7 Geologic Time
- Explain the difference between relative time and numeric time as applied to interpreting Earth history.
- Describe the five Principles of Stratigraphy and explain how each applies to interpreting geologic history of an area.
- Describe the geologic history of the Grand Canyon as interpreted using the Principles of Stratigraphy.
- Apply relative dating principles to a block diagram and interpret the sequence of geologic events.
- Explain what an isotope is and what alpha decay, beta decay, and electron capture are as mechanisms of radioactive decay.
- Describe how radio-isotopic dating is accomplished and list four key isotopes used for doing it.
- Explain how carbon-14 is formed in the atmosphere 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 radio-isotope and other techniques.
- What is a fossil? Describe ways by which fossils are preserved.
- Outline how natural selection takes place as a mechanism of evolution.
- Explain what stratigraphic correlation is and how rocks are correlated regionally and over wide geographic distances.
- Know the eras and periods of the geologic time scale and explain the purpose behind its divisions.
- Explain the relation between time units and corresponding rock units (period and system, epoch and series, age and stage).
The Geologic Time Scale and the basic outline of Earth history were worked out long before we had any scientific means of assigning numerical units of age like years to events of Earth history. Working out Earth history depended on realizing some key principles of relative time. Nicholas Steno introduced a basic understanding of stratigraphy (the study of layered rocks) in 1669 with the basic principles of stratigraphy. William Smith (1769-1839), working with the strata of the English coal mines, noticed that strata and their sequence were consistent throughout the region and eventually produced the first national geologic map of Britain becoming known as “the Father of English Geology.” Using Steno’s principles, a relative time scale was developed in the nineteenth century with names derived from areas studied and characteristics of the rocks in those areas. The figure in section 7.1 shows the names applied to units and subunits of the Geologic Time Scale. Using this time scale as a calendar, all events of Earth history can be placed in order without ever knowing the numerical age. The specific events within Earth history are discussed in the Earth History chapter, Chapter 8.
7.1 Relative Dating
Relative dating is the process of determining if one rock or geologic event is older than or younger than another, without knowing the specific age (i.e., number of 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 realized that geologic processes are slow and his ideas on uniformitarianism (i.e., “the present is the key to the past”) provided a basis for interpreting the rocks of the Earth in terms of scientific principles.
7.1.1 Relative Dating Principles
Principle of Original Horizontality: Layers of rocks deposited from above in a gravity field, such as sediments and lava flows, originally were laid down horizontally. This holds true except for the margins of basins, where the strata can slope slightly downward into the basin.
Principle of Lateral Continuity: Within the depositional basin in which they form, strata are continuous in all directions until they thin out at the edge of that basin. Of course, all strata eventually end, either by hitting a geographic barrier or by a depositional process being too far from its source, either a sediment source or a volcano. Strata that are subsequently by cut by a canyon remain continuous on either side of the canyon.
Principle of Fossil Succession: Assemblages of fossils contained in strata are unique to the time they lived and can be used to correlate rocks of the same age across wide geographic distribution. Evolution has produced a succession of life whose fossils are unique to the units of the Geologic time Scale.
7.1.2 Applying the Stratigraphic Principles
The Grand Canyon of Arizona illustrates the stratigraphic principles. Thfigure shows the South Rim separated from the North Rim by approximately 18 miles. The layers of rock lie on top of one another in order from oldest at the bottom to youngest on top based on the principle of superposition. 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 on either side by about 18 miles. These layers of rock are continuous over a wide region of the Colorado Plateau surrounding the Grand Canyon even though several canyons cut through the strata. This is an example of the principle of lateral continuity.
Rock names, like Tapeats Sandstone, Bright Angel Shale, and Muav Limestone, applied to strata in the sequence are formation names. Formation names are designated by geologists to identify rock units that have recognizable characteristics that can identify them in a region. Thus, formations are used as units for mapping purposes and communication. For convenience in mapping and understanding relationships, geologists may group formations into larger units such as groups (and larger supergroups) or subdivide them into smaller units such as members. These terms may be used in this text and other literature and maps, but the basic unit for naming rocks is the formation.
