Geologic Time

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Geologic Time

• The tools and the evidence
• that reveal Earths history.

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• Earths History
• Fossils
• Relative Time - What rocks tell us.
• Absolute Time - Radiocarbon Dating
• Eons, Eras, and Epochs
• Mass Distincition Events
• Getting to Modern Humans

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Geologic Time progresses at a rate so slow that
change is measure not in terms of hours or days,
but in millions of years. For example If the age
of the Earth is represented by the length of this
classroom, then the time that humankind has been
around be represented by ____________ . Studying
events that happened so long ago is possible
because of evidence left behind - Fossils.
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Fossils
Fossils include shells, bones, petrified trees,
footprints, impressions made by leaves, or even
burrows made by worms.
Fossils form in several ways, each method
providing different clues to Earths history.
Fossil 5 m long Xiphactinus from the Late
Cretaceous
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1. Original Remains
Fossils are rarely the original, unchaged remains
of plants or animals. However, in rare cases an
organism is preserved in its entirety.
Large woolly mammoths have been found frozen in
permafrost. Some prehistoric insects have been
preserved intact by the sticky sap that oozes
from tress. The sap hardens into amber.
Prehistoric mosquito preserved in amber
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2. Replaced Remains
The soft parts of plants or animals decay away,
and the hard parts get replaced by minerals. The
original organic matter is removed and minerals
deposited slowly over time by groundwater.
The result is an exact copy of the original
specimen. Petrified wood is a good example of a
fossil formed as replaced remains.
Petrified logs in Arizona
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3. Molds and Casts
Often fossils are preserved as molds or casts.
After an organism is buried in sediments, its
hard parts become a fossil as the sediments
become rock.
Casts and Molds of Brachiopods
Molds If the fossil later dissolves out of the
rock, the hollow depression is called a mold.
Casts If minerals seep into and fill the mold, a
cast is formed that is an exact replica of the
original fossil.
Organism falls into sediment.
Sediment becomes rock, organism decays.
Minerals fill the mold.
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4. Carbonaceous Films
Sometimes the only fossil trace is a thin film of
carbon, resembling a silouette. High temperature
and pressure cause the carbon compounds in the
tissue to undergo chemical changes, leaving a
thin film of carbon that details the remains.
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Homework

• Read pages 647-649
• Complete page 649 4

Complete the table below with 5 examples of each
type of fossil.