Transcript Relative Ages of Rock

The Relative Age of Rocks 10-2
Place in order from
youngest to oldest
B
A
C
D
E
F
Answers
youngest
to
oldest
D
F
E
C
A
B
What is the age of each?
D
F
E
C
A
B
Relative Dating

Used to determine the order of events and
age compared to surrounding materials

Can not tell exact age
Law of Superposition

Older layers of rock are beneath younger
layers of rock in the rock record
Law of Superposition
In horizontal
sedimentary
rock layers,
the oldest
layer is at the
bottom. Each
higher layer is
younger than
the layers
below it.
Igneous Rock Clues
-
Forms when magma or lava hardens
Extrusion – lava that hardens on the
surface
-an extrusion is always younger
than the extrusion below it
Extrusion
When lava hardens on the surface. It is
always younger than the rock below it.
Igneous Rock Clues
Intrusion – magma pushes into bodies of
rock then cools and hardens
- an intrusion is always younger than
the rock layers around it
Intrusion
Intrusion
A mass of igneous rock below the surface is
called an Intrusion. It is always younger
than the rock layers around and beneath it.
Fault

A fault is a break in the Earth’s crust. A
fault is always younger than the rock it
cuts through. It makes the layers not line
up.
Unconformities

Occur when there are missing layers in the
rock record
1) Erosion of existing layers
2) No new deposition for a period of
time (disconformity)
Angular Unconformity – tilted rock layers
meet horizontal rock layers indicating gaps in
rock record
Unconformity

The surface where new rock layers meet a
much older rock surface beneath them is
called an unconformity. It is a gap in the
geologic record.
Index Fossils



Index Fossils help match rock layers.
Must be widely distributed and represent a type of
organism that existed only briefly.
They tell the relative ages of the rock layers in
which they occur.