1. GEOLOGIC HISTORY THE RULES OF THE GAME
© Copyright   M. J. Krech. All rights reserved.

2. Time is… Either relative or absolute
Time is: a measured period during which an event, process, or condition exists or continues.

3. So….How do scientists tell how old rocks are anyway?
Scientists use fossils to help determine the age of rock layers.
The age of a fossil can be determined by carbon dating:
Carbon exists in all living things
Carbon decomposes at a specific rate that can be calculated
Scientists can measure how much carbon a fossil has left
in it in order to figure out how long it has been dead.
4. This is an example of absolute age (dating).
Some fossils can be used as index fossils:
These fossils are used to determine the approximate age of the
layer of rock that they are found in.
In order to be an index fossil they have to:
Have only existed for a certain short amount of time.
Have to have lived in many different places across the globe.

4. Dating Techniques
One ways in which geologists look at time: Relative Dating

5. Relative Dating is..
…the measure of a sequence of events without knowing the exact date at which the events occurred.
What happened first?
What happened next?

6. UNIFORMITARIANISM Relative Age: The age of a rock layer compared to
the other rock layers around it. ( Younger or older )
Absolute Age: The age in years (definite age….14 years old)
The rock layer that
forms first is found on
the bottom of the pile
and is the oldest layer.
The rock layer that is
on top of the pile is the
youngest.
This is called:
The Law of
Superposition.
SANDSTONE
Youngest Layer
SHALE
LIMESTONE
Oldest Layer

7. Principle of Superposition
Q#1
Which rocks are older?
Which rocks are younger?

8. The Law of Cross-cutting Relationships
When a layer cuts
across another layer
in the sequence, it has
to be younger than the
layer that it cuts apart.
Limestone
Shale
This igneous intrusion
is younger than the
conglomerate, sandstone
and shale because it
cuts through each of
those layers.
Sandstone
Granite
Intrusion
Conglomerate
This is called:
The Law of Cross-cutting Relationships

9. Principle of Cross-Cutting
Q#2
What is younger?
The rocks or the fault line?

11. Sediments laid down in layers over time lithified into rock Tilting by
Erosion of exposed
Uplifted areas
Uplift 3. 3. 3. 2. 3. 2. 2. 1. 1. 1. 5. 3. 4.
New layers of sediment
laid down on top
of the tilted and
eroded layers 3. 2. 1.

12. Principle of Inclusions
Rock layers which contain other rocks are younger than the inclusion.
The
inclusion
(the other
rocks) 
is older!

13. Principle of Inclusions
Q#3
Which is older? The rock layer or the rocks included in the layer?

14. Principle of Unconformities
Unconformities are surfaces that represent gaps in the geologic record.

15. Principle of Unconformities
missing rock layer = missing time

17. Where's the missing sediment?
Q#4
How do
you
know?

18. Where's the missing sediment?
Q#5
Where's the missing sediment?
How do
you
know?

19. What happened first?
How do
you
know?

20. Where is the youngest rock?
How do
you
know?

21. Where is the unconformity?
How do
you
know?

22. What rock layer is the oldest?
How do
you
know?

23. Which is younger? The fault or the rock layers?
How do
you know?

24. Does this diagram show an unconformity?
How do
you know?

32. BASALT SHALE SANDSTONE
GRANITE
LIMESTONE
SHALE
SANDSTONE
RHYOLITE
METAMORPHIC
CONGLOMERATE