A Trip Through Geologic Time

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A Trip Through Geologic Time
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Fossils

• Fossils are preserved remains or traces of living
  things.
• Most fossils form when living things die and are
  buried by sediments.
• The sediments slowly harden into rock and
  preserve the shape of the organisms.
• Scientists who study fossils are paleontologists.

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Fossils

• Fossils are usually found in sedimentary rocks.
• When an organism dies, its soft parts often decay
  quickly leaving only the hard parts to fossilize.
• Ex. Bones, Shells, Teeth, or Seeds

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Kinds of Fossils

• Petrified Fossils fossils in which minerals
  replace all or part of the organism. Ex
  petrified wood
• When the object is buried by sediment, water rich
  in minerals seeps into the cells. After the water
  evaporates, hardened minerals are left behind.

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Kinds of Fossils

• Molds and Casts
• A mold is a hollow area in sediment in the shape
  of an organism or part of an organism.
• A cast is a copy of the shape of an organism.

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Kinds of Fossils

• Carbon Films an extremely thin coating of carbon
  on rock that forms when materials that make up an
  organism become gases and escape leaving only
  carbon behind.
• Trace Fossils provide evidence of the activities
  of ancient organisms. Ex footprints, animal
  trails, or animal burrows.

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Kinds of Fossils

• Preserved Remains are formed when an organism is
  preserved with little or no change.
• For example when organisms become preserved in
  tar, amber (tree sap), and freezing.

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Why Study Fossils?

• Scientists study fossils to learn what past life
  forms were like.
• Paleontologists classify organisms in the order
  in which they lived.
• All the information scientists have gathered is
  called the fossil record.

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Fossil Record

• The fossil record provides evidence about the
  history of life on Earth.
• The fossil record also shows how different groups
  of organisms have changed over time.
• It also provides evidence to support the theory
  of evolution.

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Remember!

• A scientific theory is a well-tested concept that
  explains a wide range of observations.
• The fossil record shows that millions of types of
  organisms have evolved.
• However, many others became extinct.

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Finding the Age of Rocks Through Rock Dating
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D A T I N G
R O C K
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Ages of Rocks

• The relative age of a rock is its age compared to
  other rocks. Use words like older or younger
• The absolute age of a rock is the number of years
  since the rock was formed.
• Ex 358-360 mya
  

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Rock Joke!!

• What does a rock want to be when it grows up?
• A Rock Star!!

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The Position of Rock Layers

• It can be difficult to determine a rocks absolute
  age. So scientists use the law of superposition.
• According to the law of superposition, in
  horizontal sedimentary rock layers the oldest
  layer is at the bottom. Each higher layer is
  younger than the layers below it.

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Rock Joke!!

• How do rocks wash their clothes?
• The Rock Cycle!!

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Other Clues to Relative Age

• Clues From Igneous Rock
• Lava that cools at the surface is called an
  extrusion. Rock below an extrusion is always
  older.
• Magma that cools beneath the surface is called an
  intrusion. An intrusion is always younger than
  the rock layers around an beneath it.

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Other Clues to Relative Age

• Faults (a break in the rock) are always younger
  than the rock it cuts through!
• Unconformities An unconformity is a gap in the
  geological record that can occur when erosion
  wears away rock layers and other rock layers form
  on top of the eroded surface.

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Using Fossils to Date Rocks!

• Scientists use index fossils to match rock
  layers.
• An index fossil must be widely distributed and
  represent a type of organism that existed only
  briefly.
• They are useful because they tell the relative
  ages of the rock layers they are found in.

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The Trilobite

• One example of an index fossil is a trilobite.
• Trilobites were a group of hard-shelled animals
  whose bodies had three distinct parts.
• They evolved in shallow seas more than 500
  million years ago.