• The Map That Changed the World– William Smith, 1815– Geologic map of

England, Wales, part of Scotland

Dating the Earth

The Map That Changed the World

• 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

• Large divisions based on…?

• Paleozoic Era –

• Mesozoic Era –

• Cenozoic Era –

Geologic Time Scale

• Large divisions based on characteristics of fossils

• Paleozoic Era – early life dominated by invertebrate animals

• Mesozoic Era – middle life

• Cenozoic Era – recent life

Geologic Time Scale

How was the timescale created?

• Mapping in 1800s using the principles of

– Superposition

– Original Horizontality

– Original Lateral Continuity

– Cross-cutting relationships

– Also Fossil Correlation

How was the timescale created?

• Superposition– Oldest on bottom, youngest on top

Relative-Dating Principles

Chattanooga Shale, TN

• Original Horizontality– Sediment originally deposited in flat

parallel layers

• Lateral continuity– sediment extends laterally in all directions until

it thins and pinches out or terminates against the edges of the depositional basin

• Cross-cutting relationships– an igneous intrusion or a fault must be

younger than the rocks it intrudes or displaces

Relative-Dating Principles

• A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite

Cross-cutting Relationships

North shore of Lake Superior, Ontario Canada

• A small fault displaces tilted beds: the fault is younger than the beds

Cross-cutting Relationships

Templin Highway, Castaic, California

Unconformities

• What is an unconformity?

Unconformities

• What is an unconformity?

– A surface of erosion or non-deposition

– Recognizable surface in the rock record

Example of an Unconformity• Tilted sandstone and

siltstone below, conglomerate above

www.geology.sdsu.edu/visualgeology/geology101/erosion6.htm

Back to Steno

www.gly.uga.edu/railsback/1121Steno.jpg

Why are layers tilted?• Deformation of rocks

– Occurs after they are deposited– Important factor in relative dating

• Folding– Anticlines, synclines– Rock bends, but does not break

• Faulting– Normal, reverse, transform– Rock breaks

Folding

www.hill.anorak.org.uk/dhtml/glgchap5.html

Faulting

www.stmarys.ca/academic/science/geology/structural/faults.html

Relative Dating

• Principles

• Unconformities

• Deformation

Relative Dating – Cross-section

http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/

Relative Dating – Cross-section

http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/

Key:EErosionGLCHTiltingErosionMDJAErosionNKBTiltingErosionFErosion

Relative Dating – Cross-section

http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm

Relative Dating – Cross-section

http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm

Key:HFDGIErosionCErosionABMLKErosionJ

Grand Canyon: history revealed

• More than 1 billion years of history are preserved in the rock layers of the Grand Canyon

• Reading this rock book shows: – periods of mountain building– advancing and retreating shallow seas– evolution of faunas

• Determine these things by: – applying the principles of relative dating to the rocks– and recognizing that present-day processes have

operated throughout Earth history - Uniformitarianism

Grand Canyon

• Radiometric dating is the most common method of obtaining absolute ages– calculated from the natural rates of decay of

various natural radioactive elements present in trace amounts in some rocks

• Other methods?– Tree ring counting– Varves– Ice cores

Absolute Dating

• 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

• James Ussher (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

• used melted iron balls of various diameters• he estimated Earth was 75,000 years old

Changes in the Concept of Geologic Time

– Others used 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

– Or 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

• Neptunism– proposed in 1787 by Abraham Werner (1749-1817)

– all rocks, including granite and basalt, were precipitated in an orderly sequence from a primeval, worldwide ocean

– Werner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history

History of Historical Geology

• 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, he last one was the biblical flood

History of Historical Geology

• 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 – 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

History of Historical Geology

• Developed by James Hutton, advocated by Charles Lyell(1797-1875)

• Present-day processes have operated throughout geologic time

• 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 viewed Earth history as cyclical

Uniformitarianism

• He also understood that geologic processes operate over a vast amount of time

• Modern view of uniformitarianism– geologists assume that the principles or laws

of nature are constant– but the rates and intensities of change have

varied through time

erosion

deposition uplift

erosion

• 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

• 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

• 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: we have it!

• Atomic mass number= number of protons + number of neutrons

• Isotopes: different numbers of neutrons

• Different isotopes 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

Absolute-Dating Methods

• 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:– alpha decay, two protons and two neutrons (alpha

particle) are emitted from the nucleus

Radioactive Decay

– beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton

Radioactive Decay

– electron capture decay, a proton captures an electron and converts to a neutron

• Some isotopes undergo only one decay step before they become stable

– rubidium 87 decays to strontium 87 by a single beta emission

– potassium 40 decays to argon 40 by a single electron capture

Radioactive Decay

• 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

Radioactive Decay

Uranium 238 decay

• 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 – a curved graph

Half-Lives

• 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– they fit differently into the crystal structure of certain

minerals

• 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

• Closed system is needed for an accurate date– 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

• Most reliable dates use multiple methods

Sources of Uncertainty

• 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 (to other parts of the rock) 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 Daughters Half-Life (years)

Long-Lived Radioactive Isotope Pairs Used in Dating

Uranium 238 Lead 206 4.5 billionUranium 234 Lead 207 704 millionThorium 232 Lead 208 14 billionRubidium 87 Strontium 87 48.8 billionPotassium 40 Argon 40 1.3 billion

Most of these are useful for dating older rocks

Mass Spectrometer

www.mines.unr.edu/isotope/gallery.html

How do we know the Earth can’t be older than about 6-7 b.y.?

• Moderate half-life isotopes (1 b.y.)

• If Earth was > 6-7 b.y. old, there wouldn’t be many parents left

• 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 14C 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– 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• 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 19th century – and lead to radiometric dating– which allowed geologists to determine

absolute ages for geologic events

Summary

• 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

• The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairs – in igneous rocks

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