1. Earth’s History

2. How Earth “write” its history!
All sediment begin
somewhere else from
where we usually find it
Erosion = Eraser
Deposition = Deposits = Writing History
Igneous Intrusion = Writing
Eroded sediments end
Up somewhere else
Deposited in layers

3. So that we can be prepared…
Principle
of Uniformitarianism
To understand geologic
processes of the past,
study geologic processes of today.
“The present is the key to the past.”
WHY?
Catastrophic Events
e.g. Earthquakes & Volcanoes
- if it has happened before, can it happen again?
Environmental
- Climate Change
General Curiosity
***All this to help us understand how our planet got the way it is, and where it might be headed.
So that we can be prepared…

4. Strata – layers of rock
Sandstone Layers (all the same, but folded)
Colorful strata near
Capitol Reef National Park
Originally these layers were flat and horizontal but now they
Are folded and tilted…but they did start out flat

5. Law of Original Horizontality
Sedimentary rock layers were originally deposited in horizontal layers
Layers of volcanic ash becomes a part of the geologic record, and serve as excellent time markers
Volcanic ash

6. Sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.
Horizontal Layers can get squished (they won’t look horizontal anymore):
Mountain / Erosion
Body of Water
squeezing
squeezing

7. You can see the folds caused by tectonic forces
squeezing the layers of strata
Strata is often messed up some and can be a bit hard to “read”

8. Unconformities
- Indicates where layers of rocks are missing in the strata sequence.
- Strata may be missing because they were never deposited or deposited and later removed (erosion).
Main Types:
1. Disconformity - the boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface.
2. Nonconformity - An unconformity in which stratified (layers of rock) rest upon unstratified rock
3. Angular unconformity - the boundary between a set of tilted layers and a set of horizontal layers.
Image is from the website: Posters_i _.htm

9. How to tell the age of rock:
The youngest (age) layer
is on top!
How to tell the age of rock:
LAW of Superposition
Means: one thing placed upon another
Geologic Example: Layers of rock stacked by Deposition.
The cooled lava has formed a new rock
layer on top of the sandstone that
was already there
“Oldest rock layers are on the bottom”
Grand Canyon
Lava Cascades near Vulcan’s Throne

10. Relative Dating
Used to find the age of a fossil or rock compared to others!
This is not an exact method.
It does not give an exact year!
“Indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years.”
B) Index fossils
*If a particular species lived only in a certain time period then you know that whatever layer you find its fossil, that layer is the same age as the fossil

11. Comparing strata locations:
Notice that what you see on one side of the stream, you also see on the other side of the stream.
We figure that even though they are
not connected anymore, the layers on
either side of the river are the same!
The layers used to be connected,
but the river has cut them in half!
But we still understand that the layers are the same age!

12. Originally the layers continued all the way through:
If you find one layer in one place and you find the same layer on the other side of the river – it’s probably gonna be the same layer of sediments.
The layers were deposited in a shallow sea, but the sea dried up and a river cut part of the layers away
Layer C is the same rock unit separated by erosion
River
Law of Continuity of the Layers

13. Each fossil shown here is only found in the particular age that is shown:
We know how long ago these time periods are:
If you find one of these fossils in a layer of strata, you know when it lived and how old that layer of strata is!
Index Fossils
So if we found a scaphites fossil in the rock, what period and era would the rock be from?
Cretaccuous
Mesozoic Era

14. Where is the index fossil?
The layers of strata are
numbered along this side
Find the layer with
fossils that only
appears once
The types of fossils
Are indicated here
***The yellow spiral shell fossil only appears in this single layer. If we know when this critter lived, when know exactly when this layer was deposited! And we can make estimates of when the layers above and below were deposited as well!
Here it is!!!
Triassic Period

15. Why are layers sometimes “messed up”:
The soil and rock between these 3 locations has been eroded away.
Fill in the empty spaces:
Notice that layer D is totally missing in location 2
Something has caused this layer to disappear between location 2 and 4
There are two index fossils indicated here as well. They help
Us match the layers together!

16. Why are layers sometimes “messed up”: Volcanic “Intrusion”
“Intrusion” means that magma has
cut it’s way through other layers that
were already there! Now it has become igneous rock:
You can see the sandstone layers
that were already present (old)
This diagonal layer of
volcanic rock cut it’s way
through as magma (young)

17. Why are layers sometimes “messed up”: Faulting and Folding
Earthquake events can mix layers up and put them totally out of order.
Fault
Top
Bottom
The fold has caused what
should be the bottom
To become the top
The layers are
getting separated

18. An exception to the Law of Superposition
Here older rock is on
top of younger rock
YOUNG
Faults (tectonic forces)
Igneous Intrusions
Contact Metamorphism

19. Geological Structures
A fracture is a crack in rock.
A dike is a sheet of rock that formed in a fracture in a pre-existing rock body. Dikes can be either magmatic (igneous intrusion) or sedimentary in origin.
A fault is a fracture along which movement has occurred.
Fold is when sedimentary rock is bent or curved

20. The illustration below shows the most commonly used symbols to represent rocks on geologic maps and diagrams
You can also find these common map symbols on our rock scheme ID sheets

21. Law of Cross-Cutting
Any geologic event that affects a rock layer must be younger than the rock layer it affects
Layer X is an intrusion and is younger than any other rock unit.
This fault is also younger than any other rock unit

22. Relative Age
Based on what you now know about the Law of Superposition, the Principle of Original Horizontality, unconformities, and the Law of Crosscutting Relationships can you place the layers indicated in the diagram in the correct order, starting from the oldest layer?
The oldest layer is Q, followed by O, then N, then M, then L. P cuts across layers L-Q, so it is the next layer since it does not cut into layer H. Above the unconformity we then have layer H, followed by I, then J, with K being the youngest layer.

23. Imperfections in the Strata
Q: There are 15 events/features
Try to predict which layer is
next in the sequence
15 don’t forget the tree! 14
Imperfections in the Strata 13 10 9 7 2
Tilting=6 5 3
12? Hard to be sure… 11 4 8 1