7 Geologic Time
Learning Objectives
By the end of this chapter, students should be able to:
- Explain the difference between relative time and numeric time.
- Describe the five principles of .
- Apply principles to a block diagram and interpret the sequence of geologic events.
- Define an , and explain , , and as mechanisms of decay.
- Describe how radioisotopic dating is accomplished and list the four key used.
- Explain how carbon-14 forms in the and how it is used in dating recent events.
- Explain how scientists know the numeric age of the Earth and other events in Earth history.
- Explain how sedimentary sequences can be dated using radioisotopes and other techniques.
- Define a and describe types of preservation.
- Outline how natural selection takes place as a mechanism of evolution.
- Describe .
- List the , , and of the geologic time scale and explain the purpose behind the divisions.
- Explain the relationship between time units and corresponding rock units— versus .

The geologic time scale and basic outline of Earth’s history were worked out long before we had any scientific means of assigning numerical age units, like years, to events of Earth history. Working out Earth’s history depended on realizing some key principles of relative time. Nicolas Steno (1638-1686) introduced basic principles of , the study of layered rocks, in 1669. William Smith (1769-1839), working with the of English , noticed that and their sequence were consistent throughout the region. Eventually he produced the first national geologic map of Britain, becoming known as “the Father of English Geology.” Nineteenth-century scientists developed a relative time scale using Steno’s principles, with names derived from the characteristics of the rocks in those areas. The figure of this geologic time scale shows the names of the units and subunits. Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The specific events within Earth history are discussed in chapter 8.
7.1 Relative Dating

is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries. James Hutton (see chapter 1) realized geologic processes are slow and his ideas on (i.e., “the present is the key to the past”) provided a basis for interpreting rocks of the Earth using scientific principles.
7.1.1 Relative Dating Principles
is the study of layered sedimentary rocks. This section discusses principles of relative time used in all of geology, but are especially useful in .

: In an otherwise undisturbed sequence of sedimentary , or rock layers, the layers on the bottom are the oldest and layers above them are younger.
: Layers of rocks deposited from above, such as and flows, are originally laid down horizontally. The exception to this principle is at the margins of basins, where the can slope slightly downward into the .

: Within the depositional , are continuous in all directions until they thin out at the edge of that . Of course, all eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a source or a . that are cut by a canyon later remain continuous on either side of the canyon.

: events like , and intrusions that cut across rocks are younger than the rocks they cut across.
Principle of Inclusions: When one rock contains pieces or of another rock, the included rock is older than the .

Principle of Fossil Succession: Evolution has produced a succession of unique that correlate to the units of the geologic time scale. Assemblages of contained in are unique to the time they lived, and can be used to correlate rocks of the same age across a wide geographic distribution. Assemblages of refers to groups of several unique occurring together.
7.1.2 Grand Canyon Example

The Grand Canyon of Arizona illustrates the principles. The photo shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, based on the . The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops. The rock layers exhibit the , as they are found on both sides of the Grand Canyon which has been carved by the Colorado River.

The diagram called “Grand Canyon’s Three Sets of Rocks” shows a cross-section of the rocks exposed on the walls of the Grand Canyon, illustrating the , , and . In the lowest parts of the Grand Canyon are the oldest sedimentary , with and rocks at the bottom. The shows the sequence of these events. The (#16) is the oldest rock and the cross-cutting intrusion (#17) is younger. As seen in the figure, the other layers on the walls of the Grand Canyon are numbered in reverse order with #15 being the oldest and #1 the youngest. This illustrates the . The Grand Canyon region lies in Colorado Plateau, which is characterized by horizontal or nearly horizontal , which follows the . These rock have been barely disturbed from their original , except by a broad regional uplift.

The photo of the Grand Canyon here show that were originally deposited in a flat layer on top of older and “” rocks, per the principle. Because the of the rocks and the of the overlying is not continuous but broken by events of , intrusion, and , the contact between the and the older is termed an . An represents a during which did not occur or removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place. appear in cross sections and columns as wavy lines between . are discussed in the next section.
7.1.3 Unconformities

There are three types of , , , and . A occurs when is deposited on top of and rocks as is the case with the contact between the and rocks at the bottom of the Grand Canyon.
The in the Grand Canyon represent alternating and where sea level rose and fell over millions of years. When the sea level was high formed. When sea-level fell, the land was exposed to creating an . In the Grand Canyon cross-section, this is shown as heavy wavy lines between the various numbered . This is a type of called a , where either non- or took place. In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the . Disconformities are that occur between parallel layers of indicating either a of no or .

The in most of the Grand Canyon are horizontal. However, near the bottom horizontal overlie tilted . This is known as the Great and is an example of an . The lower were tilted by processes that disturbed their and caused the to be eroded. Later, horizontal were deposited on top of the tilted creating the .
Here are three graphical illustrations of the three types of .

, where is a break or stratigraphic absence between in an otherwise parallel sequence of .

, where sedimentary are deposited on crystalline ( or ) rocks.

, where sedimentary are deposited on a terrain developed on sedimentary that have been deformed by tilting, folding, and/or . so that they are no longer horizontal.
7.1.3 Applying Relative Dating Principles

In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of , sedimentary, (see chapter 4, chapter 5, and chapter 6). The sequence begins with the folded on the bottom. Next, the gneiss is cut and displaced by the labeled A. Both the and A are cut by the granitic intrusion called B; its irregular outline suggests it is an granitic intrusion emplaced as into the . Since B cuts both the and A, B is younger than the other two rock . Next, the , A, and B were eroded forming a as shown with the wavy line. This was actually an ancient landscape surface on which C was subsequently deposited perhaps by a . Next, basaltic D cut through all rocks except E. This shows that there is a between sedimentary rocks C and E. The top of D is level with the top of layer C, which establishes that flattened the landscape prior to the of layer E, creating a between rocks D and E. F cuts across all of the older rocks B, C and E, producing a , which is the low ridge on the upper-left side of the diagram. The final events affecting this area are current processes working on the land surface, off the edge of the , and producing the modern landscape at the top of the diagram.
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 7.1 via the QR code.
7.2 Absolute Dating

Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate. Based on Hutton’s (see chapter 1), early geologists surmised geological processes work slowly and the Earth is very old. principles was how scientists interpreted Earth history until the end of the 19th Century. Because science advances as technology advances, the discovery of in the late 1800s provided scientists with a new scientific tool called radioisotopic dating. Using this new technology, they could assign specific time units, in this case years, to grains within a rock. These numerical values are not dependent on comparisons with other rocks such as with , so this dating method is called . There are several types of discussed in this section but radioisotopic dating is the most common and therefore is the on this section.
7.2.1 Radioactive Decay

All on the Periodic Table of Elements (see chapter 3) contain . An is an atom of an with a different number of neutrons. For example, hydrogen (H) always has 1 proton in its nucleus (the atomic number), but the number of neutrons can vary among the (0, 1, 2). Recall that the number of neutrons added to the atomic number gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium (1H), when hydrogen has 1 proton and 1 neutron it is called deuterium (2H), and when hydrogen has 1 proton and 2 neutrons it is called tritium (3H).
Many have both stable and unstable . For the hydrogen example, 1H and 2H are stable, but 3H is unstable. Unstable , called , spontaneously decay over time releasing subatomic particles or energy in a process called decay. When this occurs, an unstable becomes a more stable of another . For example, carbon-14 (14C) decays to nitrogen-14 (14N).

