Geology 101 - Introduction to Physical Geology
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Basics -- Geologic Time


Geologic time covers the whole sweep of earth's history, from how and when the earth first formed, to everything that has happened on, in, and to the planet since then, right up to now.

Geologists analyze geologic time in two different ways: in terms of relative geologic age, and in terms of absolute (or numeric) geologic age. The combination of these two types of geologic ages makes a complete record of earth's geologic history in terms of the order of events and in terms of how many years ago each event occurred.

Relative geologic age refers to the order in which geologic events occurred. Relative geologic age is established, based on such evidence as the order in which layers of sediment are stacked, with the younger layer originally on top. By using the principles of relative geologic age, the sequence of geologic events -- what happened first, what happened next, what happened last -- can be established.

Absolute geologic age refers to how long ago a geologic event occurred or a rock formed, in numeric terms, such as 65.5 million years ago. Some rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain. The units commonly used for geologic age are mega-annum (Ma) for millions of years, giga-annum (Ga) for billions of years, and kiloannum (ka) ka for thousands of years. Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized.

Much of the most detailed and precise information that geologists have gleaned of earth's history comes from a branch of geology known as stratigraphy. Stratigraphy studies stratified rocks, - layered rocks, in other words, which are either sedimentary or volcanic - establishes their age sequence based on principles of relative geologic age, and reconstructs, from the evidence in the rocks and from their field relations as depicted on maps and cross-sections, the geologic history that they represent.

You may have already completed introductory laboratory studies of igneous, sedimentary, and metamorphic rocks. If so, you have already practiced interpreting details of earth's history from the evidence contained in rocks. By incorporating the information that can be gathered from a single rock sample into the broader context of where the body of rock that it comes from fits into the sequence of the earth's rocks, and what its age is in absolute (numeric) terms, the geologic history of that part of the earth can be reconstructed.

Stratigraphy started to become a formal science due to the work of a man who published under the name Nicolas Steno in the 17th century. Steno made careful geologic observations and illustrations. He published the results of his work and established a basic set of principles for interpreting sedimentary strata. Geologists still use Steno's principles, with some refinements and additions. They are summarized as the principles of relative geologic age determination, sometimes referred to as the principles of relative dating.

In the two centuries after Steno developed the first set of principles for determining relative geologic age, other geologists contributed major ideas that have allowed scientists to grapple with, measure, and come to terms with geologic time. James Hutton of Scotland, the same man who developed the modern concept of the rock cycle as a reflection of the endlessly dynamic and constantly changing earth, advanced the theory of unconformities. An unconformity is a buried erosional surface or non-depositional surface, a contact between the rocks below and the layer of stratified rock above that is missing a significantly large interval of geologic time. For example, deep in the Grand Canyon in Arizona, there are places where a layer of rock of Devonian age is right on top of a layer of rock of Cambrian age. That means that tens of millions of years of geologic time lapsed between those two rock layers forming, and there is no sedimentary rock, no rock record, to record the details of what happened during that geologic time. The contact between the Cambrain rock (over 488 million years old) and the Devonian rock (less than 416 million years old) is a type of unconformity, which you will read more about below.

In the early 1800s, soon after James Hutton died, William Smith in England made the scientific case for what came to be called the principle of faunal succession. The key to this principle is that during a specific geologic time, only certain types of organisms existed, so if fossils of those organisms are found in a layer of rock, the rock is of that geologic age, the age when those organisms were species that lived on earth. This principle was based on applying other methods of determining which rocks are older and which rocks are younger, which verifies that there is indeed a faunal (or fossil, if you prefer) succession that occurs in the same order in the rock layers everywhere on earth.

Charles Lyell developed a key idea known as uniformitarianism, which also underlies the geological study of earth's history. Lyell was a significant scientific presence through much of the time of Victorian England in the 1800s. He had a large influence on the development and spread of the practice of geology as a science, partly through his textbook, The Principles of Geology. This was not only the first complete geology textbook published in English, it was by far the most widely used textbook for decades, through several revised editions. A copy of The Principles of Geology that Charles Darwin read had a major influence in his thinking as he traveled around the world collecting samples, and Darwin consulted with Lyell at key stages during the time he developed and published his theory of evolution of species by natural selection. The idea of uniformitarianism is that the laws and principles that nature follows in today's world, such as gravity, also applied in the geologic past; in other words, "the present is the key to the past."

