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).
Sedimentary Rocks
 Sedimentary rocks formed from sediment
deposited millions of years ago.
Sedimentary rocks are deposited in
horizontal layers. (principle of original
horizontality)
 Sedimentary rock is deposited in
horizontal layers over geologic time with
the oldest layer on bottom and the
youngest layer at the top (principle of
superposition)
Sedimentary Rock Layers
 Remember sedimentary rock
layers are horizontal
with the oldest at the
bottom and the
young at the top
unless the layers are
disturbed by a fault
or igneous intrusion
Faults and Igneous
Intrusions
 Sometimes sedimentary rock layers are
disturbed by geological forces. A crack
in the rock layer is called a fault. When
igneous rock (volcanic lava) intrudes or
cuts through layers of sedimentary rock it
is called an igneous intrusion.
 The principle of cross-cutting
relationships states that a fault or
igneous intrusion is always younger
than the rock it cuts across.
Fault (Crack) in Sedimentary
rock
Here is a picture of a
fault or crack in the
rock
Igneous Intrusion in Rock
Layers
 Remember that
igneous intrusions
(lava) are always
younger than the rock
layers they cut
through. Note the
igneous intrusions
(volcanic rock) cutting
through the rock layer
Relative Age of Rock
 The Relative Age of the rock can be
determined by the sequence of the rock
layers using the Principle of
Superposition (oldest on the bottom and
youngest at the top).
 If the sedimentary rock layer has been
disturbed by a fault or igneous intrusion,
the fault or intrusion is always younger
than the rock layer they cut through
(cross-cutting)
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
became extinct at the end of the
Paleozoic Era)
 Trilobites are useful as index fossils
because they were widely distributed
geographically and represent an
organism that existed briefly.
Example of an index fossil
(trilobite) in layers of
sedimentary rock. How old
is the bottom layer?
Continental Drift Theory
 All continents were once joined together
in a single landmass (supercontinent)
called Pangea
 Fossil evidence supports the
Continental Drift Theory. Fossils from
a fernlike plant Glossopteris have been
found in Africa, South America, Australia,
and Antarctica. The seeds could not have
traveled across the span of the oceans
Absolute Age of Rocks
 To determine the Relative Age of Rocks
geologists use the Principles of Superposition,
Cross-Cutting Relationships, and Index fossils.
 To determine the Absolute Age (exact age) of
rock, geologists use Radiometric Dating.
 How did geologist determine the Earth was 4.6
billion years old?