3.4EARTH’S GEOLOGIC

HISTORY

Earth’s StoryJames Hutton

Considered to be the “Father of Modern Geology”.

Observed geologic processes taking place on his farm and at Siccar Point in Scotland.

Formed the idea of uniformitarianism stating current geologic processes , such as volcanism and erosion, have been at work throughout Earth’s history.

Hutton

Siccar Point

Siccar Point

Relative Age Layers of rock (strata) can show a sequence of events that took

place in the past.

Relative Age is the age of on object, such as a rock, in relation to the ages of other objects around it.

To determine the relative ages of rocks, geologists often study the layers in sedimentary rocks.

Law of SuperpositionSedimentary rocks form when

new sediments are deposited on top of old layers of sediment.

◦ They are compressed into layers known as beds, separated by boundaries called bedding planes.

The principle stating an un-deformed sedimentary rock layer is older than the layers above it and younger than the layers below it is known as the Law of Superposition.

Law of Superposition

Principle of Original HorizontalityGeologists also know

sedimentary rock generally forms in horizontal layers.

The Principle of Original Horizontality states sedimentary rocks left undisturbed will remain in horizontal layers.

◦ Proposed by Danish geological pioneer, Nicholas Steno.

◦ Some tectonic activity had to occur if rock layers are NOT horizontal.

Missing Evidence Sometimes, layers of rock can be missing

altogether, creating a gap in the geologic record (newspaper example).

An 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.

◦ Unconformities can also represent missing time – time that was not recorded in layers of rock.

3 types of unconformities:◦ Disconformity◦ Nonconformity◦ Angular Unconformity

DisconformityA disconformity is a boundary between

horizontal layers of old sedimentary rock and overlying younger layers deposited on an eroded surface.

NonconformityA nonconformity is a boundary where

stratified (layered) rock rests upon un-stratified igneous or metamorphic rock.

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Angular UnconformityAn angular unconformity is a boundary

between a set of tilted rock layers and a set of horizontal rock layers.

Cross-Cutting RelationshipsWhen rock layers have been disturbed by faults or

intrusions, determining relative age may be difficult.

◦ A fault is a break or crack in Earth’s crust along which rocks shift their position.

◦ An intrusion is a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies.

The Law of Cross-Cutting Relationships states a fault or igneous intrusion is always younger than the rock layers it cuts through.

Cross-Cutting Relationships

• The Law of Cross-Cutting Relationships tells us the igneous intrusion is younger than layers A, B, and C. • We can also determine the fault is younger than all four layers of sediment and the igneous intrusion.

Absolute AgeTo learn more about Earth’s history, geologists

often need to determine the numeric age, or absolute age, of a rock formation.

Rates of ErosionOne way to estimate absolute age is to measure

the rate at which a stream or river erodes its bed.◦ Only practical for geologic features that formed

within the past 10,000-20,000 years.

Niagra Falls

Rates of ErosionThe rocky ledge forming Niagra Falls has been

eroding at a rate of about 1.3 m per year for about 9,900 years.

Canadian Horseshoe Falls of Niagra Falls

Rates of DepositionAnother way to estimate

absolute age is to calculate the rate of sediment deposition.

◦ In general, about 30 cm. of sedimentary rock are deposited over a period of 1000 years.

Obviously, sediment deposition rates can fluctuate, therefore this method is truly an estimation and not always accurate.

Varve CountSedimentary layers showing a light-colored

band of coarse particles and a dark-colored band of fine particles are called varves.◦Common in glacial lake beds.

Summer snow/ice melt rapidly and carry large amounts of sediment into a lake (coarse, light-colored sand and silt particles settle to the bottom).

Winter surface of the lake freezes and fine, dark-colored clay particles, still suspended in the water, settle on top of the coarse sediments.

◦Each varve represents one year of deposition.

Radiometric DatingRocks generally contain

small amounts of radioactive material.◦ Radioactive isotopes have

nuclei that emit particles and energy at a constant rate.

The method of using radioactive decay to measure absolute age is called radiometric dating.

Radiometric DatingAs an atom emits particles and energy, the atom

changes into a different isotope of the same element or an isotope of a different element.

Scientists measure the concentrations of the original radioactive isotope, or parent isotope, and the newly formed stable isotope, or daughter isotope.

The time required for half of the sample of a radioactive parent isotope to decay to form a stable daughter isotope is called a half-life.

Radioactive Decay and Half-Life

Radioactive Decay and Half-Life

Radiometric DatingExample:

◦A rock sample contains a radioactive isotope with a half life of 10,000 years. Therefore every 10,000 years, half the radioactive

parent isotope will have decayed into a stable daughter isotope.

◦1/8 of your sample is parent material and 7/8 is daughter material … how old is your rock?

30,000 years (3 half-lives)

Radiometric Dating MethodsMethod Parent

IsotopeDaughter Isotope

Half-Life Effective Range

Radiocarbon Dating

14C 14N 5730 years < 70,000 years

39Argon – 40Argon 39Ar 40Ar 1.25 billion years 10,000 to 4.6 billion years

40Potassium – 40Argon

40K 40Ar 1.25 billion years 50,000 to 4.6 billion years

87Rubidium – 87Strontium

87Rb 87Sr 48.1 billion years 10 million to 4.6 billion years

235Uranium – 207Lead

235U 207Pb 704 million years 10 million to 4.6 billion years

238Uranium – 206Lead

238U 206Pb 4.5 billion years 10 million to 4.6 billion years

232Thorium – 208Lead

232Th 208Pb 14 billion years > 200 million years

The Fossil Record A fossil is the remains or physical

evidence of an organism preserved by geologic processes.

The scientific study of these fossils is known as paleontology.

Scientists studying these fossils are called paleontologists.

