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• Video

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Geologic Time, Earth History and Fossils
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How Old is the Earth?

• This has been a subject of heated debate for as
  long as modern humans have been around
• Some of the early attempts of determining Earths
  age were quite clever
• The Ancient Greek Xenophanes (570 470 B.C.)
  realized that fossils were the ancient remains of
  life and that Earth was therefore extremely old.
  
• Another Greek, Herodotus, used sediments from the
  Nile River.
• The Nile floods every year, depositing an
  additional layer of sediment
• He dug into the river bank and counted the
  layers, determining that Earth was at least many
  thousands of years old

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How Old is the Earth?

• Many scientists tried to use the saltiness of the
  ocean to determine Earths age.
• They assumed that the oceans began as fresh
  water and by estimating how much salt was
  entering the ocean from rivers, they calculated
  that the Earth must be 90 million years old.
• Their calculations were wrong b/c salt
  precipitates out (comes out of solution and falls
  to the ocean floor) when the ocean becomes
  supersaturated with it.

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How Old is the Earth?

• It wasnt until scientists became proficient in
  their work with radioactivity that we were able
  to determine that the Earth is around (well talk
  more about this later)
• 4.6 billion years old
• 4,600,000,000 years

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How do we Express the Ages of things? Two Ways.

• Relative Dating - Know Order of Events But Not
  Dates
• Civil War Happened Before W.W.II
• Bedrock in Wisconsin Formed Before The Glaciers
  Came
• Absolute (Radiometric) Dating Gives you exact
  Dates
• Civil War 1861-1865
• Glaciers Left Wisconsin About 11,000 Years Ago
• The meteorite is 4.5 billion years old

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Relative Dating

• In order to determine the relative ages of rocks,
  fossils or geologic events, a few principles must
  be understood and accepted.
• Uniformitarianism Assumes that the same natural
  laws and processes that operate in the universe
  now, have always operated in the universe in the
  past and apply everywhere in the universe. the
  present is key to the past.

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Ripple Marks, Bay Beach
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Fossil Ripple Marks, Baraboo Range
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Modern Mud Cracks
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Fossil Mud Cracks, Virginia
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Relative Dating

• Principle of Superposition- In undisturbed layers
  of rock the oldest rocks are at bottom, youngest
  on top.

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Relative Dating

• Principle of Original Horizontality Sediments
  are generally deposited as horizontal layers
  even where sediments are draped over irregular
  surface they tend toward the horizontal.
• Principle of Lateral Continuity Sediment Layers
  extend horizontally in all directions until they
  thin and pinch out.

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Relative Dating

• Principle of Cross Cutting Relationships - That
  which cuts through is younger than the object
  that is cut.

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Principle of Original Continuity
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Relative Dating

• Principle of Inclusions - Inclusions (one rock
  type contained in another rock type) are older
  than the rock they are embedded in. That is, the
  younger rock contains the inclusions.

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• Deciphering a complex rock sequence. The
  limestones must be oldest (law of superposition),
  followed by the shales. The granite and basalt
  must both be younger than the limestone they
  crosscut (note the metamorphosed zone around the
  granite). It is not possible to tell whether the
  igneous rocks predate or postdate the shales or
  to determine whether the sedimentary rocks were
  tilted before or after the igneous rocks were
  emplaced. After the limestones and shales were
  tilted, they were eroded, and then the sandstones
  were deposited on top. Finally, the lava flow
  covered the entire sequence.

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Fossils

• Remains of Ancient Plants And Animals, Evidence
  of Life

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Why do humans study fossils?

• To learn about the Earths past
• Provide clues to past geologic time events,
  climates and evolution

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• Early Ideas about Fossils
• Herodotus
• realized that fossil shells found far from oceans
  were remnants of an ancient sea.
• Aristotle
• Believed in Spontaneous Generation
• Thought that fossils grew in place in rocks.
• Leonardo da Vinci
• Argued that fossils were remnants of living
  organisms from the Earths past history.
• William Strata Smith
• Discovered a relationship among the strata from
  different areas.
• Found that each strata has a distinct group of
  fossils associated with it and that these fossils
  were different from the fossils found in other
  strata.

