shallow subduction during the Laramide orogeny. Melanie Moreno. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Shallow_subduction_Laramide_orogeny.png

Figure 2.33: Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc. USGS. 1999. Public domain. https://commons.wikimedia.org/wiki/File:Oceanic-continental_convergence_Fig21oceancont.gif

Figure 2.34: Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Oceanic-oceanic_convergence_Fig21oceanocean.gif

Figure 2.35: Two continental plates colliding. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Continental-continental_convergence_Fig21contcont.gif

Figure 2.36: A reconstruction of Pangaea, showing approximate positions of modern continents. Kieff. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Pangaea_continents.svg

Figure 2.37: The tectonics of the Zagros Mountains. Mikenorton. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:ZagrosFTB.png

Figure 2.38: Pillow lavas, which only form under water, from an ophiolite in the Apennine Mountains of central Italy. Matt Affolter (Qfl247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:ItalyPillowBasalt.jpg

Figure 2.39: Animation of India crashing into Asia. Raynaldi rji. 2015. CC BY-SA 4.0. https://en.wikipedia.org/wiki/File:India-Eurasia_collision.gif

Figure 2.40: Faulting that occurs in divergent boundaries. USGS; adapted by Gregors. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Fault-Horst-Graben.svg

Figure 2.41: The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120° apart. Koba-chan. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Topographic30deg_N0E30.png

Figure 2.42: NASA image of the Basin and Range horsts and grabens across central Nevada. NASA. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Basin_range_province.jpg

Figure 2.43: India colliding into Eurasia to create the modern day Himalayas. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Himalaya-formation.gif

Figure 2.44: Progression from rift to mid-ocean ridge. Hannes Grobe, Alfred Wegener, Institute for Polar and Marine Research; adapted by Lichtspiel. 2011. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Ocean-birth.svg

Figure 2.45: Age of oceanic lithosphere, in millions of years. Muller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest (2008) Age, spreading rates and spreading symmetry of the world’s ocean crust, Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Age_of_oceanic_lithosphere.jpg

Figure 2.46: A time progression (with “a” being youngest and “c” being oldest) showing a spreading center getting wider while recording changes in the magnetic field of the Earth. Chmee2. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg

Figure 2.47: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms. NOAA. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Main_Endeavour_black_smoker.jpg

Figure 2.48: The two types of transform/strike-slip faults. Cferrero. 2003. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Strike_slip_fault.png

Figure 2.49: Map of the San Andreas fault, showing relative motion. Kate Barton, David Howell, and Joe Vigil via USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Sanandreas.jpg

Figure 2.50: A transpressional strike-slip fault, causing uplift called a restraining bend. GeoAsh. 2015. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Restraining_Bend.png

Figure 2.51: A transtensional strike-slip fault, causing a restraining bend. In the center of the fault, a depression with extension would be found. K. Martin. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Releasing_bend.png

Figure 2.52: Wallace (dry) Creek on the Cariso Plain, California. Robert E. Wallace via USGS. 2014. Public domain. https://commons.wikimedia.org/wiki/File:Wallace_Creek_offset_across_the_San_Andreas_Fault.png

Figure 2.53: Diagram of the Wilson Cycle, showing rifting and collision phases. Hannes Grobe. 2007. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Wilson-cycle_hg.png

Figure 2.54: Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate. Los688. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Hotspot(geology)-1.svg

Figure 2.55: Map of world hotspots. Foulger. 2011. Public domain. https://commons.wikimedia.org/wiki/File:CourtHotspots.png

Figure 2.56: The Hawaii–Emperor seamount and island chain. Ingo Wölbern. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Hawaii-Emperor_engl.png

Figure 2.57: Diagram of the Hawaiian hotspot and islands that it formed. Joel E. Robinson via USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Hawaii_hotspot_cross-sectional_diagram.jpg

Figure 2.58: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago. Kelvin Case. 2013. CC BY 3.0. https://commons.wikimedia.org/wiki/File:HotspotsSRP_update2013.JPG

Figure 2.59: Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow). USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Yellowstone_volcano_-_ash_beds.svg

License

Share This Book