This is a cross section of the rocks exposed on the walls of the Grand Canyon, which illustrates some other stratigraphic principles. In the lowest parts of the Grand Canyon are the oldest formations with igneous and metamorphic rocks at the bottom. In the canyon, these are called the “Vishnu Basement Rocks.” The Principle of Cross-Cutting Relationships shows the sequence of these events. The Vishnu Schist is the oldest and the cross-cutting intrusions of Zoroaster Granite are 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 . The Colorado Plateau, on which the Grand Canyon region lies, is characterized by strata that are horizontal or nearly so. These rocks were originally deposited horizontally (Principle of Original Horizontality) and have not been disturbed very much since they were deposited except by a broad regional uplift (there are local exceptions). In the Grand Canyon, there is a gentle tilt of the strata to the south, thus the strata of the North Rim are about a thousand feet higher than those of the South Rim about 18 miles away. Applying the stratigraphic principles, one can interpret that the slight tilting of the strata occurred after their deposition and that the Grand Canyon was cut by the Colorado River after the regional tilting. This is an application of Cross Cutting Relationships to establish relative time and Lateral Continuity to correlate them across the canyon.
On top of these basement rocks, lie the strata of the Grand Canyon Supergroup (there are several formations included in this supergroup unit). These formations were originally deposited flat on top of the basement rocks (Original Horizontality) and have since been broken into tilted blocks by normal faulting (see Chapter 9) which cut through both them and the underlying basement. Because the formation of the basement rocks and the deposition of these overlying sediments is not continuous deposition but broken by events of metamorphism, intrusion, and erosion, the contact between the Grand Canyon Supergroup and the older basement is termed an unconformity. An unconformity represents a period during which deposition did not occur or erosion 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. Unconformities are shown on cross sections and stratigraphic columns as wavy lines between formations. There are three types of unconformities which will be discussed below. The first occurs when sedimentary rock lies on top of crystalline rock, and is a type of unconformity called a nonconformity. A nonconformity occurs when sediments are deposited on top of non-layered crystalline (igneous and metamorphic) rocks as is the case with the contact between the Grand Canyon Supergroup and the Vishnu basement rocks.
The Grand Canyon Supergroup is a sequence of strata representing alternating marine transgressions and terrestrial deposition (in this case regressions where the sea retreated). During formation of this sequence, sea-level rose (or the land sank) leaving marine deposits on the surface and then fell (or the land rose) leaving the land exposed to erosion and to deposition of terrestrial sediments. Heavy wavy lines between the various numbered strata on the figure show interruptions in deposition called disconformities, the second type of unconformity where either non-deposition or erosion 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 disconformity. Disconformities are unconformities that occur between parallel layers of strata indicating that there was no deformation during the period of nondeposition or erosion.
On top of the Grand Canyon Supergroup lie the horizontal layers of the canyon walls showing unconformable contacts with the tilted layers of the Grand Canyon Supergroup below (i.e. rocks representing that interval are absent). The lower strata were tilted by tectonic processes that disturbed their original horizontality (which of course also affected the underlying basement rocks). Thus there were cross-cutting processes that affected those rocks before the younger strata were deposited horizontally on top of them. After the deposition of the Grand Canyon Supergroup and the tectonic events that tilted and faulted them, there was an erosion-produced landscape with hills and valleys over which the sea transgressed again and deposited layers of three horizontal formations of sedimentary rock called the Tonto Group. The upturned and eroded edges of the tilted older rocks of the Grand Canyon Supergroup lay at angles with the overlying Tonto Group. This third type of unconformity is called an angular unconformity.
Angular unconformity, where sedimentary strata are deposited on a terrain developed on sedimentary strata that have been deformed by tilting, folding, and/or faulting. so that they are no longer horizontal.
7.1.4 Applying Relative Dating Principles
In the block diagram seen here, the sequence of events from oldest to youngest that took place can be interpreted using the stratigraphic principles and interpretations from the chapters on rocks (chapters 4-6). Here is the sequence of these events in order. The oldest rock is a body of deformed rock composed of brown and gray layers. Its deformation includes pretty severe deformation shown as folding. From the symbols used in the drawing, this rock looks like it was probably metamorphosed. The oldest event, therefore, is the formation of the brown and grey rock, followed by its deformation and metamorphism which we might call basement rock here. The brown and gray basement rock was cut by the fault (A) which cuts across and displaces it. Both the basement rock and fault A are crosscut by rock mass B. Its irregular outline suggests that it is an igneous intrusion emplaced as magma into the region. Since it cuts across both the basement rocks and the fault, it is younger than both. Next, both the basement rock and rock B were eroded forming an unconformity. This was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression. Because C is sedimentary rock that was deposited on top of crystalline igneous rock B and crystalline metamorphic rock, this unconformity is called a nonconformity. Igneous rock B, the old basement rocks, and strata C were then all cut by the igneous dike D. Deposition of sedimentary rock E suggests that there was a period of erosion or non-deposition producing a disconformity between C and E, the nonconformity between dike D and rock E establishes that erosion in fact did take place forming a quite flat landscape. Then fault F cut across all the older rocks producing a fault scarp (the low ridge on the left side of the diagram). The final events affecting this area are the current erosion processes working on the land surface, rounding off the edge of the fault scarp, and producing the modern landscape on the top of the diagram.