The decay of any individual atom is a completely unpredictable and random event. However, some rock specimens have an enormous number of , perhaps trillions of atoms, and this large group of does have a predictable pattern of decay. The decay of half of the in this group takes a specific amount of time. The time it takes for half of the atoms in a substance to decay is called the . In other words, the of an is the amount of time it takes for half of a group of unstable to decay to a stable . The is constant and measurable for a given , so it can be used to calculate the age of a rock. For example, the uranium-238 (238U) is 4.5 billion years and the of 14C is 5,730 years.
The principles behind this dating method require two key assumptions. First, the grains containing the formed at the same time as the rock, such as in an that crystallized from . Second, the crystals remain a closed , meaning they are not subsequently altered by moving in or out of them.

These requirements place some constraints on the kinds of rock suitable for dating, with being the best. rocks are crystalline, but the processes of may reset the clock and derived ages may represent a smear of different events rather than the age of original . sedimentary rocks contain clasts from separate from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with , such as , may contain suitable for radioisotopic dating. layers and flows within a sedimentary sequence can be used to date the sequence. Cross-cutting rocks and can be used to bracket the ages of affected, older sedimentary rocks. The resistant , found as clasts in many ancient sedimentary rocks, has been successfully used for establishing very old dates, including the age of Earth’s oldest known rocks. Knowing that in metamorphosed came from older rocks that are no longer available for study, scientists can date to establish the age of the pre- .

There are several ways atoms decay. We will consider three of them here—, , and . is when an alpha particle, which consists of two protons and two neutrons, is emitted from the nucleus of an atom. This also happens to be the nucleus of a helium atom; helium gas may get trapped in the crystal lattice of a in which has taken place. When an atom loses two protons from its nucleus, lowering its atomic number, it is transformed into an that is two atomic numbers lower on the Periodic Table of the Elements.

The loss of four particles, in this case two neutrons and two protons, also lowers the mass of the atom by four. For example takes place in the unstable 238U, which has an atomic number of 92 (92 protons) and mass number of 238 (total of all protons and neutrons). When 238U spontaneously emits an alpha particle, it becomes thorium-234 (234Th). The decay product of an is called its and the original is called the . In this case, 238U is the and 234Th is the . The of 238U is 4.5 billion years, i.e., the time it takes for half of the atoms to decay into the . This isotope of uranium, 238U, can be used for the oldest materials found on Earth, and even and materials from the earliest events in our solar system.

Beta Decay
is when a neutron in its nucleus splits into an electron and a proton. The electron is emitted from the nucleus as a beta ray. The new proton increases the ’s atomic number by one, forming a new with the same atomic mass as the . For example, 234Th is unstable and undergoes to form protactinium-234 (234Pa), which also undergoes to form uranium-234 (234U). Notice these are all of different but they have the same atomic mass of 234. The decay process of like uranium keeps producing and until a stable, or non-radioactive, daughter is formed. Such a series is called a . The of the 238U progresses through a series of alpha (red arrows on the adjacent figure) and beta decays (blue arrows), until it forms the stable , lead-206 (206Pb).

is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron. This produces one of two different effects: 1) an electron jumps in to fill the missing spot of the departed electron and emits an X-ray, or 2) in what is called the Auger process, another electron is released and changes the atom into an . The atomic number is reduced by one and mass number remains the same. An example of an that decays by is potassium-40 (40K). 40K makes up a tiny percentage (0.012%) of naturally occurring potassium, most of which not . 40K decays to argon-40 (40Ar) with a of 1.25 billion years, so it is very useful for dating geological events. Below is a table of some of the more commonly-used dating and their half-lives.
Elements | Parent symbol | Daughter symbol | Half-life |
---|---|---|---|
Uranium-238/Lead-206 | 238U | 206Pb | 4.5 billion years |
Uranium-235/Lead-207 | 235U | 207Pb | 704 million years |
Potassium-40/Argon-40 | 40K | 40Ar | 1.25 billion years |
Rubidium-87/Strontium-87 | 87Rb | 87Sr | 48.8 billion years |
Carbon-14/Nitrogen-14 | 14C | 14N | 5,730 years |
Table 7.1: Some common isotopes used for radioisotopic dating.
7.2.2 Radioisotopic Dating

For a given a sample of rock, how is the dating procedure carried out? The parent and are separated out of the using chemical extraction. In the case of uranium, 238U and 235U are separated out together, as are the 206Pb and 207Pb with an instrument called a .

Here is a simple example of age calculation using the daughter-to-parent ratio of . When the initially forms, it consists of 0% daughter and 100% , so the daughter-to-parent ratio (D/P) is 0. After one , half the parent has decayed so there is 50% daughter and 50% parent, a 50/50 ratio, with D/P = 1. After two half-lives, there is 75% daughter and 25% parent (75/25 ratio) and D/P = 3. This can be further calculated for a series of half-lives as shown in the table. The table does not show more than 10 half-lives because after about 10 half-lives, the amount of remaining parent is so small it becomes too difficult to accurately measure via chemical analysis. Modern applications of this method have achieved remarkable accuracies of plus or minus two million years in 2.5 billion years (that’s ±0.055%). Applying the uranium/lead technique in any given sample analysis provides two separate clocks running at the same time, 238U and 235U. The existence of these two clocks in the same sample gives a cross-check between the two. Many geological samples contain multiple parent/daughter pairs, so cross-checking the clocks confirms that radioisotopic dating is highly reliable.
Half lives (#) | Parent present (%) | Daughter present (%) | Daughter/parent ratio | Parent/daughter ratio |
---|---|---|---|---|
Start the clock | 100 | 0 | 0 | Infinite |
1 | 50 | 50 | 1 | 1 |
2 | 25 | 75 | 3 | 0.33 |
3 | 12.5 | 87.5 | 7 | 0.143 |
4 | 6.25 | 93.75 | 15 | 0.0667 |
5 | 3.125 | 96.875 | 31 | 0.0325 |
10 | 0.098 | 99.9 | 1023 | 0.00098 |
Table 7.2: Ratio of parent to daughter in terms of half-life.

