Relative Ages of Rocks
Relative Dating
* early geologists tried to determine
relative age of earth
* relative dating = placing rocks in their
correct sequence of formation
* relative dating tells whether one rock is
older than another but not how old each
is
Relative
dating just
puts these
in order
from oldest
to youngest
but does not
give a
numerical
age for any
of them
Rules and Principles of Relative
Dating
Number One: Law of Superposition
- The oldest rocks in undisturbed
sedimentary rock layers are on the
bottom
- Sediment is carried by water or wind.
When the wind or water is not moving
fast enough, the sediment falls out in
even horizontal layers.
So which of
these layers
is the oldest?
Does this still
work if your
rocks are
tilted or if
they are
volcanic?
Rules and Principles of Relative
Dating
Number Two: Principle of Original
Horizontality
- sedimentary rocks are layed down
horizontally
* However, over time, they can tilt as
plates collide or diverge. So if your
rocks aren't horizontal, they must have
been moved by plate forces.
Example of Principle of Original
Horizontality
Rules and Principles of Relative
Dating
Number Three: Principle of crosscutting relationships
- younger features (fault lines or igneous
rocks) cut through older features
- basically this says that you can't have
an earthquake crack a rock that did not
exist yet
Examples of Cross-Cutting Features
(Dikes)
Which is older, the dike or the
sedimentary layers?
When a fault goes through a rock layer, it
shifts one side up or down (depending on
the type of fault)
A is a dike (igneous intrusion) and B is a
fault line. Put the following events in order
from oldest to youngest: Dike A, Fault B,
Layer D, Layer C
The correct order from oldest to
youngest is Layer D, Layer C, Fault B,
Dike A
Unconformities
Unconformity– long period when
deposition stops, then erodes, and then
deposition starts again
- (Basically a time break in the rock
record) where a layer is missing
- These develop when running water or
glaciers wash or wear away rock.
Unconformities
There are 3 main types of unconformities.
One of them is very difficult to identify in
the field.
Type 1: Angular Unconformity: tilted
rocks overlain by flat strata (beds); tops
of tilted rocks eroded
* These are pretty easy to identify. Look
for tilted rocks with flat rocks on top of
them
The yellow
line is the
angular
unconformity.
Unconformities
Type Two: Disconformity:
- not easily identifiable
- (usually shown with a wavy line)
- deposition ceased then started again
much later
- this gives a large age gap between 2
layers
- Geologists often need to use absolute
dating to find out if there is a
disconformity
Sediments deposited on the sea bottom, plates
moved the sea bottom up, deposition stopped,
erosion began then land sank and sediments
accumulated again forming a disconformity
Unconformities
Type Three: Nonconformity:
- new (igneous) rocks intrude from below
and displace older one
- These can include the following
igneous intrusions:
A. Dike: ~Vertical intrusion of magma
B. Sill: ~ horizontal intrusion of magma
C. Batholith: Very large magma feature
underground that never reaches
surface
Putting it all together: Put these in order
from oldest to youngest. Disconformity A,
Dike B, Layer C, Layer D, Layer E, Fault F
The correct order from oldest to youngest is:
Layer E (E is older than D because it is above it),
Layer D, Fault F (the fault is younger than A
because it does not go through it), Disconformity
A, Layer C, Dike B (B is youngest because it goes
through all other layers)