7.2 Absolute Dating
Relative time allows science to tell the story of the Earth, but does not provide specific numeric ages of events, and thus, the rate at which geologic processes operate. Based on Hutton’s Uniformity Principle (Uniformitarianism), early geologists surmised that geological processes work slowly and that the Earth is very old. Because science advances as the technology of its tools advances, the discovery of radioactivity in the late 1800s provided a new scientific tool by which actual ages in years can be assigned to mineral grains within a rock. Section 7.1 shows how Earth history is understood using relative dating principles without actually knowing the numerical age of events. This was how scientists of that time interpreted Earth history, until the end of the 19th Century, when radioactivity was discovered. This discovery introduced a new dating technology that allows scientists to determine specific numeric ages of some rocks, called absolute dating . The next sections discuss this absolute dating system called radio-isotopic dating.
All elements on the Periodic Table of Elements (see Chapter 3) contain isotopes. An isotope is an atom of an element 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 isotopes (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 (2H). Note that the atomic mass of elements on the Periodic Table is usually expressed with decimal digits. This indicates that the atomic mass of that element in nature is made of all its natural isotopes so the average atomic mass including all these isotopes is a decimal value. Some isotopes are stable, like 1H and 2H, but some isotopes are unstable like 3H. Many elements like hydrogen have both stable and unstable isotopes. Unstable isotopes (called radioactive isotopes) spontaneously decay over time releasing radiation. When this occurs, that isotope becomes an isotope of another element. For example, 238U (uranium-238) decays to 234Th (thorium-234). This process of radioactivity is called radioactive decay.
The radioactive decay of any individual atom is a completely unpredictable and random event. However, given a large number of radioactive atoms (any measurable quantity of a substance contains trillions of atoms), the decay of half of the atoms in the specimen takes a specific amount of time. This amount of time for half the atoms to decay is called the half-life. In other words, the half-life of an isotope is the amount of time it take for half of an initial quantity of unstable isotopes to decay into another isotope. The half-life is constant for a given radioactive isotope and can be measured. The known half-life of an isotope can be used to calculate the age of a rock.
The principles behind this dating method require two key assumptions. First, the mineral grains containing the isotope form at the same time as the rock, such as a mineral in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed system, that is they are not subsequently altered by elements moving in or out of them.
These requirements place some constraints on the kinds of rock that are suitable for dating, igneous rock being the best. Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals (such as evaporites) may contain elements suitable for radio-isotopic dating. Igneous pyroclastic layers and lava flows within a sedimentary sequence can be used to date the sequence. Cross-cutting igneous rocks and sills can be used to bracket the ages of the affected sedimentary rocks as being older than the igneous rocks. Furthermore, the resistant mineral zircon, found as clasts in many ancient sedimentary rocks, has been successfully used for dating very old ages and has established the age of the Earth’s oldest yet known rocks. Knowing that the zircons in the metamorphosed sediments came from older rocks, their ages established the age of the source rocks, which are no longer available for study.
Alpha decay: When an atom decays by alpha decay, an alpha particle is emitted from its nucleus as an alpha ray. The alpha particle consists of two protons and two neutrons, a total of four particles. This happens also to be the nucleus of a helium atom; helium gas may thus be trapped in the crystal lattice of a mineral in which alpha decay has taken place. The loss of two protons from the nucleus of the atom lowers its atomic number by two forming an atom of an element two atomic numbers lower on the Periodic Table of the Elements.
The loss of four particles (two neutrons and two protons) also lowers the mass of the atom by four. An example of alpha decay takes place in uranium-238 (238U) with an atomic number of 92 (92 protons in its nucleus) and mass number of 238 (total of all protons and neutrons in its nucleus). When 238U spontaneously emits an alpha particle, it becomes thorium (234Th). The product of decay of a radioactive element is called its daughter isotope and the original element is called the parent isotope. In this case, 238U is the parent isotope and 234Th is the daughter isotope. The half-life of 238U is 4.5 billion years, i.e. the time it takes for half of an original amount to decay to daughter isotope. This isotope of uranium (238U) can be used to determine the age of the oldest materials found on Earth, even meteorites and materials from the earliest events in our solar system.