Another radioisotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone. Radiocarbon dating, also called carbon dating, uses the unstable carbon-14 (14C) and the stable carbon-12 (12C). Carbon-14 is constantly being created in the by the interaction of cosmic particles with atmospheric nitrogen-14 (14N). Cosmic particles such as neutrons the nitrogen nucleus, kicking out a proton but leaving the neutron in the nucleus. The reduces the atomic number by one, changing it from seven to six, changing the nitrogen into carbon with the same mass number of 14. The 14C quickly with oxygen (O) in the to form carbon dioxide (14CO2) that mixes with other atmospheric carbon dioxide (12CO2) and this mix of gases is incorporated into living matter. While an organism is alive, the ratio of 14C/12C in its body doesn’t really change since CO2 is constantly exchanged with the . However, when it dies, the radiocarbon clock starts ticking as the 14C decays back to 14N by , which has a of 5,730 years. The radiocarbon dating technique is thus useful for 57,300 years or so, about 10 half-lives back.

Radiocarbon dating relies on daughter-to-parent ratios derived from a known quantity of parent 14C. Early applications of carbon dating assumed the production and concentration of 14C in the remained fairly constant for the last 50,000 years. However, it is now known that the amount of parent 14C levels in the has varied. Comparisons of carbon ages with tree-ring data and other data for known events have allowed reliable calibration of the radiocarbon dating method. Taking into account carbon-14 baseline levels must be calibrated against other reliable dating methods, carbon dating has been shown to be a reliable method for dating archaeological specimens and very recent geologic events.
7.2.3 Age of the Earth

The work of Hutton and other scientists gained attention after the Renaissance (see chapter 1), spurring exploration into the idea of an ancient Earth. In the late 19th century William Thompson, a.k.a. Lord Kelvin, applied his knowledge of physics to develop the assumption that the Earth started as a hot molten sphere. He estimated the Earth is 98 million years old, but because of uncertainties in his calculations stated the age as a range of between 20 and 400 million years. This animation illustrates how Kelvin calculated this range and why his numbers were so far off, which has to do with unequal heat transfer within the Earth. It has also been pointed out that Kelvin failed to consider pliability and in the Earth’s as a heat transfer mechanism. Kelvin’s estimate for Earth’s age was considered plausible but not without challenge, and the discovery of provided a more accurate method for determining ancient ages.
In the 1950’s, Clair Patterson (1922–1995) thought he could determine the age of the Earth using from , which he considered to be early solar system remnants that were present at the time Earth was forming. Patterson analyzed samples for uranium and lead using a . He used the uranium/lead dating technique in determining the age of the Earth to be 4.55 billion years, give or take about 70 million (± 1.5%). The current estimate for the age of the Earth is 4.54 billion years, give or take 50 million (± 1.1%). It is remarkable that Patterson, who was still a graduate student at the University of Chicago, came up with a result that has been little altered in over 60 years, even as technology has improved dating methods.
7.2.4 Dating Geological Events

of that are common in crystals are useful for radioisotopic dating. The uranium/lead method, with its two cross-checking clocks, is most often used with crystals of the (ZrSiO4) where uranium can substitute for zirconium in the crystal lattice. is resistant to which makes it useful for dating geological events in ancient rocks. During events, crystals may form multiple crystal layers, with each layer recording the isotopic age of an event, thus tracing the progress of the several events.
Geologists have used grains to do some amazing studies that illustrate how scientific conclusions can change with technological advancements. crystals from Western Australia that formed when the crust first differentiated from the 4.4 billion years ago have been determined to be the oldest known rocks. The grains were incorporated into metasedimentary host rocks, sedimentary rocks showing signs of having undergone partial metamorphism. The host rocks were not very old but the embedded zircon grains were created 4.4 billion years ago, and survived the subsequent processes of , , , and . From other properties of the crystals, researchers concluded that not only were continental rocks exposed above sea level, but also that conditions on the early Earth were cool enough for liquid water to exist on the surface. The presence of liquid water allowed the processes of weathering and erosion to take place. Researchers at UCLA studied 4.1 billion-year-old crystals and found carbon in the crystals that may be biogenic in origin, meaning that life may have existed on Earth much earlier than previously thought.

rocks best suited for radioisotopic dating because their primary provide dates of from . processes tend to reset the clocks and smear the ’s original date. sedimentary rocks are less useful because they are made of derived from multiple parent sources with potentially many dates. However, scientists can use events to date sedimentary sequences. For example, if sedimentary are between a flow and with radioisotopic dates of 54 million years and 50 million years, then geologists know the sedimentary and its formed between 54 and 50 million years ago. Another example would be a 65 million year old that cut across sedimentary . This provides an upper limit age on the sedimentary , so this would be older than 65 million years. Potassium is common in and has been used for potassium/argon dating. Primary sedimentary containing like 40K, has provided dates for important geologic events.
7.2.5 Other Absolute Dating Techniques

Luminescence (aka Thermoluminescence): Radioisotopic dating is not the only way scientists determine numeric ages. Luminescence dating measures the time elapsed since some , such as coarse- of , were last exposed to light or heat at the surface of Earth. All buried are exposed to radiation from normal background radiation from the decay process described above. Some of these electrons get trapped in the crystal lattice of like . When exposed at the surface, ultraviolet radiation and heat from the Sun releases these electrons, but when the are buried just a few inches below the surface, the electrons get trapped again. Samples of coarse collected just a few feet below the surface are analyzed by stimulating them with light in a lab. This stimulation releases the trapped electrons as a photon of light which is called luminescence. The amount luminescence released indicates how long the has been buried. Luminescence dating is only useful for dating young that are less than 1 million years old. In Utah, luminescence dating is used to determine when coarse-grained layers were buried near a . This is one technique used to determine the interval of large earthquakes on like the Wasatch Fault that primarily cut coarse-grained material and lack buried organic for radiocarbon dating.

