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.
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
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
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.
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.
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
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
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
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.
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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.
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).
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.
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.
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.
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.
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2.2 Layers of the Earth
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.
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.
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.
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 .
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.
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.
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 .
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
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.
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.
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2.2.3 Plate Tectonic Boundaries
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.
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.
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2.3 Convergent Boundaries
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 .
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 .
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.
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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
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.
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 .
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).
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
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 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.
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.
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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.
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2.5 Transform Boundaries
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.
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
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.
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
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.
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2.6 The Wilson Cycle
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.
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.
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
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.
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
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 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.
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Video 2.4: Plate tectonics.
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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.
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URLs Linked Within This Chapter
Audio pronunciation: https://www.merriam-webster.com/dictionary/Mohorovicic%20discontinuity
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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
The theory that the outer layer of the Earth (the lithosphere) is broken in several plates, and these plates move relative to one another, causing the major topographic features of Earth (e.g. mountains, oceans) and most earthquakes and volcanoes.
A solid part of the lithosphere which moves as a unit, i.e. the entire plate generally moves the same direction at the same speed.
Place where lava is erupted at the surface.
Place where two plates come together, casing subduction or collision.
A process where an oceanic plate descends bellow a less dense plate, causing the removal of the plate from the surface. Subduction causes the largest earthquakes possible, as the subducting plate can lock as it goes down. Volcanism is also caused as the plate releases volatiles into the mantle, causing melting.
Place where two plates are moving apart, creating either a rift (continental lithosphere) or a mid-ocean ridge (oceanic lithosphere).
Area of extended continental lithosphere, forming a depression. Rifts can be narrow (focused in one place) or broad (spread out over a large area with many faults).
A divergent boundary within an oceanic plate, where new lithosphere and crust is created as the two plates spread apart. Mid-ocean ridge and spreading center are synonyms.
Place where two plates slide past each other, creating strike slip faults.
Stress within an object that causes a side-to-side movement within an internal fabric or weakness.
The cycle of opening ocean basins with rifting and seafloor spreading, then closing the basin via subduction and collision, creating a supercontinent.
The layers of igneous, sedimentary, and metamorphic rocks that form the continents. Continental crust is much thicker than oceanic crust. Continental crust is defined as having higher concentrations of very light elements like K, Na, and Ca, and is the lowest density rocky layer of Earth. Its average composition is similar to granite.
A down-warped feature in the crust.
Rising stationary magma, forming a succession of volcanism. This is reflected as islands on oceanic plates, and volcanic mountains or craters on land.
Liquid rock within the Earth.
An accepted scientific idea that explains a process using the best available information.
A feature with no internal structure, habit, or layering.
Relative balance of an object based on how it floats.
A proposed explanation for an observation that can be tested.
Any evidence of ancient life.
Submerged part of the continental mass, with a gentle slope.
Deposition and erosion tied to glacier movement.
An extensive, distinct, and mapped set of geologic layers.
The thin, outer layer of the Earth which makes up the rocky bottom of the ocean basins. Oceanic crust is much thinner (but denser) than continental crust. Oceanic crust is made of rocks similar to basalt and as it cools, becomes more dense.
A body of ice that moves downhill under its own mass.
Long term averages and variations within the conditions of the atmosphere.
Data which is out of the ordinary and does not fit previous trends.
Wobbles in the Earth's axis.
The property of unevenly-heated (heated from one direction) fluids (like water, air, ductile solids) in which warmer, less dense parts within the fluid rise while cooler, denser parts sink. This typically creates convection cells: round loops of rising and sinking material.
An acronym for SOund Navigation And Ranging, sonar uses sound waves to navigate and map surfaces. Sound waves created by an observer reflect off of surfaces and return to the observer. The amount of time it takes for the sound to return is a function of the distance the surface is from the observer. Bats use sonar to navigate through the dark. Ships use sonar to map the ocean floor.
Relatively flat ocean floor, which accumulates very fine grained detrital and chemical sediments.
Planar feature where two blocks of bedrock move past each other via earthquakes.
The location at the surface directly above the focus of an earthquake, typically associated with strong damage.
Pieces of rock that have been weathered and possibly eroded.
As a rock cools, the iron minerals within the rock align with the current magnetic field. Since the magnetic field changes (by where you are on Earth, by flips where "north" and "south" switch, and by migration of the magnetic north pole), scientists use the magnetic alignment within rocks to determine past movement or the magnetic field itself, along with the movement of rocks and plates via plate tectonics.
Rocks that are formed from liquid rock, i.e. from volcanic processes.
A natural substance that is typically solid, has a crystalline structure, and is typically formed by inorganic processes. Minerals are the building blocks of most rocks.
Liquid rock on the surface of the Earth.
The measure of degrees north or south from the equator, which has a latitude of 0 degrees. The Earth's north and south poles have latitudes of 90 degrees north and south, respectively.
