Earth’s HistoryEarth’s History

Earth’s HistoryEarth’s History

Earth’s HistoryEarth’s History Earth probably formed from an Earth probably formed from an

accumulation of rock, dust, and gases accumulation of rock, dust, and gases drawn together by its own gravity drawn together by its own gravity about 4.6 billion years agoabout 4.6 billion years ago

The rocks of Earth’s crust preserves The rocks of Earth’s crust preserves clues that help us unravel the clues that help us unravel the mystery of our changing mystery of our changing planet, its environments, planet, its environments, and the development and the development of terrestrial lifeof terrestrial life

UniformitarianismUniformitarianism

Geologists think that the forces that Geologists think that the forces that they observe today are similar to they observe today are similar to processes that occurred throughout processes that occurred throughout Earth’s historyEarth’s history

““The present is the key to the past”The present is the key to the past”

Law of SuperpositionLaw of Superposition

The rocks at the bottom of an The rocks at the bottom of an undisturbed exposure are usually the undisturbed exposure are usually the oldestoldest– There are occasional exceptions to this There are occasional exceptions to this

lawlaw

Original Original HorizontalityHorizontality

A rock is always older A rock is always older than the process that than the process that changed itchanged it

Sediments are usually Sediments are usually deposited in layersdeposited in layers– When we see sedimentary When we see sedimentary

layers, we usually assume layers, we usually assume that these layers were that these layers were deposited level, and that deposited level, and that they were tilted after they they were tilted after they had turned into had turned into sedimentary rocksedimentary rock

Igneous ExtrusionsIgneous Extrusions

An An ExtrusionExtrusion occurs when molten rock occurs when molten rock flows onto Earth’s surface, where it flows onto Earth’s surface, where it crystallizes to form igneous rockcrystallizes to form igneous rock– An extrusion is younger than the rock below An extrusion is younger than the rock below

it, but older than the rock that will form on it, but older than the rock that will form on toptop

– The rock below the extrusion will show a The rock below the extrusion will show a zone of contact metamorphism zone of contact metamorphism where the hot lava baked itwhere the hot lava baked it

Igneous IntrusionsIgneous Intrusions

An An IntrusionIntrusion is an internal process is an internal process where magma squeezes into or where magma squeezes into or between layers of pre-existing rockbetween layers of pre-existing rock– The hot molten rock changes the The hot molten rock changes the

surrounding rock immediately surrounding rock immediately above and below above and below and next to it by and next to it by contact contact metamorphismmetamorphism

Folds and FaultsFolds and Faults

FoldsFolds are bends in rock layers produced are bends in rock layers produced by movements of Earth’s crust, generally by movements of Earth’s crust, generally related to Earth’s tectonic platesrelated to Earth’s tectonic plates

FaultsFaults are breaks in the rock where are breaks in the rock where movement has occurred often associated movement has occurred often associated with earthquakeswith earthquakes

Offset layers are indications of faultingOffset layers are indications of faulting– Folds and Folds and

Faults occur Faults occur after the rock after the rock has formedhas formed

FossilsFossils The preserved remains or traces of living The preserved remains or traces of living

thingsthings– Can reveal a great deal about past life forms and Can reveal a great deal about past life forms and

environmentsenvironments– Can also provide clues about the past geological Can also provide clues about the past geological

events or relative ages of rock layersevents or relative ages of rock layers Trace FossilsTrace Fossils do not contain the remains of do not contain the remains of

the organisms that produced themthe organisms that produced them– Include the impressions of shells, Include the impressions of shells,

dinosaur footprints, oddly shaped dinosaur footprints, oddly shaped formations from sediments filling formations from sediments filling in animal burrows and in animal burrows and petrified drippingspetrified drippings

– Trace fossils reveal much about an Trace fossils reveal much about an organisms behavior and relationship to organisms behavior and relationship to its living and nonliving environmentits living and nonliving environment

Correlation of RocksCorrelation of Rocks

Geologists try to Geologists try to match similar rock match similar rock strata in different strata in different locations to see if locations to see if they formed at the they formed at the same time or under same time or under similar conditionssimilar conditions– Color, texture, Color, texture,

compositioncomposition– Compare index fossils in the strataCompare index fossils in the strata

Correlation of Rocks Correlation of Rocks ExampleExample

Which layers are the same?Which layers are the same?

Of the rock layers E and F, Of the rock layers E and F, which is the oldest? which is the oldest?

What is the correct What is the correct sequence of rock layer from sequence of rock layer from oldest to youngest? oldest to youngest?

An unconformity (buried An unconformity (buried erosional surface) is erosional surface) is represented by the interface represented by the interface between which two layers? between which two layers?

What type of rock is layer A? What type of rock is layer A?

GeologicGeologicTime ScaleTime Scale

Based on rock Based on rock formations that formations that contain contain characteristic characteristic fossil groups and fossil groups and on changes in the on changes in the kinds of organisms kinds of organisms that inhabited that inhabited EarthEarth

The scale is The scale is divided into eras, divided into eras, periods, and periods, and epochsepochs

Evolution of LifeEvolution of Life Geologists believe that life forms Geologists believe that life forms

existed in the Precambrian Periodexisted in the Precambrian Period– They had no hard parts , therefore few They had no hard parts , therefore few

left fossilsleft fossils– Precambrian fossils are very rarePrecambrian fossils are very rare

More complex organisms More complex organisms developed as time went ondeveloped as time went on– Some disappeared (went extinct) Some disappeared (went extinct)

from the fossil record from the fossil record Within each species there Within each species there

are variations in size, shape are variations in size, shape and other traitsand other traits

