Ch. 13 Section 2

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Transcript Ch. 13 Section 2

Ch. 13 Section 2
Relative Ages of Rocks
Principle of Superposition
 The
principle of superposition states
that in undisturbed layers of rock,
the oldest rocks are on the bottom
and the youngest rocks are at the
 This assumes that sedimentary rock
layers are horizontal and have not
been disturbed. Rock layers can get
turned by things like tectonic forces,
so scientists have to use other clues.
Relative Ages
 The
Relative Age of something is its
age in comparison to the ages of
other things.
 Relative age does not tell you the
age in actual years only that a layer
is younger than the layer below it, or
older than the layer above it.
Principal of Cross-Cutting Relations
 The
principal of cross-cutting states
that it rock layers are disturbed by
things like faults (breaks in the
earth’s surface), or igneous
intrusions, then you know that the
rock layers are older than the fault or
 Fossils can also help determine
relative ages of rocks.
 Most
rock layers are incomplete.
 Gaps in the rock sequences are
called unconformities.
 These develop when things like
running water or glaciers wash or
scrape rock layers away.
 There are 3 types.
1. Angular Unconformities
 Horizontal
rock layers are tilted and
 Erosion and weathering wear down
the tilted rock layers.
 New sediment layers are deposited
horizontally on top of the older tilted
and eroded layers.
2. Disconformity
layer of horizontal rock was
exposed and eroded.
 Younger rocks were formed by
deposited sediment over the old
erosion site.
 The rock layers are still horizontal,
but layers are missing.
3. Nonconformity
 Metamorphic
or Igneous rocks are
uplifted and eroded.
 Sedimentary rocks are then
deposited over the eroded rock
Matching up the rock layers
 Geologists
match up layers or rocks
over long distances.
 To do this, they can sometimes walk
along the layers to prove that they
are continuous.
 Or, if the same type or fossils are
found in the same rock layer in 2
different places, it’s possible the
rocks are the same age.