Transcript Finding Relative Age of Rocks

Relative Dating
(No, not your cousin)
Finding the Relative Age of Rocks
Relative Dating and Relative Age
1. Scientists use relative dating to
determine whether a rock is
older or younger than another
rock.
2. The relative age of a rock is its age
compared to the ages of other
rocks around it.
How do we determine the relative age of a rock?
3. Law or Original Horizontality – states that
as sediments settle out of water, they are
deposited in horizontal layers.
– This law shows where the Earth has shifted and
changed the placement of rock layers.
4. Law of Uniformitarianism – states that
events occurring today also occurred in the
past.
– All the processes we see today, such as
volcanic activity, earthquakes, and mountain
building were also occurring millions of years
ago.
How do we determine the relative age of a rock?
5. The Law of Superposition – states that
younger rock layers will be found above older rock
layers as long as the layers have not been
disturbed.
6. The Law
of Superposition
helps
determine
the of
relative age
of fossils
and rock
layers.
The Geologic Column
- The geologic column (stratigraphic
column) is a detailed sequence of rock
layers that contains all the known fossils &
rock formations on Earth, ordered from
oldest to youngest.
- Scientists use the geologic column to
help them interpret rock sequences
especially if the rock layers have been
disturbed.
- Geologists created the geologic column by
combining information from all over the
world.
Disturbed Rock
7. Gravity deposits
sediment in flat, horizontal
layers. If the layers are
NOT horizontal they have
been disturbed.
8. Folding and tilting of
rock layers is caused by
pressure and forces within
the earth.
Disturbed Rock
9. Earth may shift along
faults which causes
rock layers to not
match up.
10. The faults are
always younger than
the rock layers.
* Igneous rock can give
many clues to the
relative age of rock
Clues From Igneous Rock
11. Lava that hardens on the surface is
called an extrusion. (extrusive igneous
rock)
12. The rock layers below an extrusion are
always older than the extrusion.
13. Beneath the surface magma cools and
hardens into a mass of igneous rock called
an intrusion. (intrusive igneous rock)
14. An intrusion is always younger than
the rock layers around and beneath it.
Gaps in the Geologic Record
15. The surface where new rock layers
meet a much older rock surface beneath
them is called an unconformity.
16. An unconformity is a gap in the
geologic record.
Gaps in the Geologic Record
17. Three types of Unconformities:
a. Disconformities
b. Nonconformity
c. Angular Unconformity
18. All Unconformities are formed in one of
two ways:
1. A period of no sediment deposit.
2. A period of erosion which removed
sediment .
Unconformity (disconformity)
Unconformity (nonconformity)
Unconformity (angular unconformity)
Date That Rock Layer!
Date That Rock Layer!
Date That Rock Layer!
• Geologists must be able to read all of
these and much more difficult rock
sequences to determine relative age
• Since the earth has been changing for
billions of years this gets very tricky!