Evolution of LifeEvolution of Life The The Evolution of Life Evolution of Life

(Charles Darwin) states (Charles Darwin) states that individuals that have that individuals that have traits that better adapt traits that better adapt them to their environment them to their environment will survive longer and will survive longer and have more offspring to have more offspring to pass on these desirable pass on these desirable traitstraits

This process of This process of Evolution Evolution of Lifeof Life, leads to the , leads to the extinction of some species extinction of some species and formation of new and formation of new onesones

Evolution of LifeEvolution of Life PaleontologistsPaleontologists (geologists (geologists

who study fossils) have found who study fossils) have found remains of a large variety of remains of a large variety of plants and animals that lived plants and animals that lived in many different in many different environmentsenvironments– Some still exist, but most have Some still exist, but most have

become extinctbecome extinct Most organisms decompose Most organisms decompose

or are consumed by other or are consumed by other organisms after they die, only organisms after they die, only a very small percentage a very small percentage leave any fossil remainsleave any fossil remains– Because of this, many forms of Because of this, many forms of

life will never be knownlife will never be known

Life and The AtmosphereLife and The Atmosphere

Microscopic Microscopic organisms organisms that that developed developed about 2.2 about 2.2 billion years billion years ago changed ago changed the mixture the mixture of gasses in of gasses in our our atmosphereatmosphere

Life and The AtmosphereLife and The Atmosphere

About 3.8 billion years ago, the About 3.8 billion years ago, the atmosphere probably consisted of a atmosphere probably consisted of a mixture of carbon monoxide, carbon mixture of carbon monoxide, carbon dioxide, hydrogen, dioxide, hydrogen, nitrogen, ammonia, nitrogen, ammonia, and methaneand methane

The atmosphere todayThe atmosphere todayis 78% nitrogen is 78% nitrogen and 21% oxygenand 21% oxygen

Past Geologic EventsPast Geologic Events No single location shows a complete No single location shows a complete

record of the geologic pastrecord of the geologic past If an area was above sea level for a If an area was above sea level for a

while, it is likely that sediments were not while, it is likely that sediments were not deposited and older rocks have been deposited and older rocks have been destroyed by erosiondestroyed by erosion

When a new rock layer is layered on a When a new rock layer is layered on a surface left by erosion, it forms a buried surface left by erosion, it forms a buried erosion surface (erosion surface (unconformityunconformity))

Radioactive DatingRadioactive Dating Measurements of natural radioactivity in Measurements of natural radioactivity in

the rocks have allowed the geologic time the rocks have allowed the geologic time scale to become an absolute time scalescale to become an absolute time scale– One that gives the One that gives the

absolute ageabsolute age (numerical age) of an (numerical age) of an object (measured object (measured in years)in years)

Chemical elements Chemical elements often have several often have several forms (forms (isotopesisotopes) that ) that differ in the number of differ in the number of neutrons in their neutrons in their atomic nucleiatomic nuclei

Radioactive DatingRadioactive Dating– If the nucleus of an isotope has more or If the nucleus of an isotope has more or

fewer than the normal number of fewer than the normal number of neutrons, the isotope may be radioactiveneutrons, the isotope may be radioactive

– A radioactive isotope will break down A radioactive isotope will break down naturally into a lighter element called a naturally into a lighter element called a decay productdecay product

– In the process, it gives off radioactivityIn the process, it gives off radioactivity– A sample of a radioactive element A sample of a radioactive element

contains millions of atoms, from which we contains millions of atoms, from which we can predict a rate of decaycan predict a rate of decay

Half-LifeHalf-Life The rate of decay of a radioactive The rate of decay of a radioactive

element is measured by its half-lifeelement is measured by its half-life– Different radioactive elements have Different radioactive elements have

different half-livesdifferent half-lives A half-life is the time required for half A half-life is the time required for half

of an element’s atom in a of an element’s atom in a sample to change to sample to change to the decay productthe decay product– At the end of one half-life, At the end of one half-life,

a sample contains equal a sample contains equal amounts of the radioactive amounts of the radioactive element and its element and its decay productdecay product

Half-LifeHalf-Life In each succeeding half-life, half of the In each succeeding half-life, half of the

remaining atoms decayremaining atoms decay As the element decays, As the element decays,

fewer radioactive atoms fewer radioactive atoms remain in the sample, remain in the sample, and more decay and more decay product accumulatesproduct accumulates– The higher the ratio of The higher the ratio of

decay product to decay product to radioactive element, radioactive element, the older the samplethe older the sample

Decay-Product RatioDecay-Product Ratio

The ratio between the mass of a The ratio between the mass of a radioactive element and its decay radioactive element and its decay product in a sampleproduct in a sample

After we determine this ratio, we can After we determine this ratio, we can calculate how many calculate how many half-lives have half-lives have gone by since gone by since the sample was the sample was formed and then formed and then determine its agedetermine its age

Selecting the Best Radioactive Selecting the Best Radioactive Element for Dating a SampleElement for Dating a Sample

The sample to be dated must contain a The sample to be dated must contain a measurable quantity of a radioactive element measurable quantity of a radioactive element and its decay productand its decay product

A sample containing the remains of living A sample containing the remains of living organisms is likely to contain radioactive organisms is likely to contain radioactive carbon-14carbon-14

The sample’s age must also be consideredThe sample’s age must also be considered– Carbon-14 can only Carbon-14 can only

date samples no older date samples no older than about 50,000 yearsthan about 50,000 years

– Uranium-238 can measure Uranium-238 can measure samples of the oldest samples of the oldest rocks on our planetrocks on our planet