Fission Track: Fission track dating relies on damage to the crystal lattice produced when unstable 238U decays to the 234Th and releases an alpha particle. These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage. This is common in uranium-bearing grains such as apatite. The tracks are large and can be visually counted under an optical microscope. The number of tracks correspond to the age of the grains. Fission track dating works from about 100,000 to 2 billion (1 × 105 to 2 × 109) years ago. Fission track dating has also been used as a second clock to confirm dates obtained by other methods.
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 7.2 via the QR code.
7.3 Fossils and Evolution

are any evidence of past life preserved in rocks. They may be actual remains of body parts (rare), impressions of soft body parts, and of body parts (more common), body parts replaced by (common) or evidence of animal behavior such as footprints and burrows. The body parts of living organisms range from the hard bones and shells of animals, soft cellulose of plants, soft bodies of jellyfish, down to single cells of bacteria and algae. Which body parts can be preserved? The vast majority of life today consists soft-bodied and/or single celled organisms, and will not likely be preserved in the geologic record except under unusual conditions. The best environment for preservation is the ocean, yet processes can hard parts and scavenging can reduce or eliminate remains. Thus, even under ideal conditions in the ocean, the likelihood of preservation is quite limited. For life, the possibility of remains being buried and preserved is even more limited. In other words, the record is incomplete and records only a small percentage of life that existed. Although incomplete, records are used for , using the , and provide a method used for establishing the age of a on the Geologic Time Scale.
7.3.1 Types of Preservation

Remnants or impressions of hard parts, such as a clam shell or dinosaur bone, are the most common types of . The original material has almost always been replaced with new that preserve much of the shape of the original shell, bone, or cell. The common types of preservation are , , and , , and .
is a rare form of fossilization where the original materials or hard parts of the organism are preserved. Preservation of soft-tissue is very rare since these organic materials easily disappear because of bacterial decay. Examples of are unaltered biological materials like insects in amber or original like mother-of-pearl on the interior of a shell. Another example is mammoth skin and hair preserved in post-glacial deposits in the Arctic regions. Rare mummification has left fragments of soft tissue, skin, and sometimes even blood vessels of dinosaurs, from which proteins have been isolated and evidence for DNA fragments have been discovered.


occurs when an organism is buried, and then in completely impregnate all spaces within the body, even cells. Soft body structures can be preserved in great detail, but stronger materials like bone and teeth are the most likely to be preserved. Petrified wood is an example of detailed cellulose structures in the wood being preserved. The University of California Berkeley website has more information on .
and form when the original material of the organism dissolves and leaves a cavity in the surrounding rock. The shape of this cavity is an . If the mold is subsequently filled with or a , the organism’s external shape is preserved as a . Sometimes internal cavities of organisms, such internal of clams, snails, and even skulls are preserved as internal showing details of soft structures. If the chemistry is right, and burial is rapid, nodules form around soft structures preserving the three-dimensional detail. This is called .


occurs when the organic tissues of an organism are compressed, the are driven out, and everything but the carbon disappears leaving a carbon silhouette of the original organism. Leaf and fern are examples of .


are indirect evidence left behind by an organism, such as burrows and footprints, as it lived its life. is specifically the study of prehistoric animal tracks. Dinosaur tracks testify to their presence and movement over an area, and even provide information about their size, gait, speed, and behavior. Burrows dug by tunneling organisms tell of their presence and mode of life. Other include fossilized feces called coprolites and stomach stones called gastroliths that provide information about diet and habitat.
7.3.2 Evolution
Evolution has created a variety of ancient that are important to . (see chapter 7 and chapter 5) This section is a brief discussion of the process of evolution. The British naturalist Charles Darwin (1809-1882) recognized that life forms evolve into progeny life forms. He proposed natural selection—which operated on organisms living under environmental conditions that posed challenges to survival—was the mechanism driving the process of evolution forward.

The basic classification unit of life is the species: a population of organisms that exhibit shared characteristics and are capable of reproducing fertile offspring. For a species to survive, each individual within a particular population is faced with challenges posed by the environment and must survive them long enough to reproduce. Within the natural variations present in the population, there may be individuals possessing characteristics that give them some advantage in facing the environmental challenges. These individuals are more likely to reproduce and pass these favored characteristics on to successive generations. If sufficient individuals in a population fail to surmount the challenges of the environment and the population cannot produce enough viable offspring, the species becomes . The average lifespan of a species in the record is around a million years. That life still exists on Earth shows the role and importance of evolution as a natural process in meeting the continual challenges posed by our dynamic Earth. If the inheritance of certain distinctive characteristics is sufficiently favored over time, populations may become genetically isolated from one another, eventually resulting in the evolution of separate species. This genetic isolation may also be caused by a geographic barrier, such as an island surrounded by ocean. This of evolution by natural selection was elaborated by Darwin in his book On the Origin of Species (see chapter 1). Since Darwin’s original ideas, technology has provided many tools and mechanisms to study how evolution and speciation take place and this arsenal of tools is growing. Evolution is well beyond the stage and is a well-established of modern science.
Variation within populations occurs by the natural mixing of genes through sexual reproduction or from naturally occurring mutations. Some of this genetic variation can introduce advantageous characteristics that increase the individual’s chances of survival. While some species in the record show little morphological change over time, others show gradual or punctuated changes, within which forms can be seen.
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 7.3 via the QR code.
7.4 Correlation

is the process of establishing which sedimentary are of the same age but geographically separate. can be determined by using magnetic polarity reversals (chapter 2), rock types, unique rock sequences, or . There are four main types of : , lithostratigraphic, chronostratigraphic, and biostratigraphic.
7.4.1 Stratigraphic Correlation
is the process of establishing which sedimentary are the same age at distant geographical areas by means of their relationship. Geologists construct geologic histories of areas by mapping and making columns-a detailed description of the from bottom to top. An example of relationships and between Canyonlands National Park and Zion National Park in Utah. At Canyonlands, the Navajo Sandstone overlies the Kayenta Formation which overlies the cliff-forming Wingate Formation. In Zion, the Navajo Sandstone overlies the Kayenta Formation which overlies the cliff-forming Moenave Formation. Based on the relationship, the Wingate and Moenave Formations correlate. These two have unique names because their and outcrop pattern is slightly different. Other in the Colorado Plateau and their sequence can be recognized and correlated over thousands of square miles.

7.4.2 Lithostratigraphic Correlation

establishes a similar age of based on the lithology that is the and physical properties of that . Lithos is Greek for stone and -logy comes from the Greek word for doctrine or science. can be used to correlate whole long distances or can be used to correlate smaller within formations to trace their extent and regional .

