3.4 Earths Geologic History through the Rock Record

OVERVIEW OF THE EARTH

Unique Characteristics of the Earth

Its atmosphere contains free oxygen.It has a large amount of water.It is denser than any other planet.Presence of life.

EARTH FACTS:

Age: 4.6 billion years old.Position: 3rd planet from the sun.Size: 5th largest planet in our solar system.Surface Area: 197 million square miles, about 70% of the Earth's surface is covered with water..

EARTH FACTS:

Average Diameter: 12,742 kilometers. (7,926 miles)Average Circumference: 40,000 Kilometers

EARTH FACTS:

Average Distance from the Earth to the Sun: 149,669,180 km (93,020,000 miles)

EARTH FACTS:

Average Distance from the Earth to the Moon: 384,403.1 kmLength of Year: 365.25 daysInclination of Axis: The Earth's axis has a tilt of about 23 degrees. It is this tilt which causes the seasons.

EARTH FACTS:

Chemical Composition: The Earth is made mostly of iron, oxygen, silicon, magnesium, nickel and sulfur: 34.6% Iron, 29.5% Oxygen, 15.2% Silicon, 12.7% Magnesium, 2.4% Nickel, 1.9% Sulfur, 0.05% Titanium

Belief of ancient people about the shape of the Earth:

The early Aztecs and Mayans believed that the Earth was shaped like a cross.The corners of the cross represented the four directions on Earth.

Belief of ancient people about the shape of the Earth:

The ancient Hindus perceived the Earth as a flat coin with a layer of air on top and nirvana or heaven beyond it.Ancient people also believed that the earth was flat that when reached the end, they would fall off and be eaten by monsters or evil spirits.

Belief of ancient people about the shape of the Earth:

Aristarchus a Greek philosopher, sometime in 200 BC, was the first to have the idea that the earth is round.The travel of Magellan and Columbus during eclipses showed shadows of the Earth on the moon which serves as proof of the earth being round.

Belief of ancient people about the shape of the Earth:

They believed that the earth is flat because they saw a flat horizon.Horizon is the line seen at a distance where the land or sea seems to meet the sky. It marks the greatest distance an observer sees on Earths surface.

The earth consists of several layers and is surrounded by the atmosphere. The three main layers are the:Crust is the outer part. Until about 70 Km below from earths surfaceMantle is the middles layerCore is the inner part of the earth

Belief of ancient people about the shape of the Earth:

Belief of ancient people about the shape of the Earth:

Till this moment it hasn't been possible to take a look inside the earth because the current technology doesn't allow it. Therefore all kinds of research had to be done to find out, out of which material the earth consists, what different layers there are and which influence those have (had) on the earth's surface.

Earths interior layers.The major layers of the Earth consist of the crust, mantle, outer core and inner core.

CRUSTOutermost layerComposition varies from the continents of the ocean.Covers mostly granite like rocksOcean crust is mostly basalt rock covered with layers of sediments

THE CRUSTAL SURFACECONTINENTAL CRUST is between 20 and 60 km thick. It is composed of granitic rocks, which are less dense than basaltic rocks of the oceanic crust. So, most of continental crust is above sea level.

THE CRUSTAL SURFACEOCEANIC CRUST - is only about 10 km thick. It is composed of basaltic rocks, which are more dense than granitic rocks of the continental crust. So, oceanic crust is below sea level.

THE MANTLEThe mantle is composed of hot iron-rich silicate rocks.Flow in the mantle occurs as convection currents; hot material in the mantle rises, cools and then sinks.Mantle is elastic which means it behave in plastic manner.

OUTER COREBecause less weight is exerted on the outer core, the pressure is less there, so iron and nickel present here in liquid state.The molten outer core flows at the very slow rate which means electrons from the metals produce an electrical current. This electrical current powers the earths magnetic field.