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Where Fossils Occur

• Almost Exclusively in Sedimentary Rocks
• Heat of Melting or Metamorphism Would Destroy
  Almost Every Type of Fossil
• Rare Exceptions
• Some Fossils in Low-grade Metamorphic Rocks
• Trees Buried by Lava Flow
• To Be Preserved, Organisms Have to Be
• Buried Rapidly After Death
• Preserved From Decay
  Video

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Commonly Preserved

• Hard Parts of Organisms
• Bones
• Shells
• Hard Parts of Insects
• Woody Material

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Rarely Preserved

• Soft or Easily Decayed Parts of Organisms
• Internal Organs
• Skin
• Hair
• Feathers

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Types of Fossils
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1. Preserved Original Material

• Freezing
• Organisms buried in frozen soil or ice
• Dont readily decay ? bacteria cannot survive in
  freezing temperatures
• Ex Wooly mammoth in Siberia Alaska Video

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1. Original Material

• Amber
• Many insects preserved this way
• Insect trapped in sticky sap covered as sap
  runs preserved when sap hardens
• Excellent form of preservation even DNA
  recovered
• Video

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2. Cast or Mold

• Mold Organism is buried under sediment, the
  sediment hardens, the organisms remains decay or
  dissolve leaving empty space with imprint outside
  features of hard parts of an organism
• Cast minerals fill open space inside of shell,
  bone, etc. and makes replica of organism

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• Origin of molds and casts.
• (A) Formation of a mold.
• (B) Formation of a cast.

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Making of Cast and Mold Fossil Animation

• http//www.classzone.com/books/earth_science/terc/
  content/visualizations/es2901/es2901page01.cfm?cha
  pter_novisualization

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3. Mummification/Drying

• Often found in desert caves or buried beneath
  desert sand
• Most bacteria cannot survive without water no
  water, organism does not decay

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4. Replacement/Petrification

• Petra comes from Latin word meaning rock or
  stone
• Mineral-containing water interacts with organic
  material, replacing it with minerals
• Nearly perfect mineral replica of original
  organism
• Common petrifying mineralssilica, calcite,
  pyrite

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Petrified Forest National Park in Arizona
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5. Carbonization

• Plant leaves, and some soft body parts of fish,
  reptiles, and marine invertebrates decompose
  leaving behind only the carbon.
• This carbon creates an impression in the rock
  outlining the fossil, sometimes with great detail.

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Carbonization

• The fossil record of the hard parts is
  beautifully preserved, along with a carbon film,
  showing a detailed outline of the fish and some
  of its internal structure.

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6. Trace Fossils

• Indirect evidence of life in the rock record.
• Our only preserved record of behavior of fossil
  organisms
• Includes burrows, footprints, trails, coprolites,
  gastroliths
• Coprolites Fossilized dung or waste materials
  from ancient organisms
• Gastroliths some dinosaurs had stones in
  digestive system to help grind food survived as
  fossils
• Video

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Fossil burrows
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• This dinosaur footprint (trace fossil) is in
  shale near Tuba City, Arizona. It tells you
  something about the relative age of the shale,
  since it must have been soft mud when the
  dinosaur stepped here.

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Rubbing Rock? Wisconsin
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Pseudofossils

• Look Like Fossils But Aren't
• Pseudo- means fake

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Pseudofossils
Concretions
Dendrites
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Correlation

• Fossils help us correlate (match up), rock layers
  of similar age in locations that may be very far
  apart (even on different continents).
• Fossils helped us realize that all of the
  continents must have been connected at one time
  (Pangaea) b/c the same fossils were found in
  rocks of similar age on the edges of continents
  now very far apart.

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• Similarity of fossils suggests similarity of
  ages, even in different rocks widely separated in
  space.

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Index Fossils

• To most efficiently correlate rock layers,
  geologists use index fossils.
• 3 Criteria for a fossil to be an Index Fossil
• Must be scattered in wide area over Earths
  surface (global preferred - so can correlate lots
  of rocks over large areas)
• Must have features that clearly distinguish from
  all other fossils
• Must have lived over short span of geologic time
  (or it will be hard to figure out age of rock
  layer)

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Correlation
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Fossils are clues to the past!!

• Changes in climate and environment that occurred
  in the past

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Radiometric Dating
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How old?