For example, the Navajo Sandstone, which makes up the prominent walls of Zion National Park, is the same Navajo Sandstone in Canyonlands because the lithology of the two are identical even though they are hundreds of miles apart. Extensions of the same Navajo Sandstone formation are found miles away in other parts of southern Utah, including Capitol Reef and Arches National Parks. Further, this same is the called the Aztec Sandstone in Nevada and Nugget Sandstone near Salt Lake City because they are lithologically distinct enough to warrant new names.
7.4.3 Chronostratigraphic Correlation

matches rocks of the same age, even though they are made of different lithologies. Different lithologies of sedimentary rocks can form at the same time at different geographic locations because vary geographically. For example, at any one time in a setting there could be this sequence of from beach to deep : beach, near area, shallow , , slope, and deep . Each will have a unique . On the figure of the Permian Capitan Reef at Guadalupe National Monument in West Texas, the red line shows a chronostratigraphic time line that represents a snapshot in time. Shallow-water /back area is light blue, the main Capitan is dark blue, and deep-water is yellow. All three of these unique lithologies were forming at the same time in along this red timeline.

7.4.4 Biostratigraphic Correlation

uses to determine ages. represent assemblages or groups of organisms that were uniquely present during specific intervals of geologic time. Assemblages is referring a group of . allow geologists to assign a to an absolute date range, such as the (199 to 145 million years ago), rather than a relative time scale. In fact, most of the geologic time ranges are mapped to assemblages. The most useful come from lifeforms that were geographically widespread and had a species lifespan that was limited to a narrow time interval. In other words, can be found in many places around the world, but only during a narrow time frame. Some of the best for are microfossils, most of which came from single-celled organisms.
As with microscopic organisms today, they were widely distributed across many environments throughout the world. Some of these microscopic organisms had hard parts, such as exoskeletons or outer shells, making them better candidates for preservation.

Foraminifera, single celled organisms with calcareous shells, are an example of an especially useful for the and .

Conodonts are another example of microfossils useful for of the through . Conodonts are tooth-like phosphatic structures of an eel-like multi-celled organism that had no other preservable hard parts. The conodont-bearing creatures lived in shallow environments all over the world. Upon death, the phosphatic hard parts were scattered into the rest of the . These distinctive tooth-like structures are easily collected and separated from in the laboratory.

Because the conodont creatures were so widely abundant, rapidly evolving, and readily preserved in , their are especially useful for correlating , even though knowledge of the actual animal possessing them is sparse. Scientists in the 1960s carried out a fundamental that tied conodont zonation into ammonoids, which are ancient cousins of the pearly nautilus. Up to that point ammonoids were the only standard for , so cross-referencing micro- and macro- enhanced the reliability of for either type. That conodont study went on to establish the use of conodonts to internationally correlate located in Europe, Western North America, and the Arctic Islands of Canada.
7.4.5 Geologic Time Scale

Geologic time has been subdivided into a series of divisions by geologists. is the largest division of time, followed by , , , and age. The partitions of the geologic time scale is the same everywhere on Earth; however, rocks may or may not be present at a given location depending on the geologic activity going on during a particular of time. Thus, we have the concept of time vs. rock, in which time is an unbroken continuum but rocks may be missing and/or unavailable for study. The figure of the geologic time scale, represents time flowing continuously from the beginning of the Earth, with the time units presented in an unbroken sequence. But that does not mean there are rocks available for study for all of these time units.


