Geologic Time— Concepts and Principles Chapter 4

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 Grand Canyon

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 What is time? Ancient history involves –hundreds of years –thousands of years But geologic time involves –millions of years –even billions of years

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 Concept of Geologic Time

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 Relative Geologic Time Scale

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 Concept of Geologic Time

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 Geologic Time Scale

The concept and measurement of geologic time –has changed through human history Early Christian theologians –conceived of time as linear rather than circular James Ussher ( ) in Ireland –calculated the age of Earth based –on recorded history and geneologies 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 ( ) –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 Changes in the Concept of Geologic Time

Six fundamental geologic principles are used in relative dating Principle of superposition –Nicolas Steno ( ) –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 Relative-Dating Principles

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

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

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 ( ) –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 Relative-Dating Principles

North shore of Lake Superior, Ontario Canada A dark-colored dike has intruded into older light colored granite. Cross-cutting Relationships The dike is younger than the granite.

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

Neptunism –All rocks, including granite and basalt, –were precipitated in an orderly sequence –from a primeval, worldwide ocean. –proposed in 1787 by Abraham Werner ( ) 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 Neptunism

Catastrophism –proposed by Georges Cuvier ( ) –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 Catastrophism

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 Neptunism and Catastrophism Were Eventually abandoned

Principle of uniformitarianism –Present-day processes have operated throughout geologic time. –Developed by James Hutton, advocated by Charles Lyell ( ) 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 Uniformitarianism

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

Hutton viewed Earth history as cyclical Uniformitarianism erosion depositionuplift 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 erosion

Lord Kelvin ( ) –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

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 Crisis in Geology

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 Absolute-Dating Methods

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 Atoms

Atomic mass number = 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 Isotopes

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. Radioactive Decay –In electron capture decay, a proton captures an electron and converts to a neutron.

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 –Examples: 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 Radioactive Decay

Uranium 238 decay

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 Half-Lives

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

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

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, how old is the rock? Determining Age –25% means it is 2 half- lives old. –the rock is 57 x 2 =114 million years old.

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 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 What Materials Can Be Dated?

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

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 Not Sedimentary Rocks

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 Sources of Uncertainty

a. A mineral has just crystallized from magma. Dating Metamorphism 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.

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 DaughtersHalf-Life (years) Long-Lived Radioactive Isotope Pairs Used in Dating Uranium 238 Lead billion Uranium 234 Lead million Thorium 232 Lead billion Rubidium 87 Strontium billion Potassium 40 Argon billion Most of these are useful for dating older rocks

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 Fission Track Dating 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

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 Radiocarbon Dating Method

Carbon 14 is constantly forming –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 14 C formation rate –is fairly constant –and 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 Carbon 14

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 Tree-Ring Dating Method

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 18 th and 19 th centuries –using scientific evidence, –ages now known to be too young Neptunism and catastrophism were popular –during the 17 th, 18 th and early 19 th 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, –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 19 th 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