• At the beginning of this unit we said that there
  were two different ways to date rocks
• 1. Radiometric (absolute) Dating which tells the
  absolute age or age in years of a rock.
  Radiometric Dating uses the properties of
  unstable atoms in rocks to determine their ages
• 2. Relative Dating determines the age of rocks
  and order of events by studying the positions of
  rocks in layers

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How does radiometric dating work?

• We learned that each type of atom or element is
  characterized by the number of protons it has.
• We also learned that elements can have isotopes,
  or atoms of the same element (same number of
  protons), that have different numbers of neutrons
  in the nucleus
• Some of these isotopes are unstable or
  radioactive and want to undergo a process called
  radioactive decay, where a neutron breaks down
  into a proton and an electron (adds a proton,
  therefore changes which atom it is)

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Radiometric Dating

• Radioactive isotopes are used to date fossils.
• If an isotope is radioactive, it will break down
  (decay) naturally into an element called a
  daughter product. The original radioactive
  isotope is called the parent material

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• The isotopes decay (break down) at a constant
  known rate (we witness this) and can be used to
  determine how old an object is
• The time it takes for half of a sample of a
  radioactive (parent) element to decay into a
  stable daughter element is called the half-life.
• In each half-life only half of the remaining
  radioactive atoms decay, no matter how large the
  sample is.

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• Look at the diagram below which represents the
  radioactive decay of uranium-238. The shaded
  area represents the decay product which is
  lead-206. The half-life of uranium-238 is 4.5
  billion years, since this object has gone through
  two half-lives it is
• 9 billion years old.

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How do we pick which radioactive material we use
to date a rock or Fossil?

• A scientist must decide which parent and daughter
  elements to measure when dating a rock or fossil.
  If the object to be dated is very old, then the
  isotope with a long half-live must be used.

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• If the rock or fossil is not extremely old,
  Carbon Dating can be used.
• Organisms take in carbon from the environment to
  build tissues in their bodies. After and
  organism dies, the carbon-14 slowly decays into
  nitrogen-14.

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Radiometric Dating

• The half-life of carbon-14 is 5,730 years
• If you find an organism that originally had 10
  grams of carbon-14 and it has 5 grams when you
  find ithow old would it be?
• About 5,730 years old (1 half life of carbon-14
  means 50 of the sample will be left)
• Carbon-14 is only good for dating organisms less
  than 60,000 years old
• After that time, the carbon-14 left would be too
  small to measure (or none at all)

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• For example, if a fossil is 1 billion years old,
  there would be almost no Carbon-14 left to
  measure so wed have to use something with a
  longer half-life, such as uranium-238 which has a
  half life of 4.5 billion years, because there
  would still be enough parent and daughter
  material present to measure.

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Radiometric Dating
Radioactive Isotope Product (decays to) Half-life (years)
Potassium-40 Argon-40 1.25 billion
Uranium-235 Lead-207 7.04 million
Uranium-238 Lead-206 4.5 billion
Rubidium-87 Strontium-87 48.8 billion
Beryllium-10 Boron-10 1.5 million
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Radiometric Dating

• If 1/8 of the original amount of Potassium-40 is
  left in a sample, how old is it?
• 1/8 means 3 half lives ( ½ x ½ x ½ 1/8 )
• Half life of potassium-40 is 1.25 billion
• 3 x 1.25 billion years 3.75 billion years

VIDEO
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• Video FIND NEW VIDEO THEY ALREADY SAW EARLIER
  IN PPT

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The Geologic Time Scale
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The Earth is constantly changing!!!

• As conditions on the earths surface change,
  various organisms flourish and then become
  extinct
• Geologic Time Scale was developed by scientists
  based on the sequence and length of these major
  changes
• Represents a time line in Earths history

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Divisions of Geologic Time

• Mainly separated by major changes in Earths
  surface or climate and by major extinctions of
  various species

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Geologic Time Scale

• Worldwide changes in fossils give break points
• When did dinosaurs go mostly extinct?