The geologic time scale was developed during the 19th century using the principles of . The relative order of the time units was determined before geologist had the tools to assign numerical ages to and events. using to assign and names to sedimentary rocks on a worldwide scale. With the expansion of science and technology, some geologists think the influence of humanity on natural processes has become so great they are suggesting a new geologic time , known as the .
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 7.4 via the QR code.
Summary
Events in Earth history can be placed in sequence using the five principles of . The geologic time scale was completely worked out in the 19th Century using these principles without knowing any actual numeric ages for the events. The discovery of in the late 1800s enabled , the assignment of numerical ages to events in the Earth’s history, using decay of unstable . Accurately interpreting radioisotopic dating data depends on the type of rock tested and accurate assumptions about baseline values. With a combination of relative and , the history of geological events, age of Earth, and a geologic time scale have been determined with considerable accuracy. is additional tool used for understanding how change geographically. Geologic time is vast, providing plenty of time for the evolution of various lifeforms, and some of these have become preserved as that can be used for . The geologic time scale is continuous, although the rock record may be broken because rocks representing certain time may be missing.
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 7 via the QR code.
URLs Linked Within This Chapter
Animation 1: Why is it Hot Underground? [Video: 1:49] https://www.youtube.com/watch?v=mOSpRzW2i_4
The University of California Berkeley website: https://ucmp.berkeley.edu/paleo/fossils/permin.html
Text References
- Allison, P.A., and Briggs, D.E.G., 1993, Exceptional fossil record: Distribution of soft-tissue preservation through the Phanerozoic: Geology, v. 21, no. 6, p. 527–530.
- Bell, E.A., Boehnke, P., Harrison, T.M., and Mao, W.L., 2015, Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon: Proc. Natl. Acad. Sci. U. S. A., v. 112, no. 47, p. 14518–14521.
- Brent Dalrymple, G., 1994, The Age of the Earth: Stanford University Press.
- Burleigh, R., 1981, W. F. Libby and the development of radiocarbon dating: Antiquity, v. 55, no. 214, p. 96–98.
- Christopher B. DuRoss, Stephen F. Personius, Anthony J. Crone, Susan S. Olig, and William R. Lund, 2011, Integration of Paleoseismic Data from Multiple Sites to Develop an Objective Earthquake Chronology: Application to the Weber Segment of the Wasatch Fault Zone, Utah: Bulletin of the Seismological Society of America, v. 101, no. 6, p. 2765–2781., doi: 0.1785/0120110102.
- Dass, C., 2007, Basics of mass spectrometry, in Fundamentals of Contemporary Mass Spectrometry: John Wiley & Sons, Inc., p. 1–14.
- Elston, D.P., Billingsley, G.H., and Young, R.A., 1989, Geology of Grand Canyon, Northern Arizona (with Colorado River Guides): Lees Ferry to Pierce Ferry, Arizona: Amer Geophysical Union.
- Erickson, J., Coates, D.R., and Erickson, H.P., 2014, An introduction to fossils and minerals: seeking clues to the Earth’s past: Facts on File science library, Facts On File, Incorporated, Facts on File science library.
- Geyh, M.A., and Schleicher, H., 1990, Absolute Age Determination: Physical and Chemical Dating Methods and Their Application, 503 pp: Spring-er-Verlag, New York.
- Ireland, T., 1999, New tools for isotopic analysis: Science, v. 286, no. 5448, p. 2289–2290.
- Jackson, P.W., and of London, G.S., 2007, Four Centuries of Geological Travel: The Search for Knowledge on Foot, Bicycle, Sledge and Camel: Geological Society special publication, Geological Society, Geological Society special publication.
- Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., and others, 1971, Precision measurement of half-lives and specific activities of U 235 and U 238: Phys. Rev. C Nucl. Phys.
- Léost, I., Féraud, G., Blanc-Valleron, M.M., and Rouchy, J.M., 2001, First absolute dating of Miocene Langbeinite evaporites by 40Ar/39Ar laser step-heating:[K2Mg2 (SO4) 3] Stebnyk Mine (Carpathian Foredeep Basin): Geophys. Res. Lett., v. 28, no. 23, p. 4347–4350.
- Mosher, L.C., 1968, Triassic conodonts from Western North America and Europe and Their Correlation: J. Paleontol., v. 42, no. 4, p. 895–946.
- Oberthür, T., Davis, D.W., Blenkinsop, T.G., and Höhndorf, A., 2002, Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt: Precambrian Res., v. 113, no. 3–4, p. 293–305.
- Patterson, C., 1956, Age of meteorites and the earth: Geochim. Cosmochim. Acta, v. 10, no. 4, p. 230–237.
- Schweitzer, M.H., Wittmeyer, J.L., Horner, J.R., and Toporski, J.K., 2005, Soft-tissue vessels and cellular preservation in Tyrannosaurus rex: Science, v. 307, no. 5717, p. 1952–1955.
- Valley, J.W., Peck, W.H., King, E.M., and Wilde, S.A., 2002, A cool early Earth: Geology, v. 30, no. 4, p. 351–354.
- Whewell, W., 1837, History of the Inductive Sciences: From the Earliest to the Present Times: J.W. Parker, 492 p.
- Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001, Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, no. 6817, p. 175–178.
- Winchester, S., 2009, The Map That Changed the World: William Smith and the Birth of Modern Geology: HarperCollins.
Figure References
Figure 7.1: Nicolas Steno, c. 1670. Justus Sustermans. ca. 1666 and 1677; uploaded in 2012. Public domain. https://commons.wikimedia.org/wiki/File:Portrait_of_Nicolas_Stenonus.jpg
Figure 7.2: Geologic time scale. USGS. 1997. Public domain. https://pubs.usgs.gov/gip/fossils/numeric.html
Figure 7.3: Lower strata are older than those lying on top of them. Wilson44691. 2009. Public domain. https://commons.wikimedia.org/wiki/File:IsfjordenSuperposition.jpg
Figure 7.4: Lateral continuity. Roger Bolsius. 2013. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Grand_Canyon_Panorama_2013.jpg
Figure 7.5: Dark dike cutting across older rocks, the lighter of which is younger than the grey rock. Thomas Eliasson of Geological Survey of Sweden. 2008. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Multiple_Igneous_Intrusion_Phases_Kosterhavet_Sweden.jpg
Figure 7.6: Fossil succession showing correlation among strata. דקי 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Faunal_sucession.jpg
Figure 7.7: The Grand Canyon of Arizona. Jean-Christophe BENOIST. 2012. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Grand_Canyon_-_Hopi_Point.JPG
Figure 7.8: The rocks of the Grand Canyon. NPS. 2018. Public domain. https://www.nps.gov/articles/age-of-rocks-in-grand-canyon.htm
Figure 7.9: The red, layered rocks of the Grand Canyon Supergroup overlying the dark-colored rocks of the Vishnu schist represents a type of unconformity called a nonconformity. Simeon87. 2012. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Grand_Canyon_with_Snow_4.JPG
Figure 7.10: All three of these formations have a disconformity at the two contacts between them. NPS. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Redwall,_Temple_Butte_and_Muav_formations_in_Grand_Canyon.jpg
Figure 7.11: In the lower part of the picture is an angular unconformity in the Grand Canyon known as the Great Unconformity. Doug Dolde. 2008. Public domain. https://commons.wikimedia.org/wiki/File:View_from_Lipan_Point.jpg
Figure 7.12: Disconformity. דקי. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Disconformity.jpg
Figure 7.13: Nonconformity (the lower rocks are igneous or metamorphic). דקי. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Nonconformity.jpg
Figure 7.14: Angular unconformity. דקי. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Angular_unconformity.jpg
Figure 7.15: Block diagram to apply relative dating principles. Woudloper. 2009. CC BY-SA 1.0. https://commons.wikimedia.org/wiki/File:Cross-cutting_relations.svg
Figure 7.16: Canada’s Nuvvuagittuq Greenstone Belt may have the oldest rocks and oldest evidence life on Earth, according to recent studies. NASA. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Nuvvuagittuq_belt_rocks.jpg
Figure 7.17: Three isotopes of hydrogen. Dirk Hünniger. 2016. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Hydrogen_Deuterium_Tritium_Nuclei_Schmatic-en.svg
Figure 7.18: Simulation of half-life. Sbyrnes321. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Halflife-sim.gif
Figure 7.19: Granite (left) and gneiss (right). Fjæregranitt3 by Friman, 2007 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Fj%C3%A6regranitt3.JPG). Gneiss by Siim Sepp, 2005 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Gneiss.jpg).
