Science Sponge
1/31/2017
What Did You Learn?
1. How does the Law of Superposition help us
determine the relative age of rock layers?
2. How do fossils help us determine the
relative age of rock layers?
3. What two characteristics must a fossil have
to be considered an index fossil?
What is relative dating?
What is relative dating?
• Determine age based on the rocks or fossils
around an area.
The Principle (Law) of Superposition
• Rock layers are put down one on top of
the other over millions of years.
• The oldest layers are at the bottom, and
the youngest layers are at the top if the
rock layer is undisturbed.
Notice the
different
color
bands
that
indicate
different
layers of
rock.
Disturbing Forces
• Not all rock sequences are youngest to
oldest.
• Natural forces can move these layers.
– These forces disrupt the rock layer the same way
tearing pages from a book disrupts the book.
• Geologists, like crime scene investigators,
must use clues trapped in the rock layers to
put the rocks back in sequence and get a true
look at Earth’s history.
Disturbed Rock Layers
• Another tool aiding geologists in age rocks
is cross cutting features, such as an
intrusion.
• The intrusion or cross-cutting feature is the
youngest thing present since the other
layers had to be present before it could cut
across them.
Disturbing Forces
• Fault - break
in the Earth’s
crust where a
hanging wall
is pushed up
or down.
Disturbing Forces
• Folding - rock layers are bent into
synclines or anticlines
Disturbing Forces
• Intrusion - molten rock from Earth’s interior
squeezes into existing rock layer, cools,
and appears to have cut across it.
Disturbing Forces
• Tilting - occurs when forces inside the
Earth slants the rock layers.
Gaps in the Record:
Unconformities
• Dating rock layers with folds, faults, etc. is
difficult, but imagine dating rocks that have
missing layers.
• The missing layers are caused by erosion
or non-deposition—they weren’t put there
to start with—and they are called
unconformities.
Three types of Unconformities
• A disconformity is an unconformity between
parallel layers of sedimentary rocks which
represents a period of erosion or non-deposition.
– This is common in the Grand Canyon.
Disconformity
• Nonconformities: unconformities that separate
eroded igneous or metamorphic rocks from
overlying sedimentary rocks.
– usually indicates a long period of erosion occurred
prior to deposition of the sediments (several km of
erosion necessary).
Nonconformity
Sedimentary Rock
Metamorphic
Rock
Three types of Unconformities
• Angular unconformity: found between horizontal
layers of rock that have been tilted or folded.
Angular Unconformity
Find Some Disturbances…