Energy that radiates from fault movement via earthquakes.
Middle chemical layer of the Earth, made of mainly iron and magnesium silicates. It is generally denser than the crust (except for older oceanic crust) and less dense than the core.
A stoney and/or metallic object from our solar system which was never incorporated into a planet and has fallen onto Earth. Meteorite is used for the rock on Earth, meteoroid for the object in space, and meteor as the object travels in Earth's atmosphere.
The mineral makeup of a rock, i.e. which minerals are found within a rock.
A group of all atoms with a specific number of protons, having specific, universal, and unique properties.
The measure of the vibrational (kinetic) energy of a substance.
The outermost chemical layer of the Earth, defined by its low density and higher concentrations of lighter elements. The crust has two types: continental, which is the thick, more ductile, and lowest density, and oceanic, which is higher density, more brittle, and thinner.
General name of a felsic rock that is intrusive. Has more felsic minerals than mafic minerals.
General name of a mafic rock that is extrusive. Generally has a black groundmass color.
A property of solids in which a force applied to an object causes the object to fracture, break, or snap. Most rocks, at low temperatures, are brittle.
A property of a solid, such that when a force is applied, the solid flows, stretches, or bends along with the force, instead of cracking or breaking. For example, many plastics are ductile.
Short for Mohorovičić Discontinuity, it is the seismically-recognized layer within the Earth in which the crust ends and the mantle begins. Because the crust is very different in composition to the mantle, the moho is easy to find, since seismic waves travel differently through the two materials.
The innermost chemical layer of the Earth, made chiefly of iron and nickel. It has both liquid and solid components.
Rocks of the ocean floor, such as mid-ocean ridge rocks, which are brought to the surface.
A piece of foreign rock that has been incorporated into a magma body. This can be a different type of magma, or a mantle xenolith, a rock from the mantle brought up near the surface.
An intrusive ultramafic rock, which is the main component of the mantle. The minerals in peridotite are typically olivine with some pyroxene.
An igneous rock with extremely low silica composition, being made of almost all olivine and pyroxene. Ultramafic rocks contain very low amount of silica and are common in the mantle. Primary ultramafic rocks are komatiite (extrusive) and peridotite (intrusive).
Minerals with a luster similar to metal and contain metals, including valuable elements like lead, zinc, copper, tin, etc.
Force applied to an object, typically dealing with forces within the Earth.
The outermost physical layer of the Earth, made of the entire crust and upper mantle. It is brittle and broken into a series of plates, and these plates move in various ways (relative to one another), causing the features of the theory of plate tectonics.
Location where two plates are in contact, allowing a relative motion between the two plates. These are the locations where most earthquakes and volcanoes are found.
A ductile physical layer of the Earth, below the lithosphere. Movement within the asthenosphere is the main driver of plate motion, as the overriding lithosphere is pushed by this.
Also called lower mantle, a solid, more brittle physical layer of the Earth, below the asthenosphere.
The outer physical layer of the core, which is liquid. Movement within the outer core is believed to be responsible for Earth's magnetic field and flips of the magnetic field.
The innermost physical layer of the Earth, which is solid.
The gases that are part of the Earth, which are mainly nitrogen and oxygen.
A boundary between continental and oceanic plates that has no relative movement, making it a place where an oceanic plate is connected to a continental plate, but it is not a plate boundary.
When two continents crash, with no subduction (and thus little to no volcanism), since each continent is too buoyant. Many of the largest mountain ranges and broadest zones of seismic activity come from collisions.
Deepest part of the ocean where a subducting plate dives below the overriding plate.
Mix of sediments that form as a subducting plate descends and the overriding plate scrapes material and material is added.
A geological province which is added (accreted) to a continental mass via subduction and collision.
Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism.
The area of the mantle where volatiles rise from the slab, causing flux melting and volcanism.
Components of magma which are dissolved until it reaches the surface, where they expand. Examples include water and carbon dioxide. Volatiles also cause flux melting in the mantle, causing volcanism.
The process in which volatiles enter the mantle wedge, and the volatiles lower the melting temperature, causing volcanism.
The entire area which is related to land-sea interactions.
Two or more atoms or ions that are connected chemically.
A series of waves produced from a sudden movement of the floor of a ocean basin (or large lake), caused by events such as earthquakes, volcanic eruptions, landslides, and bolide impacts.
A measure of earthquake strength. Scales include Richter and Moment.
Any depression formed between the arc and the trench, commonly between the arc and the accretionary wedge.
Place with a chain of mountain volcanism on a continent, from oceanic-continental subduction.
A strain that occurs in a substance in which the item changes shape due to a stress.
Stresses that pull objects apart into a larger surface area or volume; stretching forces.
Area behind the arc, which can be subject to compressional (causing thrusted mountain belts) or extensional (causing back-arc basins) forces.