Beta decay: When an atom decays by beta decay, 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 atomic number by one and a new element is formed, but the atomic mass does not change. For example, 234Th is also radioactive and decays by beta decay to form protactinium-234 (234Pa) which is also radioactive and decays by beta decay to form uranium-234 (234U), a lighter isotope of uranium. The process of decay of radioactive elements like uranium leads to a series of parents and daughters, each one radioactive, until a stable (non-radioactive) daughter is formed. Such a series is called a decay chain. Uranium-238 decays through a series of alpha (red arrows) and beta decays (blue arrows) to form the stable daughter product lead-206 (206Pb).
Electron capture: In this type of radioactivity, a proton in the nucleus captures an electron from one of the electron shells to become a neutron. The result leaves two different effects: either an electron jumps in to fill the missing spot of the departed electron (emitting an X-ray), or in the Auger process, another electron is released, changing the atom into an ion. The atomic number is reduced by one and the mass number remains the same. An example of an element that decays by electron capture is potassium-40 (40K). Natural potassium is mostly not radioactive, but a tiny percent (0.012%) is. Potassium-40 decays by electron capture to form two daughter products, calcium-40 (40Ca) about 89% of the time, and argon-40 (40Ar) about 11% of the time. Both argon and calcium can be chemically separated but since calcium is very common in nature, potassium-argon is the pair that is used in dating. The half-life of 40K in its decay to 40Ar is 1.25 billion years, so it is very useful for dating geological events . Below is a table of some of the more commonly-used radioactive dating isotopes. 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|
Some common isotopes used for radio-isotopic dating .
7.2.2 Radio-Isotopic Dating
Given a sample of rock, how is the dating procedure carried out? Using chemical analysis, the parent elements and daughter products can be separated out of the mineral. Remember that elements behave chemically due to their atomic number. In the case of uranium, both the 238U and 235U isotopes are chemically separated out together, as are the 206Pb and 207Pb. An instrument called a mass spectrometer then separates the uranium isotopes from each other as well as the lead isotopes from each other by passing beams of the isotopes through a magnetic field. As these isotopic beams pass through the instrument, the path of the heavier isotope is deflected less so the two beams strike a sensor at different places. . From the intensity of each beam, the amount of parent and daughter products are determined, and from this ratio the age can be calculated.
Here is a simple example of age calculation using the ratio of daughter product to parent isotope. When the mineral initially forms, there is 100% parent isotope and 0% daughter and the ratio of daughter to parent (D/P) is 0. After one half-life, half the parent has decayed so there is 50% parent and 50% daughter. The ratio is then 1. After two half lives, there is 25% parent and 75% daughter and the ratio is 3. This can be further calculated for a series of half lives as shown in the table below. Note that after about ten half lives, the amount of parent remaining is so small that accurate chemical analysis of the parent is difficult and the accuracy of the method is diminished. Ten half lives is generally considered the upper limit for use of an isotope for radio-isotopic dating. 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%) . Considering the uranium/lead technique, in any given sample analysis, there are 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 on each other. Many geological samples contain multiple parent/daughter pairs so cross checking clocks show radio-isotopic dating to be highly reliable.
|Parent present (%)||Daughter present
|Start the clock||100||0||0||infinite|
Ratio of parent to daughter in terms of half-life.
Another radio-isotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone. Carbon dating uses the unstable isotope carbon-14 (14C) and the stable isotope carbon-12 (12C). Carbon-14 is constantly being created in the atmosphere by the interaction of cosmic particals with atmospheric nitrogen-14 (14N) . The cosmic particles include neutrons that strike the nitrogen nucleus kicking out a proton but leaving the neutron in the nucleus. The atomic number is reduced by one from 7 to 6 forming carbon and the mass number remains the same at 14. The 14C quickly bonds with oxygen in the atmosphere to form carbon dioxide which mixes with the other atmospheric carbon dioxide and is incorporated into living matter. Thus, while an organism is alive the ratio of 14C/12C in its body doesn’t really change since it is constantly exchanging with the atmosphere. However, when it dies, the radiocarbon clock starts ticking as the 14C decays back to 14N by beta decay with a half-life of 5,730 years. The radiocarbon dating technique is thus useful for about ten half lives back 57,300 years or so.
