Finding the Relative Age of Rocks

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Finding the Relative Age of
Rocks
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.
Relative Age
Which is the “Oldest” & which is the “Newest”?
How do you know?
Real World Example
Position of Rock Layers
It is difficult to determine absolute age, so
geologists use methods to find relative age. One
such method is 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.
Other Clues to Relative Age
Clues from Igneous Rocks:
Lava that hardens on the surface is called an
Extrusion
(ex: 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.
The Extrusion is in Black
Now, the Extrusion is the Youngest Layer.
Clues from Igneous Rocks:
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.
The intrusion (in red) is now younger than the
surrounding rocks.
Draw this in your notes and label each of the parts listed below.
1. Intrusion
2. Extrusion
3. Oldest rock
4. Youngest rock
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
GAPS IN THE GEOLOGIC RECORD
The 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’s surface.
Unconformity – is a gap in the geologic record. An
unconformity shows where some rock layers have
been lost because of erosion.
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.
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.
Adapted from Greenville Schools: greenville.k12.sc.us
Unlabeled images courtesy of google images
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