Figure 7.20: An alpha decay: Two protons and two neutrons leave the nucleus. Inductiveload. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Alpha_Decay.svg
Figure 7.21: Periodic table of the elements. Sandbh. 2017. CC BY-SA 4.0. https://en.wikipedia.org/wiki/File:Periodic_Table_Chart_with_less_active_and_active_nonmetals.png
Figure 7.22: Decay chain of U-238 to stable Pb-206 through a series of alpha and beta decays. ThaLibster. 2017. CC BY-SA 4.0. https://en.wikipedia.org/wiki/File:Decay_Chain_of_Uranium-238.svg
Figure 7.23: The two paths of electron capture. Pamputt. 2015. CC BY-SA 4.0. https://en.wikipedia.org/wiki/File:Atomic_rearrangement_following_an_electron_capture.svg
Figure 7.24: Mass spectrometer instrument. Archives CAMECA. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:IMS3F_pbmf.JPG
Figure 7.25: Graph of the amount of half life versus the amount of daughter isotope. Krishnavedala. 2015. Public domain. https://en.wikipedia.org/wiki/File:Half_times.svg
Figure 7.26: Schematic of carbon going through a mass spectrometer. Mike Christie. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Accelerator_mass_spectrometer_schematic_for_radiocarbon.svg
Figure 7.27: Carbon dioxide concentrations over the last 400,000 years. Robert A. Rohde. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Carbon_Dioxide_400kyr.svg
Figure 7.28: Artist’s impression of the Earth in the Hadean. Tim Bertelink. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Hadean.png
Figure 7.29: Photomicrograph of zircon crystal. Denniss. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Zircon_microscope.jpg
Figure 7.30: 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
Figure 7.31: Thermoluminescence, a type of luminescence dating. Zkeizars. 2008. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Keizars_TLexplained2.jpg
Figure 7.32: Apatite from Mexico. Robert M. Lavinsky. Before March 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Apatite-(CaF)-280343.jpg
Figure 7.33: Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. H. Raab. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Archaeopteryx_lithographica_(Berlin_specimen).jpg
Figure 7.34: The trilobites had a hard exoskeleton, and is an early arthropod, the same group that includes modern insects, crustaceans, and arachnids. Wilson44691. 2010. Public domain. https://commons.wikimedia.org/wiki/File:ElrathiakingiUtahWheelerCambrian.jpg
Figure 7.35: Mosquito preserved in amber. Didier Desouens. 2010. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Ambre_Dominique_Moustique.jpg
Figure 7.36: Permineralization in petrified wood. Moondigger. 2005. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Petrified_forest_log_2_md.jpg
Figure 7.37: External mold of a clam. Wilson44691. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Aviculopecten_subcardiformis01.JPG
Figure 7.38: Carbonized leaf. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:ViburnumFossil.jpg
Figure 7.39: Dinosaur tracks as a record of its passing. Ballista. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Cheirotherium_prints_possibly_Ticinosuchus.JPG
Figure 7.40: Fossil animal droppings (coprolite). USGS. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Coprolite.jpg
Figure 7.41: Variation within a population. Inglesenargentina. 2006. Public domain. https://en.wikipedia.org/wiki/File:Bell-shaped-curve.JPG
Figure 7.42: 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 7.43: Correlation of strata along the Grand Staircase from the Grand Canyon to Zion Canyon, Bryce Canyon, and Cedar Breaks. NPS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Grand_Staircase-big.jpg
Figure 7.44: View of Navajo Sandstone from Angel’s Landing in Zion National Park. Diliff. 2004. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Zion_angels_landing_view.jpg
Figure 7.45: Stevens Arch in the Navajo Sandstone at Coyote Gulch some 125 miles away from Zion National Park. G. Thomas. 2007. Public domain. https://en.wikipedia.org/wiki/File:StevensArchUT.jpg
Figure 7.46: Cross-section of the Permian El Capitan Reef at Guadalupe National Monument, Texas. Kindred Grey. 2022. Adapted under fair use from Garber, R.A., Grover, G.A., & Harris, P.M. (1989). Geology of the Capitan Shelf Margin – Subsurface Data from the Northern Delaware Basin (DOI:10.2110/cor.89.13.0003).
Figure 7.47: The rising sea levels of transgressions create onlapping sediments, regressions create offlapping. Woudloper. 2009. CC BY-SA 1.0. https://commons.wikimedia.org/wiki/File:Offlap_%26_onlap_EN.svg
Figure 7.48: Index fossils used for biostratigraphic correlation. David Bond. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Biostratigraphic_index_fossils_01.svg
Figure 7.49: Foraminifera, microscopic creatures with hard shells. Hans Hillewaert. 2011. CC BY-SA 4.0. https://en.wikipedia.org/wiki/File:Quinqueloculina_seminula.jpg
Figure 7.50: Conodonts. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Conodonts.jpg
Figure 7.51: Artist reconstruction of the conodont animal. Philippe Janvier. 1997. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Euconodonta.gif
Figure 7.52: Geologic time on Earth, represented circularly, to show the individual time divisions and important events. Woudloper; adapted by Hardwigg. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Geologic_Clock_with_events_and_periods.svg
Figure 7.53: Geologic time scale with ages shown. USGS. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Geologic_time_scale.jpg
Figure 7.54: Names from the geologic time scale applied to taxonomical diversity of some major animal taxa. Frederik Lerouge. 2015. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Geologic_Time_Scale.png
The study of rock layers and their relationships to each other within a specific area.
Determining a qualitative age of a geologic item in relation to another geologic item.
An atom that has different number of neutrons but the same number of protons. While most properties are based on the number of protons in an element, isotopes can have subtle changes between them, including temperature fractionation and radioactivity.
Radioactive decay where two protons and two neutrons leave the isotope.
A radioactive decay process where a neutron changes into a proton, releasing an electron.
A type of radioactive decay where an electron combines with a proton, making a neutron.
The process of atoms breaking down randomly and spontaneously.
The gases that are part of the Earth, which are mainly nitrogen and oxygen.
Any evidence of ancient life.
Matching disconnected rock strata over large distances.
The largest span of time recognized by geologists, larger than an era. We are currently in the Phanerozoic eon. Rocks of a specific eon are called eonotherms.
The second largest span of time recognized by geologists; smaller than an eon, larger than a period. We are currently in the Cenozoic era.
A unit of the geologic time scale; smaller than an era, larger than an epoch. We are currently in the Quaternary period.
A type of stratigraphic correlation which is based on similar ages.
A type of stratigraphic correlation in which the physical characteristics of rocks are used to correlate.
Discernible layers of rock, typically from a sedimentary rock.
Former swamp-derived (plant) material that is part of the rock record.
Place where material is extracted from the Earth for human use.
Idea championed by James Hutton that the present is the key to the past, meaning the physical laws and processes that existed and operated in the past still exist and operate today.
In an undisturbed sequence of strata, the rocks on the bottom are older than the rocks on the top.
Layered rocks are generally laid down flat at their formation.
Pieces of rock that have been weathered and possibly eroded.
Liquid rock on the surface of the Earth.
A down-warped feature in the crust.
Layered rocks can be assumed to continue if interrupted within its area of deposition.
Place where lava is erupted at the surface.
A geologic object can not be altered until it exists, meaning, the change to the object must be younger than the object itself.
A strain that occurs in a substance in which the item changes shape due to a stress.
A rock layer that has been bent in a ductile way instead of breaking (as with faulting).
Planar feature where two blocks of bedrock move past each other via earthquakes.
Rocks that are formed from liquid rock, i.e. from volcanic processes.
An extensive, distinct, and mapped set of geologic layers.
A piece of a rock that is caught up inside of another rock.
Rocks and minerals that change within the Earth are called metamorphic, changed by heat and pressure. Metamorphism is the name of the process.
Rock more metamorphosed than phyllite, to the point that mica grains are visible. Larger porphyroblasts are sometimes present.
