The Rock and Fossil Record

Chapter 6Earth’s Story and Those Who First

ListenedRelative Dating: Which Came First?Absolute Dating: A Measure of Time

Looking at FossilsTime Marches On

Terms You Must Know• Uniformitarianism• Catastrophism• Paleontology• Relative dating• Superposition• Geologic column• Unconformity• Absolute dating• Isotope• Radioactive decay• Radiometric dating• Half-life

• Fossil• Trace fossil• Mold• Cast• Index fossil• Geologic time scale• Eon• Era• Period• Epoch• extinction

People To Know

• James Hutton• Charles Lyell• Georges Cuvier

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

• James Hutton wrote Theory of the Earth• Hutton stated that present-day processes, such as erosion and deposition, have operated throughout geologic time

• Uniformitarianism is a principle that states that geologic processes that occurred in the past can be explained by current geologic processes

• 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

Siccar Point• Hutton’s theories sparked a scientific debate

• In Hutton’s time, people believed that the Earth was only a few thousand years old.

• What Hutton proposed could not happen in just a few thousand years

• He formed his theories by observing the geologic processes at Siccar Point

• Deposition and folding were observed

Unconformity• Hutton first described the evidence for this break in the rock record. He was impressed by a placed called Siccar Point in Scotland where some of the rocks were laid down horizontally, but the rocks underneath them were at an angle. The only explanation Hutton felt was plausible was that the underlying rocks had originally been laid down horizontally, then were tilted and eroded and more rocks were then deposited on top of them. Hutton realized that this scenario represented a large gap in time between the first set of rocks being laid down, then tilted, and the second set of rocks deposited. These gaps in the rock record are called unconformities.

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

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 plant and animal species

• 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

• 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

Catastrophism

Uniformitarianism vs Catastrophism

• Catastrophism remained the guiding principle until the work of Charles Lyell

• 1830-1833 Lyell published Principles of Geology, in which he reintroduced uniformitarianism.

• He believed that geologic change happened at the same rate in the past as it happens in the present--

graduallygradually!

Lyell and Darwin• Charles Darwin and Charles Lyell were good friends.

• Darwin accepted and supported uniformitarianism

• Darwin had read Lyell’s book Principles of Geology before his famous 1831 voyage on HMS Beagle

• Despite being friends, Lyell did not embrace Darwin’s theories of natural selection.

• Much later, Lyell finally accepted Darwin’s theories.

Modern Geology

• Modern scientists like Stephen Gould have challenged Lyell’s uniformitarianism.

• Today scientists beleive that catastrophes do at times play an important role in shaping Earth’s history.

• Most geologic change is gradual and uniform but catastrophes have caused geologic change.

• Ex. Craters formed due to asteroids and comets

Paleontology• Paleontology is thee scientific study of fossils- plant and animal.

• Fossils are the remains of organisms preserved by geologic processes.

• Vertebrate paleontologists study fossils of animals that have backbones.

• Invertebrate paleontologists study fossils of animals that do not have backbones.

• Paleobotanists study fossils of plants

Relative Dating

• "Relative Dating" This phrase may conjure up odd jokes and images of kissing cousins to some, but to geologists the phrase refers to distinguishing the age relationships between contiguous rock layers.

• Superposition– Oldest on bottom, youngest on top

Relative-Dating Principles

Chattanooga Shale, TN

Relative dating is any method of determining whether an event or object is older or younger than other events or objects.

Relative Dating

Relative datingCan only be used when the rock layers have been preserved in their original sequence – top layer being the youngest layer

Helps scientists determine whether one fossil is older than the other

Disturbing Forces• Not all rock formations are arranged with the oldest layers on the bottom.

• Natural forces can fold, tilt, break, or remove parts of the rock layer

• Geologists use a geologic column to help them

• Relative dating assumes that if rock layers are not horizontal, then something must have disturbed them after they formed.