The idea of uniformitarianisms is commonly misinterpreted in two different ways. The first incorrect interpretation is that it states that only slow changes occur on earth, the second misinterpretation is that it states that catalclysmic events cannot have happened in the past. Uniformtarianism does not require that all geological processes are slow. Some are abrupt, such as an explosive volcanic eruption, an earthquake, or a landslide. It is true, and needs to be kept in mind to understand the earth geologiclly, that many geologic processes are slow, accumulating into larger effects over the course of geologic time. For example, we now know that continents move across the face of the earth a few centimeters a year as part of tectonic plates. A few centimeters a year, about the rate your fingernails grow, may not seem like much but, as millions of years of geologic time unfold, it adds up to thousands of kilometers . Fast or slow, abrupt or taking millions of years, uniformitarianism includes natural processes of all different rates, from abrupt and cataclysmic to barely detectable and very slow.

In addition, uniformitarianism does not rule out the possibility that larger cataclysms could have happened in the geologic past than humans have witnessed in the modern world. For example, the evidence is strong that a catastrophic meteorite impact at the end of the Cretaceous period was the cause, or precipitating factor, of a major mass extinction, when the last of the dinosaurs passed from existence on earth, along with many other species. This is consistent with uniformitarianism. The key is that the meteorite and its effects on the ocean, the atmosphere, and on life happened according to the laws and behaviors of physics, chemistry, and biology, the same laws and behaviors we can observe in nature and verify by experimentation today.

The Role of Stratigraphy in Understanding Geologic History

Stratigraphy is the study of rock layers and reconstruction of the original sequence in which they were deposited. The stratigraphy of an area provides the basis for putting together the geologic history of an area.

The details of a region's stratigraphic story are revealed by:

Ask yourself how the things that are happening in the world today might end up being recorded in the sediments that are now or soon will be deposited. How would today's sediments appear to a geologist millions of years in the future examining outcrops of sedimentary rock that originated in our time? What would the geologist be able to deduce about the world we live in, based on what was left in the strata?

The Principles of Relative Geologic Age Determination

  1. The principle of original horizontality - sedimentary strata are initially deposited as horizontal or nearly horizontal layers.
    Note: If sedimentary strata dip at an angle other than horizontal, or are folded into various angles of tilt, then the layers of rock have been tilted or folded after the layers originally formed.

  2. The principle of lateral continuity - sedimentary strata extend sideways for some distance.
    Note: If a sedimentary stratum occurs on one side of a stream valley and a seemingly identical stratum occurs at a corresponding level on the other side of the valley, then presumably they were once a single, laterally continuous layer that was later partly eroded away as the valley was eroded.

  3. The principle of superposition - In a sequence of sedimentary strata, the stratum that is underneath is older, the stratum that is on top is younger.
    Note: This is probably the simplest and yet most powerful principle of relative age determination. However, to make sure it correctly applied, you need to be sure which way was up when the sediments were initially deposited, because in some geologic structures (faults or folds) it is possible for a layer of rock to be turned completely upside-down.

  4. The principle of inclusions - A piece of rock that is included in (completely surrounded by) sedimentary rock is older than the sedimentary rock in which it is included.
    Note: If rounded pieces of granite are pebbles in a layer of conglomerate that lies on top of the granite, then the granite must have been exposed, weathered and eroded prior to the conglomerate being deposited.

  5. The principle of cross-cutting relationships - A rock body or geologic structure that cuts off other layers or structures that would otherwise tend to continue is younger than the layers or structures that it cuts off.
    Note: If sedimentary beds are cut off by a fault, then the fault must be younger than the layers of sediment.

  6. Principle of faunal succession - Within a geologic era, period, or epoch there are certain fossil types that occur in strata of that age that are not found in strata of other ages.
    Note: This principle is a powerful tool for determining the age of sedimentary rocks. Index fossils are ones that only occur within limited intervals of geologic time. Much geological research has been done to determine the extent of geologic time through which particular index fossils occurred.

By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale. They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.

In the 20th century, radiometric methods of absolute age determination were developed. These methods allow the ages of certain types of rocks and minerals to be quantified in terms of years. By the 1960s absolute dating methods had been used to determine the ages of many rocks from all the continents and ocean floors. Repeatedly, the absolute age determinations confirmed what geologists already knew, for example that the Cambrian period occurred before-is older than-the Ordovician period. The absolute dating methods proved that the relative dating methods had been correct, and now geologists can say not only state the sequence of geologic time, they can also estimate fairly accurately how many years ago each division in the sequence occurred.

How Do Unconformities Mark Missing Time?

Another essential concept in stratigraphy is the unconformity. An unconformity is a surface upon which no new sediments were deposited for a long geologic interval. During this interval, erosion may have occurred before more deposits of sediments covered the surface. An unconformity marks a "gap in geologic time" because the rocks below and above it come from widely separated geologic times. There are no sedimentary strata to record what happened during the intervening interval. Instead, there is just an unconformity, a buried erosional or non-depositional surface.

