History of the Earth

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History of the Earth
A matter-of-time scale If the age of the Earth
(4.6 billion years) were condensed into one year
... Jan. 1 Earth was born Early Feb Oldest
known rocks formed Late Mar First primitive
life formed Mid-Nov... First complex life with
shells or skeletons formed Late-Nov First
land animals Dec 25 Extinction of the dinosaurs
Dec 31 Humans evolved in the evening Dec
31 One second before midnight, humans first set
foot on the Moon
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BIG Ideas

1. Scientists use several methods to learn about
   Earths long history.
2. The oceans and atmosphere formed and life began
   during the three eons of the Precambrian, which
   spans nearly 90 of Earths history.
3. Complex life developed and diversified during the
   three eras of the Phanerozoic as the continents
   moved into their present positions.

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• HOW have scientists investigated and understood
  the history of Earth and the ages of rocks?

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• By studying rocks and fossils

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Uniformitarianism

• geologic processes occurring today have been
  occurring since Earth formed
• the present is the key
  to the past

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I. Determining Relative Age

• Relative Age
• The age of a rock or an event as compared with
  some other rock or event
• A comparison (older vs. younger) does NOT
  indicate an exact age

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• Principle of Original Horizontality most
  sedimentary layers of rock are deposited in a
  horizontal position

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• A. Law of Superposition
• an undeformed sedimentary rock layer is older
  than the one above it and younger than the layer
  below it.

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How would you explain this?
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B. Unconformities

• breaks in the geologic record

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An UNCONFORMITY is

• a GAP in the sedimentary rock record (like
  missing pages from a book).
• a BURIED
• erosional
• surface.

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Is this an unconformity?
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• NOjust heavily eroded rocks (they remain
  HORIZONTAL)

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Law of Crosscutting Relationships a fault or
intrusion is always younger than the rock layer
it cuts through.
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Put these rock layers in order, oldest to
youngest
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II. Determining Absolute Age

• Absolute Age
• The actual age of the rock.

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A. Varve Count
How can you tell how old this tree is?
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Varves annual layers of sedimentary rock-
consist of a light-colored band of coarse
particles- and a darker band of fine particles
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B. Radioactive Decay

• As elements age, they change into new elements
• Uranium (U) will decay into lead (Pb) - in about
  4.5 billion years

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1. Half Life

• the time it takes for half the mass of a given
  amount of a radioactive element to decay into its
  daughter elements.

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• U-238 decays into Pb-206
• U-238 has a half-life of 4.5 billion years
• If we were to begin with 10g of U-238, after 4.5
  billion years you would have 5g of U-238 left
• After another 4.5 billion years, how much U-238
  would remain?
• How much Pb-206 would you have?

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2. Carbon Dating

• Organic materials (remains of once-living things)
  decay at a measurable rate
• Can be used to estimate the ages of samples from
  0 to 70,000 years old

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Would you use half life or carbon dating to date
this tooth? Why?
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III. The Fossil Record

• Paleontologist Scientist that studies fossils.
• Fossils the remains or traces of animals or
  plants from a previous geologic time.

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• What type of rock are nearly all fossils found
  in?
• WHY?

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Fossils may be preserved in a number of ways

1. Molds sediment preserves the form of a fossil
2. Casts created by filling a mold with mineral or
   other material
3. Imprints (trace fossils) tracks or footprints
4. Original bone or shell

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Trace Fossils
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Amber

• the fossilized remaining part of tree resin
  (which is very thick and sticky when it first
  flows from a tree)
• resin runs down the trunk and may trap insects,
  spiders, and sometimes larger organisms (like
  lizards)
• these organisms can be preserved for millions of
  years with details of their soft tissue, such as
  muscles and hair-like bristles, still intact

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• Most Virginia fossils are of marine organisms.
• What does this suggest about the past environment
  of our state?

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• Large areas of the state have been periodically
  covered by seawater.

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• Where are most fossils found in Virginia?

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• Coastal Plain
• Valley and Ridge
• Appalachian Plateau

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• How are fossils used to determine the ages of
  rocks?

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Index Fossils fossils found exclusively in rock
layers of a particular geologic age.
Phacops (a species of Trilobite) lived 400
million years ago in oceans
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Requirements for Index Fossils

1. Must be present in rocks scattered over a wide
   area of Earths surface
2. Must have unique, distinguishing features
3. Organisms must have lived during a relatively
   short span of geologic time
4. Must occur in fairly large numbers within the
   rock layers

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IV. The Geologic Time Scale

• Divided into units
• EON the LARGEST unit of geologic time
• Eons are divided into Eras
• Eras are divided into Periods
• Periods are divided into Epochs

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PRECAMBRIAN

• The three earliest EONS make up 90 of geologic
  time
• Known together as PRECAMBRIAN
• During this time, Earth was formed and became
  hospitable to modern life.
• 4.6 billion-540 million years ago

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ERA based on relative ages of life forms found
in rocks

• Paleozoic (paleo old) 540 million-248 million
  years ago
• Mesozoic (meso middle) 248 million-65 million
  years ago
• Cenozoic (ceno recent) 65 million years ago to
  present
• (zoic of life)

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• During what ERA did humans appear on Earth?
• What ERA are we in right now?

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