Since radio-isotopic dating relies on parent and daughter ratios and the amount of parent 14C needs to be known, early applications of 14C dating assumed the production and concentration of 14C in the atmosphere for the last 50,000 years or so was the same as today. But production of CO2 since the Industrial Revolution by combustion of fossil fuels (in which 14C long ago decayed) has diluted 14C in the atmosphere leading to potential errors in this assumption. Other factors affecting the estimates of composition of parent carbon in the atmosphere have also been studied. Comparisons of carbon ages with tree ring data and other data for known events have allowed calibration for reliability of the radiocarbon method which is primarily used in archaeology and very recent geologic events. Taking into account these factors, carbon-14 dating has been shown to be a reliable dating method in this range.
7.2.4 The Age of the Earth
After the Renaissance and with the work of Hutton and others gaining attention, the idea of an ancient Earth began to be explored. In the late 19th century William Thompson (a.k.a. Lord Kelvin) applied his knowledge of physics and the assumption that the Earth started as a hot molten sphere to estimate that the Earth is 98 million years old, but because of uncertainties in his calculations, he stated it as between 20 and 400 million years. (Here is an animation illustrating Kelvin’s attempt and factors involved in heat transfer in the Earth). This estimate of an old Earth was considered plausible but not without challenge, and the discovery of radioactivity provided a better method for determining ancient ages. It has also been pointed out that Kelvin failed to consider pliability and convection in the Earth’s mantle as a heat transfer mechanism .
As a graduate student at the University of Chicago in the 1950’s, Clair Patterson thought he could determine the age of the Earth using radioactive isotopes from meteorites that he considered to be remnants of the early solar system at the time Earth was forming. Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer. Using the uranium/lead dating technique, he determined 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, a graduate student in the 1950s, came up with a result that has been little altered in over 60 years even as technology has improved the methods.
7.2.5 Dating Geological Events
Radioactive isotopes of elements that are common in mineral crystals are useful for radio-isotopic dating. The uranium/lead method, with its two cross-checking clocks, is most often used with crystals of the mineral zircon (ZrSiO4) where uranium can substitute for zirconium in the crystal lattice. Zircon is resistant to both mechanical and chemical weathering, but during metamorphic events, zircon crystals may even form multiple crystal layers, with each layer recording the isotopic age of an event thus tracing the progress of the several metamorphic events . Some amazing work on zircon grains has been done. Zircon crystals from Western Australia that formed when the earliest crust differentiated from the mantle 4.4 billion years ago have been studied, the oldest known rocks . The zircon grains were incorporated in younger host rocks (metasedimentary) that were not that old, but the zircon grains themselves were dated at 4.4 billion years ago, and had survived subsequent processes of weathering, erosion, deposition, and metamorphism. From other properties of the zircon crystals, these researchers concluded that not only were continental rocks present, but that conditions on the early Earth were cool enough for liquid water to exist on the surface and for processes of weathering and erosion to take place . Researchers at UCLA studied 4.1 billion year old zircon crystals and found carbon in the zircon crystals that may be biogenic in origin, meaning that life may have existed on Earth much earlier than previously thought . These studies illustrate that both science and its conclusions advance as technology-driven advancements in scientific tools and ideas generate new knowledge.
The rocks best suited for radio-isotopic dating are igneous, which provide dates on crystallization of primary minerals from magma. Metamorphic processes tend to reset the clocks and smear the dates over the metamorphic events. Detrital sedimentary rocks are made of minerals derived from multiple parent sources with potentially many dates. However, there are igneous events that do allow dating of sedimentary sequences. For example, a lava flow that occurs on top of sedimentary strata and is later buried by more sediments may provide a date within the sequence. A layer of volcanic ash (tuff) deposited within a sedimentary sequence likewise may allow dating of the sequence. A sill intruded within a sedimentary sequence provides an upper limit age on the sequence. Potassium is common in evaporite sediments and has been used to date them by potassium/argon dating . Use of primary sedimentary minerals, with radioactive isotopes like 40K, has provided dates for important geologic events.
7.2.6 Other Absolute Dating Techniques
Luminescence: Radio-isotopic dating is not the only way scientists determine numeric ages. Luminescence dating stimulates the release of electrons that are trapped in mineral grains as radioactive isotopes decay over time. The accumulation of electrons is governed by the rate of background radiation. The electrons are released when exposed to heat or light (depending on the technique). This technique shows the last time mineral grains in a sediment or rock were exposed to light or heat. Luminescence dating is generally only useful for dating sediments that are less than 1 million years old .
Fission Track: Fission track dating relies on damage to the crystal lattice produced when the unstable 238U decays to the daughter product 234Th releasing 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 mineral 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 0.1 Ma to 2000 Ma. Fission track dating has also been used as a second clock to confirm dates obtained by other methods .
