Geologic Time—Concepts and Principles Chapter 4 Geologic Time—Concepts and Principles -How do we tell time in geology?...relative dating, absolute dating Absolute age dating: 4 methods: 1. Radiometric age dating: parents…daughters….isotopes… 2. Carbon 14 age dating: 3. Tree Rings: 4. Fission track dating -Basic principles: horizontality, superposition, cross cutting relationships -unconformities- 3 types….

Grand Canyon When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth

Grand Canyon More than 1 billion years of history are preserved, like pages of a book, in the rock layers of the Grand Canyon Reading this rock book we learn that the area underwent episodes of mountain building advancing and retreating shallow seas We know these things by applying the principles of relative dating to the rocks and recognizing that present-day processes have operated throughout Earth history

What is time? We are obsessed with time, using clocks calendars appointment books Mostly we don’t have enough of it. Our common time units are seconds hours days weeks months years Ancient history involves hundreds of years thousands of years But geologic time involves millions of years even billions of years

Concept of Geologic Time Geologists use two different frames of reference when discussing geologic time Relative dating involves placing geologic events in a sequential order as determined from their position in the geologic record It does not tell us how long ago a particular event occurred only that one event preceded another For hundreds of years geologists have been using relative dating to establish a relative geologic time scale

Relative Geologic Time Scale The relative geologic time scale has a sequence of eons eras periods epochs but no numbers indicating how long ago each of these times occurred

Concept of Geologic Time The second frame of reference for geologic time is absolute dating Absolute dating results in specific dates for rock units or events expressed in years before the present It tells us how long ago a particular event occurred giving us numerical information about time Radiometric dating is the most common method of obtaining absolute ages Such dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks

Geologic Time Scale The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale The geologic time scale is a dual scale a relative scale and an absolute scale

Changes in the Concept of Geologic Time The concept and measurement of geologic time has changed through human history Early Christian theologians conceived of time as linear rather than circular James Usher (1581-1665) in Ireland calculated the age of Earth based on recorded history and genealogies in Genesis He announced that Earth was created on October 22, 4004 B.C. A century later it was considered heresy to say Earth was more than about 6000 years old.

Changes in the Concept of Geologic Time During the 1700s and 1800s Earth’s age was estimated scientifically Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning using melted iron balls of various diameters Extrapolating their cooling rate to an Earth-sized ball, he estimated Earth was 75,000 years old

Changes in the Concept of Geologic Time Others used different techniques Using rates of deposition of various sediments and thickness of sedimentary rock in the crust gave estimates of <1 million to more than 2 billion years. Using the amount of salt carried by rivers to the ocean and the salinity of seawater John Joly in 1899 obtained a minimum age of 90 million years

Relative-Dating Principles Six fundamental geologic principles are used in relative dating Principle of superposition Nicolas Steno (1638-1686) In an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top This method is used for determining the relative age of rock layers (strata) and the fossils they contain

Principle of Superposition Illustration of the principles of superposition and original horizontality Superposition: The youngest rocks are at the top of the outcrop and the oldest rocks are at the bottom

Relative-Dating Principles Principle of original horizontality Nicolas Steno Sediment is deposited in essentially horizontal layers Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification

Principle of Horizontality Illustration of the principles of superposition and original horizontality Horizontality: These sediments were originally deposited horizontally in a marine environment This outcrop is Chattanooga Shale, Tennessee

Principle of lateral continuity In 1669, Nicolas Steno proposed his principle of lateral continuity, meaning that layers of sediment extend outward in all directions until they terminate Terminations may be abrupt at the edge of a depositional basin where eroded where truncated by faults

Gradual Terminations or they may be gradual where a rock unit becomes progressively thinner until it pinches out or where it splits into thinner units each of which pinches out, called intertonging where a rock unit changes by lateral gradation as its composition and/or texture becomes increasingly different

Principle of cross-cutting relationships James Hutton (1726-1797) An igneous intrusion or a fault must be younger than the rocks it intrudes or displaces North shore of Lake Superior, Ontario Canada A dark-colored dike has intruded into older light colored granite. The dike is younger than the granite.

Cross-cutting Relationships Templin Highway, Castaic, California A small fault displaces tilted beds. The fault is younger than the beds.