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Geologic Time Scale

• The Geologic Time scale is broken down into 3
  major subdivisions
• Eon The most broad of the subdivisions. They
  can be broken down into Eras.
• Era (4 of them precambrian, paleozoic,
  mesozoic, cenozoic). Each Era is broken down into
  Periods
• Periods can be broken down into Epochs (most
  specific)

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Precambrian Era

• The Precambrian Era begins at the beginning of
  Earths formation, about 4.6 billion (4600
  million) years ago and lasted until about 544
  mya.
• Beginning of Precambrian time - Earths crust was
  just beginning to solidify
• Very early rocks of this era contain no evidence
  of life

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Precambrian Era

• A few fossils of bacteria and algae, thought to
  be earths first life forms, found in rocks about
  3.5 billion years old
• Late Precambrian time rocks contain fossils of
  primitive worms, sponges, corals
• Lead scientists to believe life began in the ocean

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Fossils

• Single-celled organisms range?

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Paleozoic Era

• Followed Precambrian time
• Paleozoic from Greek word for ancient life
• Lasted 299 million years (from 544 million years
  ago 245mya)
• Fossils consist of variety of both marine land
  plants animals

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Fossils

• Clams (Mollusks) range?

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Paleozoic Era

• Broken up into 7 periods
• Cambrian
• Ordovician
• Silurian
• Devonian
• Pennsylvanian
• Mississippian
• Permian

• End of the Paleozoic largest mass extinction
  recorded (90 marine/70 land species
  disappeared)

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Late Paleozoic

• All continental plates come together to form the
  single landmass Pangaea.
• Major Glaciers form.

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Mesozoic Era

• Span of about 180 million years (245 mya 65
  mya)
• Mesozoic means middle life
• Divided into 3 periods
• Triassic
• Jurassic
• Cretaceous

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Triassic Period
Jurassic Period

• Mammals first appeared on Earth (small and
  mouse-like)
• Dinosaurs (which are reptiles) appeared at this
  time
• Small initially
• Pangaea separated into 2 separate landmasses

• Began 208 mya
• The Age of Dinosaurs
• Modern birds evolved at the end of this period
  (from dinosaurs)

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Fossils

• Dinosaur range?

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Cretaceous Period

• 144 mya 65mya
• New types of mammals and first flowering plants
  appeared on Earth
• A mass extinction of the dinosaurs marked the end
  of the Cretaceous Period (also Mesozoic Era)
• M.I.T. (Meteor Impact Theory)
• 2/3 of all living species became extinct

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• Extinction Video

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Cenozoic Era

• Our current (present) geologic era (65 mya to
  present)
• Cenozoic means recent life
• Fossils of mammals become common

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Cenozoic Era

• Among the mammals that first appeared was a group
  of animals to which humans belong, the primates
• Primates first appeared 30 mya
• Homo Sapiens, probably appeared about 500,000
  years ago
• Homo Sapiens became dominant only 10,000 years
  ago.

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Fossils

• Where do humans appear?

Humans 4 M.y.a - Present
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Cenozoic Era

• 2 periods
• Tertiary (65 mya 1.6 mya)
• Quaternary (1.6 mya - present)
• Separated by an Ice Age

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End
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Some Geologic Rates

• Cutting of Grand Canyon
• 2 km/3 m.y. 1 cm/15 yr
• Uplift of Alps
• 5 km/10 m.y. 1 cm/20 yr.
• Opening of Atlantic
• 5000 km/180 m.y. 2.8 cm/yr.
• Uplift of White Mtns. (N.H.) Granites
• 8 km/150 m.y. 1 cm/190 yr.

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Some Geologic Rates

• Movement of San Andreas Fault
• 5 cm/yr 7 m/140 yr.
• Growth of Mt. St. Helens
• 3 km/30,000 yr 10 cm/yr.
• Deposition of Niagara Dolomite
• 100 m/ 1 m.y.? 1 cm/100 yr.

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1 Second 1 Year

• 35 minutes to birth of Christ
• 1 hour to pyramids
• 3 hours to retreat of glaciers from Wisconsin
• 12 days 1 million years
• 2 years to extinction of dinosaurs
• 14 years to age of Niagara Escarpment
• 31 years 1 billion years

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Were The Dinosaurs Failures?

• Dinosaurs 150,000,000 years
• Recorded History 5000 years
• For every year of recorded history, the dinosaurs
  had 30,000 years
• For every day of recorded history, the dinosaurs
  had 82 years
• For every minute of recorded history, the
  dinosaurs had three weeks

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Making of an Angular Unconformity Animation

• http//www.classzone.com/books/earth_science/terc/
  content/visualizations/es2902/es2902page01.cfm?cha
  pter_novisualization