7.3 Fossils and Evolution
Fossils are any evidence of past life preserved in the rocks. They may be actual remains of body parts (rare), impressions of soft body parts, casts and molds of body parts (more common), or evidence of animal behavior such as footprints and burrows. Life today has body parts ranging from hard bones and shells to the soft cellulose of plants, to soft-bodied organisms like jellyfish, down to single celled bacteria and algae. Which body parts can be preserved? The vast majority of life today is soft-bodied and/or single celled, and would not likely be preserved in the geologic record except under very unusual conditions. The best environment for preservation is the ocean, yet marine processes can dissolve hard parts, scavenging can reduce or eliminate remains, and most marine life is soft-bodied and/or microscopic anyway. Thus, even in the ocean, the likelihood of preservation is quite limited. For terrestrial life, possible burial and preservation of remains is even more limited. The fossil record is incomplete, and records only a small percentage of life that existed. Although incomplete, fossils are used for stratigraphic correlation (the Principle of Faunal Succession) and provode a method used for establishing the age of a formation on the Geologic Time Scale.
7.3.1 Types of preservation
Remnants or impressions of hard parts such as a marine clam shell or dinosaur bone are the most common types of fossil . The original material of these hard parts has almost always been replaced with new minerals. These minerals preserve much of the shape but the original material is gone. The following are types of fossil preservation.
Actual preservation is a rare form of fossilization where actual materials of the organism or hard parts are preserved.
This can be unaltered preservation in amber, or preservation of original minerals like mother-of-pearl on the interior of a shell. Another is the preservation of mammoth skin and hair in post-glacial deposits in the Arctic regions . Mummification of dinosaurs has left fragments of soft tissue and skin sometimes even including blood vessels, and the isolation of proteins and evidence for DNA fragments have been discovered . Preservation of soft-tissue is very rare since these organic materials can easily disappear by bacterial decay .
Permineralization occurs when an organism is buried, then elements in groundwater completely impregnate all spaces within the body, even cells. 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 where details of cellulose structures in the wood are preserved. This link to the University of California Berkeley website has more information.
Casts and molds form when the original material of the organism dissolves and a cavity is left in the surrounding rock. The shape of this cavity is an external mold. If the mold is then filled with a subsequent deposit of mineral or sediment, the external shape of the organism is preserved as a cast. Sometimes internal casts of the internal cavities of organisms are preserved. Such internal casts of clams, snails, even skulls may show
details of soft structures. If the chemistry is right, and burial is rapid, mineral nodules may form around soft structures preserving three-dimensional detail. This is called authigenic mineralization.
Carbonization occurs when the organic tissues of an organism are compressed, the volatiles are driven out, and everything but the carbon disappears. A carbon silhouette of the original organism remains. Examples are leaf and fern fossils .
Dinosaur tracks testify of 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 trace fossils include fossilized feces called coprolites and stomach stones called gastroliths . These provide information about diet and lifestyle of the organism.
That ancient life forms evolved to produce the variety of fossils we see in the rocks is important to stratigraphic correlation. Here is a brief discussion of evolution to provide some understanding of the process. Darwin recognized that life forms evolve into progeny life forms. The mechanism he proposed for this process was Natural Selection operating on species that live within environmental conditions that pose challenges to survival.
The basic unit of classification of life on Earth is the species, a population of organisms within which individuals can mutually reproduce to produce fertile offspring. Within that population exists variations, differences in physical and behavioral characteristics. Just think of all the different variations that exist among human beings in a classroom or community. For the species to survive, each individual within that population has the biological purpose to survive long enough to reproduce. But each individual is faced with the challenges to survival posed by the environment and must survive to reproduce within those challenges. If within the variations present in the population there are individuals that possess characteristics giving them some advantage in facing the environmental challenges, those individuals will be favored in reproducing and those favored characteristics will be passed on in successive generations. Sufficient such favored changes in characteristics over time may cause reproducing populations to become geographically or even genetically isolated from one another eventually resulting in separate species. This is the process of Natural Selection elaborated by Darwin in On the Origin of Species . 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 hypothesis stage and is a well-established Theory of modern science.
Variation within populations occurs by natural mixing of the genes through reproduction, and also by mutations which are spontaneous changes within the genetic material (DNA) caused by many natural agents and processes. Most mutations are not advantageous and soon disappear, but some cause a dramatic change in the characteristics even introduction of something novel that may be advantageous. While fossils of some species in the fossil record show little morphological change over time, others show gradual or punctuated changes within which all intermediate forms can be seen. If sufficient individuals in a population fail to surmount the challenges of the environment and don’t reproduce generations of viable offspring, the species becomes extinct. The average lifespan of a species in the fossil 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.