INNER COREThe core is composed mainly of iron and nickel.Although the inner core is very hot, pressure from the weight of the rest of the Earth doesnt allowed the material to melt. Irons normal temperature of melting is 15350C, but in the earth inner core it could stand 40000C with no melting.

Composition of the Earth

THEORIES ABOUT THE EARTHPermanent magnet TheoryThat the center of the earth is one huge magnet of Fe and Ni.

THEORIES ABOUT THE EARTHNegative Experiment (Blackett)From an English physicist. That the magnetism of the sun, stars and earth is caused by the rotation.He developed the magnetometer, a sensitive instrument that measure magnetism.

THEORIES ABOUT THE EARTHDynamo theory ( Walter Elsasser) and Edward Bullard)That the earth is an electromagnet rather than a permanent magnet.The movement of the liquid core produces an electric current which in turn produces magnetism.

EARTHS GEOLOGIC HISTORYEarths Story

Catastrophismis thetheory(now known to be factual from the fossil record) that massive catastrophes occurred in earth's past, substantially altering theEarthand its life via mountain uplift, rapid deposition, and mass extinctions.Earths StoryJames Hutton

Scottish physician and farmer who published Theory of the Earth Father of Modern GeologyObserved geologic processes taking place on his farm and at Siccar Point in Scotland.Formed the idea of uniformitarianism

HuttonStates that the physical chemical and biological laws that operate today have also operated in the geological past.The present is the key to the past.Two Type of DatesLaw of SuperpositionPrinciple of original HorizontalityRates of ErosionPrinciple of Cross -cutting RelationshipsInclusions and UniformitiesRates of DepositionVarve CountRadiometric DatingRELATIVE DateABSOLUTE AgeFossil RecordGeologic Time ScaleRELATIVE AGE

Relative AgeLayers 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.

Scientists were seeking for numerical date.Relative dating - means placing sequence of formation

Law of SuperpositionNicolaus Steno, Danish anatomist, geologist, and priest (1636-1686) applied the first law.

Sedimentary 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.

Principle of Cross-Cutting RelationshipWhen rock layers have been disturbed by faults or intrusions, determining relative age may be difficult.

A fault is a break or crack in Earths 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 states that 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.

Missing EvidenceSometimes, 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.

Missing Evidence3 types of unconformities:DisconformityNonconformityAngular 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.

ABSOLUTE AGE

Absolute AgeTo learn more about Earths history, geologists often need to determine the numeric age, or absolute age, of a rock formation.

Absolute AgeA significant problem in assigning numerical dates to units of time is thatnot all rocks can be dated radiometrically. A sedimentary rock may contain particles of many ages that have been weathered from different rocks that formed at various times. One way geologists assign numerical dates to sedimentary rocks is to relate them to datable igneous masses, such as dikes and volcanic ash beds.

Rates of ErosionOne way to estimate absolute age is to measure the rate at which a stream or river erodes its bed.

Niagra FallsRelative dates can be established usingOnly practical for geologic features that formed within the past 10,000-20,000 years.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 FallsRates 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 MethodsMethodParent IsotopeDaughter IsotopeHalf-LifeEffective RangeRadiocarbon Dating14C14N5730 years< 70,000 years39Argon 40Argon39Ar40Ar1.25 billion years10,000 to 4.6 billion years40Potassium 40Argon40K40Ar1.25 billion years50,000 to 4.6 billion years87Rubidium 87Strontium87Rb87Sr48.1 billion years10 million to 4.6 billion years235Uranium 207Lead235U207Pb704 million years10 million to 4.6 billion years238Uranium 206Lead238U206Pb4.5 billion years10 million to 4.6 billion years232Thorium 208Lead232Th208Pb14 billion years> 200 million yearsThe Fossil RecordA 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 Earths 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, CAFossil 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 FossilTypes 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.

MoldCast

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.

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:

Must be widespread geographically.

Must have distinguishing features from other fossils.

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

Must occur abundantly within the rock layer.Index FossilsScientists 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.

BE READY FOR A QUIZ !END OF CHAPTER