Earth’s History

Earth’s History Planet Earth is approximately 4.5 X 109 years old Rocks of the crust provide clues to Earth’s past By analyzing these clues we can infer events from the past

Earth’s History Principle of Uniformitarianism Major assumption in geology Events in the past occurred the same way that they are occurring today. Examples Include: Weathering/erosion Deposition Volcanism Plate tectonics

Geologic Time Plate motions and mountain building events can be placed on the geologic time scale

Geologic Time Geologists have divided Earth’s history into time units based on the the fossil record

Geologic Time A study of the fossil record shows A great variety of plants, animals, and simpler life forms have lived on Earth in the past That life forms have evolved through time Most life forms of the geologic past have become extinct

Geologic Dating

There are two types of Geologic Dating Absolute Determines how many years old something is Relative Used to determine if one thing is younger or older than another

Tools used to determine Absolute Age Relative Age Using radiometric dating Law of superposition Use of index fossils Correlation of rock layers

Relative Dating

Law of Superposition Sediments are laid down underwater in horizontal layers and form sedimentary rocks

Law of Superposition In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest Lower layers must be in place before younger rocks can be deposited on top of them Exception: when something occurs to overturn layers

Grand Canyon

GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM

Law of Superposition Rock layers are older than folds found in them Layers were there before they were folded

Folds/Tilted

Law of Superposition Rock layers are older than faults found in them This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

Law of Superposition Fossils are generally the same age as the rock layers in which they are found Animal remains are deposited along with the sediments that will turn into sedimentary rocks

Law of Superposition Igneous intrusions are younger than the rock that they cut through or flow out of

Igneous Intrusion - Cross Cutting

Unconformity When a new rock layer is formed atop an eroded surface

Taconic Unconformity Upper Silurian Carbonates February 19 Upper Silurian Carbonates Tilted Ordovician Shales and Sandstones We stopped along the side of the road where engineers had cut into the rock formations to make the road. The unconformity was clearly visible showing banded carbonate rock overlying tilted shales and sandstones. unconformity Taconic Unconformity Allen

Practice: what happened here? February 19 Practice: what happened here? Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the next slide. Allen

Correlation Matching similar rock layers in different locations to see if they formed at the same time

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1. Which layers are the same? 2. Which layer is older E or F? 3. What is correct sequence of rock layers from oldest to youngest? 4. An unconformity is represented by the interface between which 2 layers?

Which fossil might be found in Devonian rock layers?

Volcanic Ash Falls Can also be used to correlate rock layers over a large area Ash is a good indicator because: The ash from one explosion has distinct characteristics ash can be deposited around the globe The event occurs at one, geologically brief, time

Fossils

February 19 Fossils Naturally occurring preserved remains or impressions of living things Generally only hard parts get preserved Bones, teeth, shells Hard parts are replaced by naturally occurring minerals Allen

February 19 Fossils Can be a mold – a hollow area in sediment in the shape of an organism or part of an organism Ex: imprint of a shell Or a cast – a copy of the shape of an organism by minerals and sediment Ex: filled in shell Allen

Fossils (Cont) Preserved remains – organisms trapped in tar, amber, or ice remain unchanged Carbon film – a thin coating of carbon on rock from remains of an organism

Fossils (Cont) Petrified – minerals replace all or part of an organism Trace fossils – footprints, animal trails, or other clues left behind by animals that have been filled with sediment and turned to rock

Fossils Why don’t fossils exist in most igneous or metamorphic rock? February 19 Fossils Generally found in sedimentary rock layers Why don’t fossils exist in most igneous or metamorphic rock? Allen

February 19 Fossils The living organism that made the fossil lived during the time the rock layer was forming i.e. when the sediment was being deposited Allen

Fossils Fossils can provide information about ancient environments Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…

Fossils A study of the fossil record shows: February 19 Fossils A study of the fossil record shows: A great variety of plants, animals and simpler life forms have lived on Earth in the past That life forms have changed through time Most life forms of the geologic past have become extinct Allen

Index Fossils Index Fossils--used to date rock layers (strata) Fossils from creatures that existed for a geologically short period of time Ex. less than 2.0 x 107 Fossils from creatures that had a wide geographic distribution

Which letter would make a good index fossil? Location A Location B Location C Rock layer 1 W W Z Rock layer 2 W Z Y Z Rock layer 3 W X X X Z Which letter would make a good index fossil?

Absolute Dating

What is Absolute Dating? Determining the actual age, in numbers of years, of something (rock, object, event). How do you do it? Index Fossils Radioactive isotopes Atoms of the same element that have different # of neutrons

Radioactive Isotopes Radioactive isotopes are isotopes that are unstable and break down into other stable isotopes by a process called radioactive decay.

Radioactive Decay Alpha and Beta particles are emitted during decay The Beta particles are what is measured to determine the amount of decay

Radioactive Isotopes The radioactive isotope is called the parent isotope, and the stable isotope formed by its breakdown is called the daughter isotope.

Example Unstable Radioactive Isotope Stable Decay Product Carbon 14 (C14) Nitrogen 14 (N14) Uranium 238 (U238) Lead 206 (Pb206) Parent Daughter

Radiometric Dating Half life: The time required for 1/2 of a parent material to break down to daughter material

Another look

Radiometric Dating The age of objects can be determined by measuring the relative amounts of radioisotope (parent) and decay products (daughter)

Radiometric Dating The half lives of radioisotopes vary depending upon the isotope Examples: C14 = 5,770 years U238 = 4,500,000,000 years Rb87 = 47,000,000,000 years

Carbon 14 is used to date biological remains Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies Uranium 238 is used to date rocks Larger half life

Half Life Practice Problems

If half the K -40 of a rock has decayed into Ar-40, how old is the rock? How many years would it take for 75 % of a sample of K-40 to disintegrate?

A sample that has ½ c-14 and ½ n-14? How old are: A sample that has ½ c-14 and ½ n-14? A sample that has ¼ K-40 and ¾ Ar-40? A sample that has ½ U-238 ½ Pb-206?

How old are: A sample that has 1/8 Rb-87 and 7/8 Sr-87? A sample that has ¼ C-14 and ¾ N-14? A sample that has ¾ C-14 and ¼ N-14?

What fraction remains of: February 19 A 50 billion year old sample has ____ Rb-87 and ____ Sr-87. A 2.6 billion year old sample has____ K-40 and ____Ar-40. Allen

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