At any given time on Earth, preservable fossils represent a sample assemblage of organisms living at that time. While the ranges in geologic time of individual fossil species vary, the assemblage of fossils is unique to the time in which it lived. Assemblages of fossils thus can be used to identify rocks of similar age at geographically dispersed locations on Earth and for assigning rocks to the systems of the Geologic Time Scale. The process of relating rocks to each other is called correlation. Correlation can be used with magnetic polarity reversals, rock types, rock assemblages (i.e. a unique sequence of rocks), or via fossils to establish temporal relations among rocks of diverse geographic regions on the Earth.
7.4.1 Stratigraphic Correlation
Geologic histories of local regions are constructed by geologists in the field doing mapping and study of the rocks using the Principles of Stratigraphy outlined above. This is done with stratigraphic correlation. Since absolute ages require laboratory analysis, ages of strata and events are determined in the field from fossils and cross-cutting relationships. Using fossils, strata are correlated across large regions. While the details of Earth history continue to be studied, the basic outline of Earth history is known (see Chapter 8). The time periods of the Geologic Time Scale are represented by rock layers on the Earth’s surface, and individual rock layers may extend for hundreds or thousands of miles. Their correlation within a region and with rocks in other regions is important in piecing together Earth’s history. For example, the layers of rock covering the Colorado Plateau and their sequence can be recognized and correlated over thousands of square miles of the Plateau and correlated with rocks of similar age in Europe and other parts of the world.
7.4.2 Lithostratigraphic Correlation
Lithostratigraphic correlation is the comparison and correlation of strata within a region by examining the physical properties of the rock and its sequence with other strata. Lithostratigraphic correlation is done within formations to trace their extent and the environment in which they were deposited in a region.
For example, the Navajo Sandstone (called the Aztec Sandstone in Nevada and Nugget Sandstone in Wyoming) is a deposit of sand dunes in a Jurassic desert that makes the prominent walls of Zion National Park and is the same Navajo Sandstone found miles away in other parts of southern Utah, including Capitol Reef, Canyonlands, and Arches National Parks. This was a large desert on the order of the Sahara desert of today.
As another example, the Dakota Sandstone is a recognizable sedimentary stratum over broad areas of the American upper Midwest, representing deposition in the Cretaceous Interior Seaway, an incursion of marine water over the middle of the North American around 105 million years ago.
7.4.3 Chronostratigraphic Correlation
Another factor in lithostratigraphic correlation is the fact that a lithology can change with distance (lithology reflects the environmental conditions in the depositional basin), but the rocks can be temporally related because they were formed at the same time (different facies). In this type of correlation, called chronostratigraphy, different rocks of the same age are matched in time even though they show different lithological characteristics. As depositional environments change, shift, or migrate, the timing within the same formation can change. Using the examples above, the Navajo or Dakota Sandstones are correlated by lithostratigraphic means, but they do not necessarily correlate based on chronostratigraphy. And the Tapeats Sandstone is older in its western exposures and gets younger in its eastern exposures because of the marine transgression involved in its formation.
7.4.4 Biostratigraphic Correlation
Biostratigraphic correlation uses fossils to correlate strata. Biostratigraphic correlation is usually done using assemblages of organisms that are unique to specific intervals of geologic time. Using fossils allows geologists to assign a formation to a geological time period such as the Jurassic Period (199 to 145 million years ago). Most of the geologic time names used on geologic maps are assigned using fossils. The most useful fossils represent lifeforms that were widespread geographically and limited to narrow time intervals.
Such individual fossils are called index fossils and are especially useful for biostratigraphic correlation. The intervals defined by index fossils and fossil assemblages are called zones. Some of the best fossils for biostratigraphic correlation are microfossils, most of which are prolific single celled organisms. As with microscopic organisms today, they lived in widespread environments. Some microscopic organisms have hard parts.
Foraminifera, single celled organisms with calcareous shells, are a good example and are especially useful for correlation in the Cretaceous Period and Cenozoic Era . Conodonts are another example of microfossils useful for biostratigraphic correlation, which lived from the Cambrian through the Triassic. Conodonts are the only hard parts of an extinct, possibly eel-like multi-celled organism. The conodont animal apparently had no other preservable hard parts except these tooth-like structures. It was a creature that lived in shallow marine environments all over the world. Upon death, it’s phosphatic hard parts were released and scattered into the sediments and are easily collected and separated from limestone in the laboratory for study. Microfossils like conodonts can be separated from drill cuttings in the search for oil and are thus have special use in economic geology like petroleum exploration.