Principle of Inclusions According to the principle of inclusions, which also helps to determine relative ages, inclusions or fragments in a rock are older than the rock itself Light-colored granite in northern Wisconsin showing basalt inclusions (dark) Which rock is older? Basalt, because the granite includes it

Relative-Dating Principles Principle of fossil succession discussed later in the term

Early Geologic Concepts/Thoughts: Neptunism All rocks, including granite and basalt, were precipitated in an orderly sequence from a primeval, worldwide ocean. proposed in 1787 by Abraham Werner (1749-1817) Werner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history

Neptunism Werner’s geologic column was widely accepted Alluvial rocks unconsolidated sediments, youngest Secondary rocks rocks such as sandstones, limestones, coal, basalt Transition rocks chemical and detrital rocks, some fossiliferous rocks Primitive rocks oldest including igneous and metamorphic

Catastrophism Catastrophism proposed by Georges Cuvier (1769-1832) dominated European geologic thinking The physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area Six major catastrophes occurred, corresponding to the six days of biblical creation The last one was the biblical flood

Neptunism and Catastrophism Were Eventually abandoned These hypotheses were abandoned because they were not supported by field evidence Basalt was shown to be of igneous origin Volcanic rocks interbedded with sedimentary and primitive rocks showed that igneous activity had occurred throughout geologic time More than 6 catastrophes were needed to explain field observations The principle of uniformitarianism became the guiding philosophy of geology

Uniformitarianism Principle of uniformitarianism Present-day processes have operated throughout geologic time. Developed by James Hutton, advocated by Charles Lyell (1797-1875) Term uniformitarianism was coined by William Whewell in 1832 Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point Scotland We now call what he observed an unconformity but he properly interpreted its formation

Angular Unconformity at Siccar Point Hutton explained that the tilted, lower rocks resulted from severe upheavals that formed mountains these were then worn away and covered by younger flat-lying rocks the erosional surface represents a gap in the rock record= Hiatus

3 Types of Unconformities Unconformities of regional extent may change from one type to another They may not represent the same amount of geologic time everywhere

A Disconformity A disconformity between sedimentary rocks in California, with conglomerate deposited upon an erosion surface in the underlying rocks

A Nonconformity A nonconformity in South Dakota separating Precambrian metamorphic rocks from the overlying Cambrian-aged Deadwood Formation

Uniformitarianism Hutton viewed Earth history as cyclical erosion erosion Hutton viewed Earth history as cyclical deposition uplift He also understood that geologic processes operate over a vast amount of time Modern view of uniformitarianism Today, geologists assume that the principles or laws of nature are constant but the rates and intensities of change have varied through time

Crisis in Geology Lord Kelvin (1824-1907) knew about high temperatures inside of deep mines and reasoned that Earth is losing heat from its interior Assuming Earth was once molten, he used the melting temperature of rocks the size of Earth and the rate of heat loss to calculate the age of Earth as between 400 and 20 million years- too Young!!

Absolute-Dating Methods: 1. Radiometric Age Dating The discovery of radioactivity destroyed Kelvin’s argument for the age of Earth and provided a clock to measure Earth’s age Radioactivity is the spontaneous decay of an atom’s nucleus to a more stable form The heat from radioactivity helps explain why the Earth is still warm inside Radioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events

Atoms Understanding absolute dating requires knowledge of atoms and isotopes All matter is made up of atoms The nucleus of an atom is composed of protons – particles with a positive electrical charge neutrons – electrically neutral particles with electrons – negatively charged particles – encircling the nucleus The number of protons (= the atomic number) helps determine the atom’s chemical properties and the element to which it belongs

Isotopes Atomic mass number The different forms of an element’s atoms = number of protons + number of neutrons The different forms of an element’s atoms with varying numbers of neutrons are called isotopes Different isotopes of the same element have different atomic mass numbers but behave the same chemically Most isotopes are stable, but some are unstable Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

Radioactive Decay Radioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element Three types of radioactive decay: In alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus.

Radioactive Decay In beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton. In electron capture decay, a proton captures an electron and converts to a neutron.

Radioactive Decay Some isotopes undergo only one decay step before they become stable. Examples: rubidium 87 decays to strontium 87 by a single beta emission potassium 40 decays to argon 40 by a single electron capture But other isotopes undergo several decay steps uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps

Uranium 238 decay

Half-Lives: key to understanding The half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope The half-life of a specific radioactive isotope is constant and can be precisely measured

Half-Lives The length of half-lives for different isotopes of different elements can vary from less than 1/billionth of a second to 49 billion years Radioactive decay is geometric not linear, so has a curved graph

Geometric Radioactive Decay In radioactive decay, during each equal time unit one half-life, the proportion of parent atoms decreases by 1/2

Determining Age By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined in the laboratory geologists can calculate the age of a sample containing the radioactive element The parent/daughter ratio is usually determined by a mass spectrometer an instrument that measures the proportions of atoms with different masses

Determining Age For example: how old is the rock? If a rock has a parent/daughter ratio of 1:3 = a parent proportion of 25%, and the half-live is 57 million years, how old is the rock? 25% means it is 2 half-lives old. the rock is 57 x 2 =114 million years old.

What Materials Can Be Dated? Most radiometric dates are obtained from igneous rocks As magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atoms Because they fit, some radioactive parents are included in the crystal structure of certain minerals

Not Sedimentary Rocks Generally, sedimentary rocks cannot be radiometrically dated because the date obtained would correspond to the time of crystallization of the mineral, when it formed in an igneous or metamorphic rock, not the time that it was deposited as a sedimentary particle Exception: dating the mineral glauconite, because it forms in certain marine environments as a reaction with clay during the formation of the sedimentary rock

Sources of Uncertainty In glauconite, potassium 40 decays to argon 40 because argon is a gas, it can easily escape from a mineral A closed system is needed for an accurate date that is, neither parent nor daughter atoms can have been added or removed from the sample since crystallization If leakage of daughters has occurred it partially resets the radiometric clock and the age will be too young If parents escape, the date will be too old. The most reliable dates use multiple methods.

