Determining the
Age of Rocks
Relative Age of Rocks
Types of Rock
§  Sedimentary rock forms from sediment
(sediment- solid particles of rock
produced by weathering and erosion by
water and wind)
* Igneous rock forms from volcanic lava
flows
* Metamorphic rock- forms from intense
heat and pressure
Relative Dating of Rocks
§  The relative age of rocks determines
the age of rock layers as younger or
older, but does not give the exact age.
§  The principle of uniformitarianism
states that forces that shaped the Earth
in the past continues to shape the earth
today (volcanoes and weathering and
erosion by wind and water).
Rules of Relative Dating
1.  Law of Superposition: When sedimentary
rock layers are deposited, younger layers
are on top of older deposits.
Rules of Relative Dating
2. Law of Original Horizontality: Sedimentary
rock layers are deposited horizontally. If
they are tilted, folded, or broken, it
happened later.
Rules of Relative Dating
3. Law of Cross-Cutting Relationships: If an
igneous intrusion or a fault cuts through
existing rocks, the intrusion/fault is
YOUNGER than the rock it cuts through
Correlation
§  Rock layers in different places and be
correlated or matched up by matching up
the rocks and fossils in the layers.
SWBAT: Describe what an index fossil is:
Finding the Relative Age of Rocks using Index Fossils
§  Certain fossils called Index fossils help
geologists determine the Relative Age of
rocks.
§  To be a useful Index fossil:
- the fossil must be widely distributed
different geographic areas
- and represent an organism that
existed only briefly
Index fossils
§  Trilobites were a group of hard shelled
animals that evolved in shallow sea more
than 500 million years ago
§  Trilobites are useful as index fossils
because they were widely distributed
geographically and represent an
organism that existed briefly.
Index Fossils
§  Some species of
organisms only lived for
a short period of time
before they became
extinct. If you use
radiometric dating to get
an age for the fossil, then
you know that the rock
the fossil is found in is
also that age.
Trilobite: Index fossil for the
Paleozoic Era
Age: 590-250 mya
Example of an index fossil
(trilobite) in layers of
sedimentary rock. How old
is the bottom layer?
http://reynolds.asu.edu/glg103/relative_age_principles.htm