Download Relative Age of Rocks and

Relative Age of Rocks and the
Geologic Time Scale
Absolute and Relative Age
The absolute age of a rock is how long it has been since
the rock has formed. The relative age of a rock is how
old it is compared to the rocks around it.
How Scientists Determine
Relative Age
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The Law of Superposition
Intrusions and Exclusion
Fault line changes
Index Fossils
The Law of Superposition
According to the law of
superposition, in horizontal
sedimentary rocks, the
lowest layer is at the
bottom, and each higher
layer is younger than the
layers below it.
The lowest layer of rock will
have the oldest fossils.
The highest layer of rock will
have the most recent fossils.
Intrusions
Magma is molten, flowing rocky material beneath the earth’s
surface. Magma can push into sedimentary layers and
harden into solid rock. This rock is called an intrusion.
Intrusions are younger than the layers they push through.
Intrusion Formation
Layer E – Scientists need to use other clues to find out if this layer is older or
younger than the intrusion.
Intrusion
DAn
intrusion
happens
after
Layer C is
formed
Layer C is made of sediment that
builds up on Layer B
Layer B is made of sediment that
builds on Layer A
Layer A forms – it is the oldest layer
Extrusions
When magma flows to
the surface it is called
lava. It can harden into
rock.
When lava hardens on the
surface of the Earth, it can
even form a horizontal
layer in the same way
sediment can.
This is called an extrusion.
An extrusion is always
younger than the rocks
below it.
Extrusion Formation
Layer E – This layer is definitely younger than the lava extrusion
Extrusion
DLava cuts
through
the first
three
layers and
forms a
layer on
top of C
Layer C is made of sediment that
builds up on Layer B
Layer B is made of sediment that
builds on Layer A
Layer A forms – the oldest layer
Fault Line Changes
A fault is a break in the Earth’s
crust. The fault is always
younger than the rock it cuts
through.
To determine the relative age of
the fault, geologists find the
relative age of the youngest
layer cut by the fault.
The fault causes layers to no
longer line up. Solid layers, like
layer E, can be built up evenly
after the fault has changed.
Uncomformities
(not nonconformities)
Unconformities appear when something
happens to the horizontal layers of sediment.
The layers can be changed or some layers can
be missing.
When the top layer of sedimentary rock is
eroded and then new sediment is deposited
where the layer used to be, there is a gap in
what geologists can learn.
The rock can be folded by converging plates
or molten magma can shift the layers so they
are angled or wavy.
Index Fossils
Index fossils are specific fossils that occur across
large areas and only lived for a short time. Index
fossils are helpful because they tell the relative
ages of the rock layers in which they are found.
Scaphites only lived between 145
and 65 million years old. Layers
with Scaphites and the fossils in
those layerscan only be that old.
The Geologic Time Scale
The Geologic Time Scale is a record of life forms
and geologic events through the Earth’s history.
Scientists first created the Geologic Time Scale
by studying rock layers and index fossils
worldwide.
The Geologic Time Scale is divided into units of
time called eras and periods.