Relative Age Dating
Geology Unit
• Until the late 1800's scientists were
unable to calculate reliable ages of
rocks
• With the discovery of radioactivity
geologists were given a new reliable
dating tool to actually calculate a
more reliable age of rocks, however,
we still cannot determine exact dates
• Before this discovery scientists used
relative age dating to place rocks and
geologic events in their proper
sequence.
Geology is one of the oldest sciences!
Youngest
• Relative age dating is the task of
placing rock units and geologic
events in their proper sequence.
• putting rock layers in order of
creation, creating a sequence.
Oldest
• Like a brick
wall, you have
to deposit the
bottom layer
before you can
deposit the
next layer.
What is Relative Age Dating?
Youngest
Oldest
Two features that are
always younger than
the rock layers they are
found in are:
1. Faults
2. Igneous Intrusions
(or Dikes)
Why are they younger?
• The rock layers had
to be there before
the fault or dike
could be formed in it.
Relative age dating is used on more
than just rock layers...
Sediments are typically deposited flat (horizontally)
Rock layers are disturbed over the years becoming:
Tilted
Pushed
Weathering and Erosion
Folded
Moved
Weathering and Erosion can destroy the rock record by erasing
layers of rock.
When an eroded surface is buried it is called an unconformity
Conformable: layers of rock
that have been deposited
essentially without
interruption
no place on
Earth has a
complete set
of conformable
strata
VS
Unconformity: represents a long
period during which deposition
ceased, erosion removed previously
formed rocks, and then deposition
resumed
Angular
Unconformity
older rock strata
dip at an angle
different then
younger rocks
bottom beds are
tilted or folded
making them
angular
UNCONFORMITY
-removed rocks
Disconformity
breaks in rock
record where strata
on both sides of
unconformity are
parallel
rocks on top and
bottom of the
unconformity are still
flat
Nonconformity
Break separates
older metamorphic
or igneous rocks
from younger
sedimentary rocks
separates
sedimentary from
non-sedimentary
Principle of
Original Horizontality
Sedimentary rocks are
initially deposited in
nearly horizontal layers
Principle of
Superposition
An undisturbed rock
sequence will have the oldest
rocks at the bottom and the
youngest rocks at the top.
Four Principles of Geology (Cont’d)
Principle of Crosscutting
Principle of Inclusion
Relationships
When rock layer includes pieces of
rock from a layer below
• Ex. If lava flows over a bed of
loose rocks the old rocks are now
in the lava layer
An intrusion or a fault is younger
than the rock layers it cuts
through
• dike—an intrusion of igneous
rock
Four Principles of Geology (Cont’d)
2.
1.
A
D
3.
E
C
B
4.
A
C
B
C
B
D
A
E
D
On your paper: label each of the four
principles shown and label each layer from
oldest to youngest
1.
Principle of
Superposition
A E
C
D
3.
2.
Principle of
Crosscutting
Relationships
B
A Principle of Inclusion 4.
C
B
D
Check your answers!
C
Principle of
B
Original Horizontality
A
E
D