Stresses that push objects together into a smaller surface area or volume; contracting forces.
A low-angle reverse fault, common in mountain building.
A low-angle reverse fault, common in mountain building.
Faulting that is not deep into the crust, and typically only involves sedimentary cover, not basement rocks.
Faulting that is deep into the crust, and typically involves crystalline basement rocks.
The process of uplifting mountains and creating mountain belts, primarily via tectonic movement. Orogenic belts are the mountain belts that result from these movements, and orogenesis is the name for the process of forming mountain belts.
The last period of the Mesozoic, 145-66 million years ago.
A unit of the geologic time scale; smaller than an era, larger than an epoch. We are currently in the Quaternary period.
Where an ocean plate subducts beneath a continental plate, causing a volcanic arc to form.
Where a dense oceanic plate subducts beneath a less dense oceanic plate, causing an island arc to form on the overriding plate.
Place where oceanic-oceanic subduction causes volcanoes to form on an overriding oceanic plate, making a chain of active volcanoes.
An arrangement of many continental masses collided together into one larger mass. According to the Wilson Cycle, this occurs every half billion years or so.
The most recent supercontinent, which formed over 300 million years ago and started breaking apart less than 200 million years ago. Africa and South America, as well as Europe and North America, bordered each other.
The supercontinent that existed before Pangea, about 1 billion years ago. North America was positioned in the center of the land mass.
Process which allows a continental plate to bring up oceanic plate, frequently occurring in collision zones.
A valley formed by normal faulting.
Uplifted mountain block caused by normal faulting.
A valley formed by normal faulting on just one side.
The stable interior part of a continent, typically more than a billion years old, and sometimes as old as 2.5-3 billion years. When exposed on the surface, a craton is called a shield.
A break within a rock that has no relative movement between the sides. Caused by cooling, pressure release, tectonic forces, etc.
A section of a rift that starts but does not complete. This typically occurs at 120° angles to the active rift.
A depression that occurs in an area that was subject to earlier rifting.
Place where three plate boundaries (typically divergent) extend from a single point at 120° angles.
Steep spire carved by several glaciers.
Term for the extensional tectonic province that extends from California’s Sierra Nevada Mountains in the west, to Utah’s Wasatch Mountains to the east, to southern Oregon and Idaho to the north, to northern Mexico to the south. Known as a wide rift, each graben (basin) is bounded by horsts (ranges).
Melting that occurs as material is moved upward and pressure is released, typically found at divergent plate boundaries or hot spots.
An igneous composition or rock containing more than 50% carbonate minerals (e.g. calcite). Magma of this composition is very low temperature (500-600 C) relative to other magmas.
Mineral group in which the carbonate ion, CO3-2, is the building block. This can also refer to the rocks that are made from these minerals, namely limestone and dolomite (dolostone).
Can refer to a volcanic rock with lower silica composition, or the minerals that make up those rocks, namely olivine, pyroxene, amphibole, and biotite. Mafic rocks are darker in color and contain more minerals that are dark in color, but can contain some plagioclase feldspar. Primary mafic rocks are basalt (extrusive) and gabbro (intrusive).
The deep, flat part of the ocean. Also known as the ocean floor.
Metamorphism which occurs with hot fluids going within rocks, altering and changing the rocks.
The process in which solids (like minerals) are disassociated and the ionic components are dispersed in a liquid (usually water).
A biologic process of gaining energy from chemicals from within the Earth, similar to using the energy of the sun in photosynthesis.
Faulting that occurs with shear forces, typically on vertical fault plaines as two fault blocks slide past each other.
Movement in a transform or strike-slip setting which is toward the right across the fault. As viewed across the fault, objects will move to the right.
A strike-slip or transform motion in which the relative motion is to the left. As viewed across the fault, objects will move to the left.
Fault, or movement along a fault, that does not have earthquake activity.
A segment along a transform or strike-slip fault which has a compressional component, sometimes creating related thrust faulting and mountains.
A place along a transform or strike-slip fault with an extensional component, sometimes including normal faulting, basin formation, and volcanism.
An object that is cut by a fault which allows the amount of movement to be determined. This is useful for all faults, but more commonly used in strike-slip faults.
When a species no longer exists.
Rising material and heat derived from the mantle. These may be responsible for hot spots.
Opening of a volcano where lava can erupt.
An eroded island. Since wave and weather action does not extend deep into the ocean, the root of the island is preserved as a seamount. Reefs can grow around seamounts.
A process of using 3D seismic arrays to get subsurface images.
Rare very low viscosity eruption that covers vast areas. None have been observed in human history.
Rocks made from pyroclastic tephra: either ash, lapilli, and/or bombs. Tephra type can be used as an adjective, i.e. ash-fall tuff. If deposited hot, where material can fuse together while hot, the rock is then called a welded tuff.
Volcanic tephra that is less than 2 mm in diameter.