Some specialize in particular organisms:

Invertebrate Paleontologists – study fossils of animals without backbones

Vertebrate Paleontologists – study fossils of animals with backbones

Paleobotanists – study plant fossils

Fossil Formation – MummificationMummified remains often found in dry places because

bacteria, which cause decay, cannot survive in this environment.◦ Some ancient civilizations mummified their dead by extracting

the internal organs then wrapping the body in strips of cloth.

Fossil Formation – AmberHardened tree sap is called amber.

Insects become trapped in the sticky sap and are preserved when it hardens.

Fossil Formation – Tar SeepsWhen thick petroleum oozes to the Earth’s surface, it forms

a tar seep.◦ Commonly covered by water.

◦ Animals came to drink the water and became trapped in the sticky tar.

La Brea Tar PitsLos Angeles, CA

Fossil Formation – FreezingThe low temperature of frozen soil and ice can

protect and preserve organisms.◦ Since most bacteria cannot survive freezing

temperatures, organisms buried in frozen soil or ice do not decay.

Fossil Formation – PetrificationMinerals that precipitate from groundwater

solutions replace original organic materials buried under layers of sediment.◦ Common petrifying minerals are silica, calcite, and pyrite.

Petrified Wood

Petrified Fossil

Types of Fossils – Carbon FilmsCarbonized residue of leaves, stems, flowers, and

fish have been found preserved in sedimentary rock made of soft mud or clay.◦ When original organic material partially decays, it leaves

behind a carbon-rich film displaying the surface features of the organism.

Types of Fossils – Molds and CastsShells often leave empty spaces called molds within

hardened sediment. ◦ When a shell is buried, its remains eventually dissolve and

leave an empty space.

◦ When sand or mud fills a mold and turns into a rock, a natural cast forms – a replica of the original organism.

Mold Cast

Types of Fossils – CoprolitesFossilized dung or waste materials from ancient

animals are called coprolites.

◦ Observable through a microscope.

◦ The materials identified throughout reveal the feeding habits of ancient animals, such as dinosaurs.

Types of Fossils - GastrolithsSome dinosaurs had stones in their

digestive systems to help grind their food.◦ In many cases, these stones which are called

gastroliths, survive as fossils.

http://upload.wikimedia.org/wikipedia/commons/e/ef/Psittacosaurus_stomach_stones.jpg

Trace FossilsFossilized evidence of

past animal movement such as tracks, footprints, and burrows are called trace fossils.

◦ Provide information about prehistoric life.

Index FossilsFossils that occur only in rock layers of a

particular geologic age are called index fossils.

To be considered an index fossil, it must meet certain requirements:

1. Must be widespread geographically.

2. Must have distinguishing features from other fossils.

3. The organisms from which the fossil formed must have lived during a short span of geologic time.

4. Must occur abundantly within the rock layer.

Index Fossils Scientists can use index fossils to estimate the absolute age of

specific rock layers.

◦ For example, the AMMONITE fossils pictured below show the rock in which the fossils were observed formed between 180 and 206 million years ago thus giving the approximate age of the ammonites.

◦ Helps locate rock layers that are likely to contain resources such as oil and natural gas deposits.

Geologic Time By studying fossils and

applying various principles, scientists determined the relatives ages of rock in different areas throughout the world.

◦ No single area on Earth contained a record of all geologic time, so they combined their observations.

◦ This ordered arrangement of rock layers based on the relative ages of the rocks in which the oldest rocks are at the bottom is known as the geologic column.

Grand Canyon, Arizona

Zion National Park, Utah

Geologic TimeThroughout Earth’s history, there have been

several changes in Earth’s surface, climates, and organism types.

Geologists use certain indicators to divide the geologic time scale into smaller units.◦Generally characterized by fossils of a dominant

life-form.

◦Because Earth’s history is so long, geologists commonly use abbreviations such as Ma, standing for mega-annum, which means “million years.”

Divisions of TimeThe largest unit of geologic time is called an eon.

There are 4 eons:

◦ Hadean – the only rocks found are meteorites and rocks from the moon.

◦ Archean – earliest known rocks on Earth formed during this eon.

◦ Proterozoic – first organisms with well-developed cells appeared during this eon.

◦ Phanerozoic – most recent eon in which we live.

The first 3 eons are part of a time interval known as Precambrian time 4 billion-year interval with very few fossils.

Divisions of TimeAfter Precambrian time, the Phanerozoic

eon began.

Eons are further divided into smaller units called eras, which are the second largest divisions of geologic time.◦Phanerozoic Eon:

Paleozoic – oldest era of the Phanerozoic eon. Mesozoic – dinosaur appearance and extinction. Cenozoic – most recent era in which we live.

Divisions of TimeEras are divided into shorter time units called periods,

which are the third largest divisions of geologic time.

Characterized by specific fossils and are usually named for the location in which the fossils were first discovered.

Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian &

Pennsylvanian), Permian

Mesozoic Era: Triassic, Jurassic, Cretaceous

Cenozoic Era: Tertiary, Quaternary

Divisions of TimeThe most complete portion of the rock

record where periods are divided into shorter time units called epochs, the fourth largest divisions of geologic time.◦Cenozoic Era:

Tertiary Period: Paleocene, Eocene, Oligocene, Miocene, Pliocene

Quaternary Period: Pleistocene, Holocene (most recent)

Cretaceous Mass ExtinctionDinosaurs are believed to

have gone extinct around 65 MYA when an asteroid hit off the coast of the Yucatan peninsula.

◦ Scientists have discovered levels of Iridium (Ir), which is very rare on Earth’s surface but far more common on asteroids. Found to be 30 times greater

than average in the Cretaceous-Tertiary (KT) boundary of sedimentary rock.