Sources of Uncertainty During metamorphism, some of the daughter atoms may escape leading to a date that is too young. However, if all of the daughters are forced out during metamorphism, then the date obtained would be the time of metamorphism—a useful piece of information. Dating techniques are always improving. Presently measurement error is typically <0.5% of the age, and even better than 0.1% A date of 540 million might have an error of ±2.7 million years or as low as ±0.54 million

Long-Lived Radioactive Isotope Pairs Used in Dating The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable Such isotopes include: Parents Daughters Half-Life (years) Most of these are useful for dating older rocks Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion

2. Fission Track Dating Uranium in a crystal will damage the crystal structure as it decays The damage can be seen as fission tracks under a microscope after etching the mineral The age of the sample is related to the number of fission tracks and the amount of uranium with older samples having more tracks This method is useful for samples between 1.5 and 0.04 million years old

3. Radiocarbon Dating Method Carbon is found in all life It has 3 isotopes carbon 12 and 13 are stable but carbon 14 is not Carbon 14 has a half-life of 5730 years Carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living The short half-life of carbon 14 makes it suitable for dating material < 70,000 years old It is not useful for most rocks, but is useful for archaeology and young geologic materials

4. Tree-Ring Dating Method The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk) The width of the rings are related to climate and can be correlated from tree to tree a procedure called cross-dating The tree-ring time scale now extends back 14,000 years

Tree-Ring Dating Method In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

Summary Early Christian theologians viewed time as linear and decided that Earth was very young (about 6000 years old) A variety of ages for Earth were estimated during the 18th and 19th centuries using scientific evidence, ages now known to be too young Neptunism and catastrophism were popular during the 17th, 18th and early 19th centuries because of their consistency with scripture, but were not supported by evidence

Summary James Hutton viewed Earth history as cyclical and very long His observations were instrumental in establishing the principle of uniformitarianism Charles Lyell articulated uniformitarianism in a way that soon made it the guiding doctrine of geology Uniformitarianism holds that the laws of nature have been constant through time and that the same processes operating today have operated in the past, although not necessarily at the same rates

Summary The principles of superposition, Radioactivity was discovered original horizontality, lateral continuity and cross-cutting relationships are basic for determining relative geologic ages and for interpreting Earth history Radioactivity was discovered during the late 19th century and lead to radiometric dating, which allowed geologists to determine absolute ages for geologic events

Summary Geologists determine how many half-lives of a radioactive parent isotope have elapsed since the sample crystallized Half-life is the length of time it takes for one-half of the radioactive parent isotope to decay to a stable daughter isotope of a different element

Summary The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairs in igneous rocks Common pairs include: uranium 238 – lead 206 uranium 235 – lead 207 thorium 232 – lead 208 rubidium87 – strontium 87 potassium 40 – argon 40

Summary The most reliable radiometric ages are obtained using two different pairs in the same rock Carbon 14 dating can be used only for organic matter such as wood, bones, and shells and is effective back to about 70,000 years

Sedimentary Facies On a continental shelf, sand may accumulate in the high-energy nearshore environment while mud and carbonate deposition takes place at the same time in offshore low-energy environments

Uniform Linear Change In this example of uniform linear change, water is dripping into a glass at a constant rate

Carbon 14 Carbon 14 is constantly forming When a high-energy neutron in the upper atmosphere When a high-energy neutron a type of cosmic ray strikes a nitrogen 14 atom it may be absorbed by the nucleus and eject a proton changing it to carbon 14 The 14C formation rate is fairly constant and has been calibrated against tree rings

Carbon 14 The carbon 14 becomes While the organism lives part of the natural carbon cycle and becomes incorporated into organisms While the organism lives it continues to take in carbon 14 but when it dies the carbon 14 begins to decay without being replenished Thus, carbon 14 dating measures the time of death

What Materials Can Be Dated? The daughter atoms are different elements with different sizes and, therefore, do not generally fit into the same minerals as the parents Geologists can use the crystals containing the parents atoms to date the time of crystallization

Dating Metamorphism a. A mineral has just crystallized from magma. b. As time passes, parent atoms decay to daughters. c. Metamorphism drives the daughters out of the mineral as it recrystallizes. d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time. Dating the whole rock yields a date of 700 million years = time of crystallization.

Crisis in Geology This age was too young for the geologic processes envisioned by other geologists at that time, leading to a crisis in geology Kelvin did not know about radioactivity as a heat source within the Earth

Igneous Crystallization Crystallization of magma separates parent atoms from previously formed daughters This resets the radiometric clock to zero. Then the parents gradually decay.