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TOPIC 13 –
GEOLOGIC HISTORY
DATING ROCKS
There are two ways to get the ‘age’ of a rock:
1.) RELATIVELY
ex) Rock layer ‘A’ is older than rock layer ‘B’
2.) ABSOLUTELY
ex) Rock layer ‘A’ is 860 million years old
Dating Method #1 - RELATIVE
DATING
The determination of an age of a rock or
event in relation to other rocks.
To do this, use:
PRINCIPLE OF SUPERPOSITION
When rocks are layered, the bottom layer is:
OLDEST
And they get progressively younger as you
rise through the rock column
There are exceptions…
FOLDING:
FAULTING:
How can we tell this has
happened?
1.) Look for surface
features
Ex) ripple marks, mud
cracks, etc
2.) Check for vertical
sorting patterns “out of
whack”
3.) Check fossil patterns –
are they all upside down?
DATING LEFTOVER MAGMA
INTRUSION –
When molten rock squeezes into pre-existing
rocks and crystallizes
EXTRUSION –
When lava flows on Earth’s surface and
solidifies.
KEY – extrusions are YOUNGER than rocks
below it, and OLDER than any rocks that
form on top of it.
UNCONFORMITY
Missing time in the geologic rock record
Occurs when layers are deposited, then
eroded, and new layers are later deposited.
Where could there be an unconformity in this picture?
Whenever the Regents includes an
unconformity in a rock column, they
ALWAYS give you a HUGE clue:
Dating Method #2 CORRELATION
The process of showing that rocks or
geologic events from different places are the
same or similar age
Accomplished by using:
1.) OUTCROPS – exposed layers of bedrock
Seen in roadcuts, etc…
SAME LAYER
2.) Similarities between rock layers:
Ex) red shale <-> red shale, large breccia <->
large breccia
3.) Similarities in fossils:
3.) INDEX FOSSIL – an incredibly useful tool!
- They must have lived over a huge area
-Must have lived over a very short time
-Ex) TRILOBITE
Occurred in the CAMBRIAN
period (500 mya)
So if a rock has this fossil in
it, we know how old it is
So what can be done with these fossils?
1.) If you find a rock with a trilobite in it, it is
approximately 500 myo
2.) If you find rock layers at different levels in
the rock column with trilobites, they had to
come from the same time period.
3.) If an older index fossil is on TOP of a
younger one:
Then the Law of Superposition has been
disturbed
Fossils typically get more
complex over time:
4.) Volcanic ash layers They are spread through a very large area –
they act as an AGE MARKER (since they
happened over a very short time)
5.) Impact debris A certain element is rarely found on Earth
but frequently in space:
IRIDIUM
If this occurs in the rock record, it was most
likely from an asteroid impact – and this can
be matched to other layers around the Earth.