Geologic Time—Concepts and Principles

Chapter 4

-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….

• 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 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

Changes in the Concept of Geologic Time

• 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

• 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 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

• 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

Principle of lateral continuity

• 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

– 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.

Principle of cross-cutting relationships

• Templin Highway, Castaic, California

• A small fault displaces tilted beds.

Cross-cutting Relationships

• The fault is younger than the beds.

• 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

Principle of Inclusions

• Light-colored granite – in northern Wisconsin – showing basalt

inclusions (dark)

• Which rock is older?– Basalt, because the

granite includes it

• Principle of fossil succession – discussed later in the term

Relative-Dating Principles

• 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

Early Geologic Concepts/Thoughts: 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 (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

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 (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

Uniformitarianism

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

• Unconformities of regional extent – may change from one type to another

• They may not represent the same amount – of geologic time everywhere

3 Types of Unconformities

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

A Disconformity

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

A Nonconformity

• Hutton viewed Earth history as cyclical

Uniformitarianism

erosion

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

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- too Young!!

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:1. Radiometric Age Dating

• 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: key to understanding

• 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 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?

• 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

• 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

• 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

2. 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

3. Radiocarbon 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

4. 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 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, – 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

• On a continental shelf, sand may accumulate – in the high-energy nearshore environment

Sedimentary Facies

– while mud and carbonate deposition takes place – at the same time – in offshore low-energy environments

• In this example – of uniform

linear change, – water is

dripping into a glass

– at a constant rate

Uniform Linear Change

• 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– 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 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?

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.

• 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

• Crystallization of magma separates parent atoms – from previously formed daughters

• This resets the radiometric clock to zero.• Then the parents gradually decay.

Igneous Crystallization