Unconformities separate chapters in the geologic history of a given region. For instance, an orogenic episode (a long geologic episode of mountain building) may finally come to end and the eroded mountains may be buried beneath a new sequence of sediments. A major unconformity would mark the change from the building up of mountains to the wearing down of those same mountains and the subsequent blanketing of the area with sediments.

There are several specific types of unconformities. The three major, specific types of unconformities are included here.

The key to identifying each specific type of unconformity is recognizing what the unconformity is on top of. The possibilities for what is in the rocks immediately beneath the unconformity are (1) layers of sedimentary or volcanic rock (strata) that have been tilted or folded prior to development of the unconformity; (2) a stratum is parallel to the unconformity and parallel to the stratum above the unconformity; or (3) plutonic or metamorphic rocks, which originated much deep in the earth's crust rather than at its surface.

An angular unconformity is an unconformity beneath which the strata were tilted or folded before deposition of the younger layers of sediment above the unconformity. After being tilted or folded, the older layers of sediment were eroded. Then younger layers of sediment were deposited on them. The angular unconformity is the contact between the younger layers of sediment and the older, tilted strata beneath.

angular unconformity diagram

A nonconformity is an unconformity with sedimentary or volcanic strata on top and, beneath it, either plutonic rock such as granite or metamorphic rock such as schist. Because granitic and metamorphic rocks form deep in the earth's crust, a significant amount of time is required for uplift and erosion to expose them. Nonconformities mark major chapter breaks in the geologic history of an area.

In the example below, the contact between the conglomerate and the granite beneath it appears likely to be a nonconformity. However, it is possible that the granite may have intruded as a magma within the crust, beneath conglomerate, after the conglomerate formed. If so, the granite is younger and the boundary between the granite and the conglomerate is an intrusive contact rather than a nonconformity. To determine the nature of the contact - whether it is an intrusive contact or a nonconformity - further evidence from field investigations would be needed. Evidence such as angular pieces of conglomerate surrounded by the granitic intrusion, and contact metamorphism of the conglomerate adjacent to the granite, would indicate that the granite is younger and intruded the older conglomerate. Evidence such as rounded pebbles of the granite within the conglomerate would indicate that the granite is older and underwent erosion prior to the conglomerate forming, and the contact is a nonconformity.

nonconformity diagram

A disconformity is an unconformity with a sedimentary stratum beneath it that is not folded or tilted relative to the unconformity. Because there is a layer of sedimentary rock below a disconformity that is parallel to the layer above it, a disconformity may be difficult to recognize. The existence of a disconformity is indicated by the geologic ages of the sedimentary strata. If there is a significant gap in geologic time between the two layers - for example, if the layer beneath is Cambrian in age and the layer above is Devonian in age - then it can be inferred that the contact between the layers is a disconformity. Confirming evidence of a disconformity may include signs of erosion into the lower layer, and soil development on top of it, prior to deposition of the sediment of the upper layer.

In the example below, it appears that the contact between layers b and c may be a buried erosional surface. If the geologic ages of the strata show a significant gap in geologic time between stratum b and stratum c, then the contact between them is a disoncormity.

disconformity diagram

Radiometric dating methods

In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years. Most absolute age determinations in geology rely on radiometric methods.

The earth is billions of years old. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials.

An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus. By definition, all atoms of a given element have the same number of protons. However, they do not all have the same number of neutrons. The different numbers of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element.

For example, all carbon atoms have 6 protons. Carbon-12 is the isotope of carbon that has 6 neutrons. Carbon-13 is the isotope of carbon that has 7 neutrons. Carbon-14 has 8 neutrons in its nucleus, along with its 6 protons, which is not a stable combination. That is why carbon-14 is a radioactive isotope-it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product - a different isotope, which is not radioactive.

Each type of radioactive isotope has a half-life, a length of time that it will take for half of the atoms in a sample of that isotope to decay into the stable daughter product. Physicists have measured the half-lives of most radioactive isotopes to a high level of precision.

The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle. If a granite crystallizes with minerals containing radioactive isotopes, it is as though the rock crystallizes with a built-in batch of stopwatches that begin ticking away as soon as the granite has cooled.

Radiometric age determination

Radiometric age determinations are expensive and time-consuming. A geologist has to be sure that an age of a rock will help answer an important research question before he or she devotes time and money to making a radiometric age measurement.

