Finding the Relative Age of Rocks

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Finding the Relative Age of Rocks
Standard 8-2.6 Infer the relative age of rocks
and fossils from index fossils and the ordering
of the rock layers.
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Relative and Absolute Ages

• Relative Age The age of a rock compared to the
  ages of rock layers.
• Absolute Age The age of a rock given as the
  number of years since the rock formed.

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Position of Rock Layers

• Its difficult to determine the absolute age so
  geologists use method to find a rocks relative
  age. Use 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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Other Clues to Relative Age

• Clues from Igneous Rocks
• 1. Lava that hardens on the surface is called
  an Extrusion

(example an eruption would put a layer of
igneous rock on top of sedimentary rocks. Rock
layers below an extrusion are always older than
the extrusion.
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The extrusion is in black
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Now the extrusion is the youngest layer.
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• Clues from Igneous Rocks
• 2. Magma that cools and pushes into bodies of
  rock and hardens is called an Intrusion

An intrusion is always younger than the rock
layers around and beneath it.
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The intrusion (in red) is now younger than the
surrounding rocks.
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• Draw this in your notes and label each of the
  parts listed below.
• Intrusion
• Extrusion
• Oldest rock
• Youngest rock

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• Clues from Faults
• Fault is a break in the Earths crust.
• Forces inside the Earth cause movement of the
  rock on opposite sides of a fault.
• Fault is always younger than the rock it cuts
  through.

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REVIEW

• Geologists use the Relative and Absolute
• Age of rocks to determine age.

• Using the Law of Superposition

• Other clues are from Igneous rocks
• Extrusion
• Intrusion

• Clues from Faults

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GAPS IN THE GEOLOGIC RECORD - Record of
sedimentary rock layers is not always complete -
Deposition slowly builds layers upon layer of
sedimentary rock, BUT some of these layers may
erode away, exposing an older rock surface.
Unconformity is a gap in the geologic record.
An unconformity shows where some rock layers have
been lost because of erosion.
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USING FOSSILS TO DATE ROCKS
To date rock layers, geologists first give a
relative age to a layer of rock at one location.
THEN they can give the same age to matching
layers of rock at other locations. Certain
fossils, called Index Fossils help geologist
match rock layers. INDEX FOSSILS Fossils of
widely distributed organisms that lived during
only one short period.
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• Example of an Index Fossil
• Trilobites (hard shelled animals whose bodies had
  three distinct parts.
• Trilobites evolved in shallow seas more than 500
  million years ago.
• Over time, many types have appeared.
• They became extinct about 245 million years ago.
• They have been found in many different places.

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• To become a Index Fossil a trilobite must be
  different in some way from other trilobites.
• Example type with large eyes
• These large-eyed ..bites survived for a time
  AFTER other bites became extinct.
• If a geologist finds large-eyed Trilobites in a
  rock layer, the geologist can infer that those
  rocks are younger than rocks containing other
  types of trilobites

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• The World's Biggest Trilobite
• A team of Canadian paleontologists working along
  Hudson Bay in northern Manitoba has discovered
  the world's largest recorded complete fossil of a
  trilobite, a many-legged, sea-dwelling animal
  that lived 445 million years ago. The giant
  creature is more than 70 cm long (about 28
  inches), 70 percent larger than the previous
  record holder. "This is an important and amazing
  find," says Bob Elias, a professor in the
  department of geological sciences at the
  University of Manitoba. "It looks like a huge
  bug!"

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• You can use INDEX FOSSILS to match rock layers.
• Page 117 G
• Figure 13
• Adobe Info sheet on Trilobites