Unconformities

• What is unconformity?– Unconformity is a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

– A surface of erosion or non-deposition

– Recognizable surface in the rock record

Unconformities

• Unconformities involve time gaps, typically on the order of tens of millions of years or more. A time gap refers to missing time (as in taking a vacation). A time gap may be due to a time of "nondeposition", meaning that no sediments were deposited for an interval of time. More likely, unconformities indicate a time when uplift and erosion have occurred such that layers deposited at an earlier time have been stripped away. Typically, unconformities involve:– Major sea level changes– Major tectonic events

Unconformity

• When a geologists finds an unconformity, they must question whether the “missing layer” was never present or whether is was somehow removed

• Nondeposition- stopage of deposition when a supply of sediment is cut off

• Erosion can create unconformities.

3 Types of Unconformity

• Disconformities– Most common– Part of a sequence of parallel rock layers is missing

• Nonconformities• Angular unconformities

disconformity

• Disconformities are much harder to recognize in the field, because often there is no angular relationship between sets of layers. Disconformities are usually recognized by correlating from one area to another and finding that some strata is missing in one of the areas.

nonconfomity

• Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface. The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion.

Nonconfomity

• Notice that there is an "intersection" of a vertical rock butting up against a horizontal sock. In an unconformity, it is two of the same type of rock (e.g. sedimentary & sedimentary). A nonconformity is two layers of different types (e.g. igneous & sedimentary).

Angular unconformity

• Angular unconformities are easy to recognize in the field because of the angular relationship of layers that were originally deposited horizontally.

Example of an Unconformity

• Tilted sandstone and siltstone below, conglomerate above

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

Dating rocks• Relative dating

•Using a set of principles to put rocks in their proper sequences of formation

• Absolute dating•Using radioactive decay to determine the exact age of rocks

Absolute Dating:any method of measuring the age of an

event or object in years Most common:based on Radioactive

DecayParent daughter

Why does it work?

1. The decay rate is CONSTANT, independent of external conditions in the earth.2. The daughter/Parent ratio can be precisely measured.

Radioactive Decay• The process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.

• Sounds great but what is an isotope?

• An isotope is an atom that has the same number of protons (atomic #) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)

Isotopes• Most isotopes are stable, meaning they stay in their original form

• But some are unstable• Unstable isotopes are radioactive

• Radioactive decay is the process in which a radioactive isotope tends to bread down into a stable isotope of the same or another element

How does it work?What does this have to do with the age

of rocks?• Unstable isotope is called parent isotopeparent isotope

• The stable isotope produced by radioactive decay is the daughter isotopedaughter isotope.

• Decay is constantDecay is constant• The more daughter isotope- the older the rock!

Radiometric datingRadiometric dating• A method of determining the age of an object by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope

• Ratio or parent material to daughter material

– Absolute dating•Helps scientists determine actual age of fossils•Rocks near fossils contain radioactive elements – unstable elements that break down into different elements

•Half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay

•Scientists compare the amount of radioactive element in a sample to the amount of the element into which it breaks down

•Scientists use this info to calculate the age of the rock, which then tells the age of the fossil

Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

Dating with carbon-14Dating with carbon-14 (radiocarbon dating)

– Carbon is normally found in three forms: stable C-12, stable C-14 & radioactive C-14

– All combine with oxygen to form CO2

– Half-life of only 5730 years– Used to date very recent events– Carbon-14 is produced in the upper atmosphere

– Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

Carbon-14Carbon-14• Carbon-14 is continuously created in the atmosphere by cosmic radiation.

• There is one atom of radioactive C-14 for every trillion atoms of C-12 in the atmosphere

• Plants absorb C-14 directly through their leaves in the form of carbon dioxide

• Animals take in C-14 indirectly when they eat plants

• Although C-14 disintegrates at a constant rate, it is continuously renewed as long as an organism remains alive.

• When an organism dies, it stops absorbing new C-14 and its radiocarbon “clock” is set.

Types of radiometric dating• Potassium-argon method

– K-40 half-life 1.3 billion years– Decays into argon and calcium– Used to date rocks older than 100,00 years old

• Uranium-lead method– U-238 half-life 4.5 billion years– Decays into lead-206– Used for rocks more than 10 million years old

• Rubidium-strontium method– Rb-87 half-life 49 billion years– Decays into strontium-87– Used for rocks more than 10 million years old

• Carbon-14 method

• 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

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

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?

• 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

• 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

• 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

• 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