General name of a felsic rock that is intrusive. Has more felsic minerals than mafic minerals.
Sediment gathering together and collecting, typically in a topographic low point.
Term for the underlying lithified rocks that make up the geologic record in an area. This term can sometimes refer to only the deeper, crystalline (non-layered) rocks.
The transport and movement of weathered sediments.
Missing time in the rock record, either because of a lack of deposition and/or erosion.
Layered rocks on top of a non-layered rock, such as crystalline basement.
Two layered rocks that may seem conformable, but an erosional surface exists between them.
Angular discordance between two sets of rock layers. Caused when sedimentary strata are tilted and eroded, followed by new deposition of horizontal strata above.
Rocks that are formed by sedimentary processes, including sediments lithifying and precipitation from solution.
Places that are under ocean water at all times.
Sea level rise over time.
Sea level fall over time.
Meaning "visible life," the most recent eon in Earth's history, starting at 541 million years ago and extending through the present. Known for the diversification and evolution of life, along with the formation of Pangea.
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.
Rocks formed via heat and/or pressure which change the minerals within the rock.
A very high grade metamorphic rock, higher grade than schist, with a separation of light and dark minerals.
Used to describe a large mass or chain of many plutons and intrusive rocks.
Liquid rock within the Earth.
A narrow igneous intrusion that cuts through existing rock, not along bedding planes.
Place where fault movement cuts the surface of the Earth.
How smooth or rough the edges are within a sediment.
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.
Quantitative method of dating a geologic substance or event to a specific amount of time in the past.
Initiation point of an earthquake or fault movement.
A group of all atoms with a specific number of protons, having specific, universal, and unique properties.
The calculated amount of time that half of the mass of an original (parent) radioactive isotope breaks down into a new (daughter) isotope.
An interconnected set of parts that combine and make up a whole.
The process of liquid rock solidifying into solid rock. Because liquid rock is made of many components, the process is complex as different components solidify at different temperatures.
Sedimentary rocks made of mineral grains weathered as mechanical detritus of previous rocks, e.g. sand, gravel, etc.
The rocks that existed before the changes that lead to a metamorphic rock, i.e. what rock would exist if the metamorphism was reversed.
The act of a solid coming out of solution, typically resulting from a drop in temperature or a decrease of the dissolving material.
A chemical sedimentary rock that forms as water evaporates.
Rocks (or rock textures) that are formed from explosive volcanism.
A sheet-like igneous intrusion that has intruded parallel to bedding planes within the bedrock.
ZrSiO4. Relatively chemically inert with a hardness of 8.5. Common accessory mineral in igneous and metamorphic rocks, as well as detrital sediments. Uranium can substitute for zirconium, making zircon a valuable mineral in radiometric dating.
A rock that contains material which can be turned into petroleum resources. Organic-rich muds form good source rocks.
The atom that is made after a radioactive decay.
A radioactive atom that can and will decay.
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.
to move in a circular or curving course or orbit. Not to be confused with rotate, when something spins on an axis
A series of several radioactive decays which eventually leads to a stable isotope.
An atom or molecule that has a charge (positive or negative) due to the loss or gain of electrons.
A device that can determine the amounts of different isotopes in a substance.
A measure of a geologic plane's orientation in 3-D space. Used for beds of rocks, faults, fold hinges, etc. Using the right hand rule, dip is perpendicular, and to the right 90° of the strike.
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.
Two or more atoms or ions that are connected chemically.
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.
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.
Breaking down rocks into small pieces by chemical or mechanical means.
Volcanic tephra that is less than 2 mm in diameter.
A specific layer of rock with identifiable properties.
Mineral group in which the silica tetrahedra, SiO4-4, is the building block.
SiO2. Transparent, but can be any color imaginable with impurities. No cleavage, hard, and commonly forms equant masses. Perfect crystals are hexagonal prisms topped with pyramidal shapes. One of the most common minerals, and is found in many different geologic settings, including the dominant component of sand on the surface of Earth. Structure is a three-dimensional network of silica tetrahedra, connected as much as possible to each other.
Average time between earthquakes calculated based on past earthquake records.
A type of non-eroded sediment mixed with organic matter, used by plants. Many essential elements for life, like nitrogen, are delivered to organisms via the soil.
Material filling in a cavity left by a organism that has dissolved away.
Organic material making a preserved impression in a rock.
The process in which solids (like minerals) are disassociated and the ionic components are dispersed in a liquid (usually water).
Depositional environments that are on land.
The fossils found at any time are unique, and the fossils in layers of different ages have progressed and changed as time has moved forward. Fossils found in layers that are not as old have organisms that more resemble organisms that are alive today.
Unchanged materials preserved in the fossil record. This is rare, and is exceedingly less likely with soft materials and older materials.
Style of fossilization where materials are replaced by minerals in groundwater fluids.
A type of fossilization where only a carbon-rich film is preserved, common in plants.
Evidence of biologic activity that is preserved in the fossil record, but it not the organism itself. Examples include footprints and burrows. Ichnology is the study of trace fossils.
Water that is below the surface.
Specialized mineralization around organic material which produces highly precise molds and casts.
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.
When a species no longer exists.
An accepted scientific idea that explains a process using the best available information.
A proposed explanation for an observation that can be tested.
A volcanic rock with medium silica composition, equally rich in felsic minerals (feldspar) and mafic minerals (amphibole, biotite, pyroxene). Intermediate rocks are grey in color and contain somewhat equal amounts of minerals that are light and dark in color. Primary intermediate rocks are andesite (extrusive) and diorite (intrusive).
Matching rocks of similar ages, types, etc.
A fossil with a wide geographic reach but short geologic time span used to match rock layers to a specific time period.
The mineral makeup of a rock, i.e. which minerals are found within a rock.
An interpretation of the rock record which describes the cause of sedimentation (i.e. ancient beach, river, swamp, etc.).
The part of the coastline which is directly related to water-land interaction, specifically the tidal zone and the range of wave base.
Interior body of ocean water, at least partially cut off from the main ocean water.
A topographic high found away from the beach in deeper water, but still on the continental shelf. Typically, these are formed in tropical areas by organisms such as corals.
A rock made of primarily silt.
The last period of the Paleozoic, 299-252 million years ago.
A type of stratigraphic correlation in which fossils are used to match different rock layers.
The middle period of the Mesozoic era, 201-145 million years ago.
The last period of the Mesozoic, 145-66 million years ago.
The last (and current) era of the Phanerozoic eon, starting 66 million years ago and spanning through the present.
The first period of the Paleozoic, 541 million years ago-485 million years ago.
The first period of the Mesozoic era, from 252-201 million years ago.
A chemical or biochemical rock made of mainly calcite.
A unit of geological time recognized by geologists; smaller than a period. We are currently in the Holocene epoch.
A newly-proposed time segment (an epoch) that would be representative of time since humans have changed (and left evidence behind within) the geologic record.