The actual animal that made conodonts is known from a few impressions preserved in unusual circumstances. Because these conodont hard parts were so abundant, readily preserved, rapidly evolving, and widespread in sediments of Paleozoic and Triassic age, they are especially useful for correlating those strata even though knowledge of the actual animal possessing them is sparce. A fundamental biostratigraphic zonation of Triassic conodonts was carried out in the 1960s that tied the conodont zonation in with ammonoids (extinct ancient cousins of the pearly nautilus), up to that point the standard for Triassic correlation . That study went on to establish the use of conodonts for correlation of Triassic strata internationally between Europe, Western North America, and the Arctic Islands of Canada . Correlating zonations of both microfossils and macrofossils enhances the use of both for biostratigraphic correlation.
7.4.5 Geologic Time Scale
Geologic time has been subdivided into a series of divisions by geologists. Eon is the largest division of time, followed by era, period, epoch, and age. The geologic record shows processes that started and stopped, yet time flowed continuously. The partitions of the Geologic Time Scale occurred everywhere on Earth, however, rocks representing processes of deposition and rock formation during a particular period of time may or may not be present at a given location depending on the active geologic processes. Thus, we have the concepts of Time vs. Rock; e.g. Periods of geologic time flowed continuously but the rocks that formed during that period are called Systems of rock (see table below). The Tonto Group is the Cambrian System in the Grand Canyon deposited during the Cambrian Period. System refers to rock; period refers to time. Looking back at the Geologic Time Scale, the names shown for the various units of the Geologic Time Scale represent time flowing continuously from the beginning of the Earth. But only the rocks formed during these time units are available for study. Comparative terms for time and rock are given in the table below. Subdivisions of the time units are given as early, middle, and late. These terms are capitalized when formally defined, but left lowercase when used informally. Corresponding subdivisions of rock units are expressed as lower, middle, and upper reflecting depositional position in the superpositional sequence of strata.
Time divisions vs. rock divisions.
With the expansion of science and technology into the center of the developed world, some geologists think the influence of humanity on natural processes has become so great that the geologic community is now considering designating a new geologic time period, known as the Anthropocene . As improvements to our understanding of the human impact on Earth, geology uses the understanding of the history of the Earth system to make predictions about the future and how it will unfold under the influences of people.
The application of the Principles of Stratigraphy by 18th and 19th century geologists resulted in the development of the Geologic Time Scale. The origin of the period names have interesting stories and involve interesting personalities mostly in Europe . The development of the Geologic Time Scale occurred as strata and their included fossils were studied and correlated in Europe and North America. Most of the names have European origins. Cambrian was named for the Roman name of an area in north Wales. Silurian was named for an ancient tribe that lived in south Wales. Jurassic was named for strata exposed in the Jura Mountains in Switzerland and France. Triassic was named for the three-fold expression of its strata in Germany. Cretaceous came from the Chalk (Latin creta) layers of northern Europe, dramatically exposed in the Cliffs of Dover. The Geologic Time Scale with its familiar names was completely developed during the 19th century using the Principles of Stratigraphy and placed in relative order before geology had the tools to assign numerical ages to its periods and events. Fossils allowed these names to be applied worldwide through biostratigraphic correlation. A geologist armed with knowledge of the fossils characteristic of the intervals represented on the Geological Time Scale can identify the geologic ages of rocks in any region and correlate them with rocks in other regions of the world.
All the events of earth history can be placed in sequence using the principles of relative time called the five Principles of Stratigraphy. The Geologic Time Scale was completely worked out in the 19th Century using these principles and used as a calendar for telling the story of earth. Hutton’s Uniformity Principle, knowing how natural processes work in the present is key to understanding the past, provided a means to study and understand the processes involved in the events that have shaped the earth to its present form. All of this was and could continue to be done without knowing any actual ages for the events. However, the discovery of radioactivity in the late 1800s provided a tool to measure the actual ages of the events of earth history. However, certain types of rocks and their minerals are better suited for dating and certain assumptions about those rocks and minerals require care and precaution in interpreting ages. With this care and precautions, both the history of events, the Geologic Time Scale itself, and the actual age of the earth have been determined with considerable accuracy. Geologic time is now known to be vast and to have provided plenty of time for the evolution of the planet and life upon it to have taken place to produce the earth as we see it and all the life forms on it.