Before determining the age of the granite, it must be analyzed under a powerful microscope, and with an electron microprobe, to make sure that its original minerals have not been cracked and altered by metamorphism since the rock first formed. Separating the minerals from the granite is the next step in determining its age. High-precision laboratory analyses are then used to measure the amounts of radioactive parent isotope and stable daughter product in the minerals. Once these quantities have been measured, the half-life of the radioactive isotope is used to calculate absolute age of the granite.

Parent Isotopes, Daughter Isotopes, and Half-Lives

The dots in the cartoon below represent atoms of a parent isotope decaying to its stable daughter product through two half-lives. At time zero in the diagram, which could represent the crystallization of minerals in a rock, there are 32 red dots. After one half-life has passed, there are 16 red dots and 16 green dots. After two half-lives have passed, there are 8 red dots and 24 green dots.

schematic half-lives diagram with three boxes, from left to right:
0 half-lives, contains 32 red dots; 1 half-life, contains 16 red dots and 16 green dots; 2 half-lives, contains 8 red dots and 24 green dots

The following graph illustrates radioactive decay of a fixed amount of an isotope. You can see how the proportions of the isotopes from the cartoon above are graphed as percentages at half-lives 0, 1, and 2 below.

The following table lists a selection of isotope pairs that are used in making radiometric age determination. Note that carbon-14 has a relatively short half-life, which makes it useful only for young, carbon-rich geologic materials, less than about 70,000 years old. Igneous rocks and high-grade metamorphic rocks are the most likely to be entirely formed of minerals that crystallized when the rocks formed. As most fossils are found in clastic sedimentary rocks, which are made of weathered and eroded minerals and bits of rock of various ages, it is unlikely to be able to make an radiometric age determination of a rock in which a fossil is found. The age of a rock containing fossils can usually be narrowed down by measuring the ages of metamorphic or igneous rocks in stratigraphic relation to it, such as a lava flow on top of a layer of sedimentary rock.

Parent Daughter Half Life (years) Dating Range (years) Minerals/materials
4.5 billion 10 million -
4.6 billion
Minerals include zircon, uraninite. Igneous or metamorphic rocks.
1.3 billion 0.05 million -
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Volcanic rocks.
47 billion 10 million -
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Igneous or metamorphic rocks.
5,730 years 100 - 70,000 years Not used for dating rocks, except carbonates from earth's surface such as recent coral reefs. Used for young organic materials, or surface-water samples: Wood, charcoal, peat, bone, tissue, carbonate minerals from surficial environments, water containing dissolved carbon.

The Geologic Time Scale

The timescale used by geologists as a framework for earth's history, its sequence of rocks and fossils and the events they record, was largely established during the 1800s using Steno's principles of relative geologic age, Smith's principle of faunal succession, and the theory of unconformities by Hutton and others. The names of some of the periods, such as Jurassic period and Cambrian period, are familiar even to many non-geologists. This geologic time scale was assembled entirely on the basis of relative geologic ages, without knowing the absolute ages of any of the events, eons, or periods.

The types of fossils that occur in the rocks are the main criterion used to separate the Phanerozoic eon from the Precambrian eons, to divide the Phanerozoic eon into the Paleozoic, Mesozoic, and Cenozoic periods, and to define each of the geologic periods and epochs. The two most abrupt and pervasive changes in the types of fossils found in the rocks - the two greatest mass extinctions - occur at the end of the Paleozoic eon, when as many as 90% of animal species met their demise, and at the end of the Mesozoic eon, which is famous as the mass extinction that includes the last of the dinosaurs.

In the late 1800s, radioactive isotopes of the elements were discovered by such scientists as Marie Curie of France. Scientists quickly realized they could use radioactive isotopes in geologic materials to measure how old the materials were, and began developing techniques for doing so. Over the course of the 1900s these techniques become more accurate and precise as as new technologies were applied and theoretical advances were made in understanding how isotopes yield information on different aspects of the rock cycle. During the 1900s thousands of geologic ages were determined by measuring radioactive isotopes and their daughter products in rocks, minerals, or other geologic materials, and now thousands of absolute geologic ages are measured every year. With these radiometric methods, the ages of the eons, periods, and epochs have been determined in terms of absolute ages. This has allowed the number of years ago that each interval of geologic time began or ended to be added to the geologic time scale. As a by-product of the application of absolute age measurements to the geologic timescale, the absolute ages have confirmed the validity of the relative ages.

The Age of the Earth

The earth formed between 4.5 and 4.6 billion years ago, at the same time as the other planets in the solar system were forming. How do we know this? By measuring the age of the earth's oldest rocks and minerals, the earth as a whole, the solar system as a whole, and the Moon. All those ages converge on an age of the earth, and the solar system, of between 4.5 and 4.6 billion years. All these ages have been measured by analyzing radiometric isotopes such as U-Pb and Sm-Nd in samples of rocks and minerals - except for the age of the sun, which is based on theoretical models of how stars form and evolve, derived from the laws of physics.

