Chapter 4

Geologic Time— Concepts and Principles

Grand Canyon

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

Concept of Geologic Time

► Geologists use two different frames of reference when discussing geologic time 

Relative dating

sequential order as determined from their position in the geologic record involves placing geologic events in a ► ► 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

or events results in specific dates for rock units ► expressed in years before the present ►

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 ► 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

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

Illustration of the principles of original horizontality

Illustration of the principles of superposition

Relative-Dating Principles

► Principle of lateral continuity  Nicolas Steno ► Sediment extends laterally in all direction until it thins and pinches out or terminates against the edges of the depositional basin ► 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

Relative-Dating Principles

► Principle of inclusions  discussed later in the term ► Principle of fossil succession  discussed later in the term

Cross-cutting Relationships

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.

Neptunism

►

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

► ► ►

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

► 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) ► ► Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point Scotland We now call what he observed an interpreted its formation unconformity but he properly ► Term uniformitarianism was coined by William Whewell in 1832

Unconformity at Siccar Point

► Hutton explained that  the tilted, lower rocks resulted from severe upheavals that formed mountains   The mountains were then worn away and covered by younger flat-lying rocks the erosional surface represents a gap in the rock record

Uniformitarianism

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

Crisis in Geology

► For the geologic processes envisioned by other geologists at that time, this age was too young!

 What was the flaw in Kelvin’s calculation?

► Kelvin did not know about radioactivity as a heat source within the Earth

Absolute-Dating Methods

►

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 ► ► The

nucleus



protons

of an atom is composed of – particles with a positive electrical charge  

neutrons electrons

nucleus – electrically neutral particles – the negatively charged particles – encircling the atomic number   Equal to the number of protons helps determine the atom’s chemical properties and the element to which it belongs

Isotopes

►

Atomic mass number

neutrons  The different forms of an element’s atoms with varying numbers of neutrons are called

isotopes

► = number of protons + number of 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 determine absolute ages of rocks to

Radioactive Decay

►

Radioactive decay

-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 potassium 40

decays to strontium 87 by a single beta emission decays to argon 40 by a single electron capture But other isotopes undergo several decay steps  Examples: ►

uranium 235

►

uranium 238

decays to lead 207 by 7 alpha steps and 6 beta steps decays to lead 206 by 8 alpha steps and 6 beta steps

Uranium 238 decay

Half-Lives

► 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 Can vary from less than 1/billionth of a second to 49 billion years Is geometric not linear, so has a curved graph

Uniform Linear Change

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

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:  If a rock has a parent/daughter ratio of 1:3 = a parent proportion of 25%,  and the half-live is 57 million years,  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 Some radioactive parents are included in the crystal structure of certain minerals

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

Igneous Crystallization

► ► ► Crystallization of magma separates parent atoms  from previously formed daughters This resets the radiometric clock to zero.

Then the parents gradually decay.

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

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 .

Dating the whole rock yields a date of 700 million years = time of crystallization.

d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time.

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 Uranium 238 Uranium 234 Thorium 232 Rubidium 87 Potassium 40 Daughters Lead 206 Lead 207 Lead 208 Strontium 87 Argon 40 Half-Life (years) 4.5 billion 704 million 14 billion 48.8 billion 1.3 billion

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  the amount of uranium

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

Carbon 14

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

Carbon 14

► ► ► The carbon 14 becomes 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

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

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

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