Here are the lines of evidence that, combined, tell us how old the earth is:

The oldest rocks on earth: The oldest rock so far found on earth has an age of 4.0 billion years. We would not expect to find any rocks from the very beginning of earth history, because the earth appears to have gone through a largely molten stage soon after it formed. In addition, the earth is a dynamic planet that erodes, buries, metamorphoses, and recycles its rocks in ways described in the rock cycle. Few, if any, rocks from the earth's original solid crust would be expected to still exist.

The oldest minerals on earth: There are some mineral grains in ancient sandstone from the continent of Australia that have radiometric ages going back to 4.3 billion years old. This indicates that when the sand was being deposited (which is though to be about 3.5 billion years ago by other age measurements), some rocks in the mountains undergoing erosion into sand were 4.3 billion years old.

The age of the earth's crust and mantle, theoretically combined. The earth's mantle and crust can be considered a "bulk rock," distinct from the partly molten core, which is made mostly of iron. Adding up the isotope ratios of uranium and lead from the earth's crust and mantle as a whole gives isotope ratios that converge at an age of between 4.5 and 4.6 billion years ago. This age is based on some assumptions about how the crust formed from melted mantle rocks and evolved separately from the mantle over the course of earth history, so it is an indirect age measurement rather than a direct one like the age of a single rock or mineral.

The evidence that the earth formed as part of the solar system. The ratios of isotopes and chemical elements of all components of the solar system that we have been able to measure - Sun, several planets, earth's Moon, asteroids, and comets - show the same characteristics, the same "fingerprint," which indicates they all formed from the same batch of chemical elements. This batch of chemical elements is different from the ratios of isotopes and chemical elements that other star systems are made of, which shows that these components of the solar system share a common origin.

The ages of the oldest meteorites, taken as representing the first stage in the formation of the planets. Most meteorites are pieces of asteroids that have crashed to earth. The oldest meteorites have ages between 4.6 and 4.5 billion years. The asteroids, and these oldest meteorites, are thought to be pieces of planetesimals. Planetesimals are small, solid bodies that formed early in solar system history, most of which combined together to become the planets. Asteroids are the remains of planetesimals that did not condense into a larger planet, and thus have not changed much since that early stage of solar system history. One type of meteorite, known as chondrites, is thought to represent the first type of solid that condensed during the initial formation of the solar system, and chondrites have ages close to 4.6 billion years. Other types of meteorites appear to represent stages of planetesimal formation, with ages tapering down to about 4.55 billion years.

The age of the oldest Moon rocks. The Apollo astronauts brought back samples of moon rocks thought to have formed soon after the Moon originated, as it solidified from a largely molten state. The oldest Moon rocks are about 4.4 billion years old.

The theoretical age of the Sun. Stars are, in a sense, a lot less complicated than planets such as earth, because earth is made of solids, liquids, and gases, whereas the Sun is essentially just a big ball of gas. This makes it possible for astrophysicists to write mathematical models, based on the laws of physics, which duplicate how stars like the Sun form and evolve, and ultimately meet their own natural fate as either a supernova or else a white dwarf, depending on how big the star is. The Sun, according to these calculations, is not a big enough star to explode as a supernova once it has burned out; instead, it will shed its outer parts and leave behind a dense, hot, white dwarf star that no longer produces any new heat. Based on these calculations, and the current composition and temperature of the Sun, the Sun is now between 4.5 and 5.5 billion years old. This seems a bit older than the age of the solar system, but it is an approximate age based on theoretical calculations, not a precisely measured age. It overlaps with the 4.6 billion year age of the solar system derived from direct measurements of meteorite ages.

Material Age Methods
Oldest earth rock 4.0 billion years radiometric
Oldest earth mineral 4.3 billion years radiometric
Earth's mantle and crust combined 4.5-4.55 billion years radiometric
plus some theoretical assumptions
Oldest meteorites 4.54-4.58 billion years radiometric
Oldest Moon rock 4.45 billion years radiometric
Sun 4.5-5.5 billion years theoretical calculations

Open Source Web Links
University of California Museum of Paleontology description of principles of relative dating and some of the geologists who developed them
UCMP interactive geologic time scale

To learn more about the geologic time scale, go to the University of California Museum of Paleontology at

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Geology 101 - Introduction to Physical Geology
Basics--Geologic Time
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 9/11/13

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.

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