Earth

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Earths History

• Butthere is no book!
• Ohrats

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Why do we want to know Earths History?

• Catastrophic Events
• If it has happened before, can it happen again?
• Environmental/Climate Changes
• Global Warming?
• Earthquake activity
• Where are the active earthquake areas?
• Building Project Safety
• Where can we build our homes without them being
  destroyed?
• 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
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How does 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
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Q How do we see Earths history?
Main Concept We can use layers of sediment to
determine what the area used to be like and
even figure out what has happened in the area
over periods of time.
There are at least three different layers in
this picture

• A Strata
• Stacked layers of rock and soil.
• Each layer is made of slightly different stuff
  which make them look just a bit different

Normally, the layers are found flat And
horizontal Like these
They go like this
But sometimes they can be tilted like these or
even have their order totally mixed up as we
will see!
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Horizontal Layers in a Depositional Environment
Layers of Sediment are deposited one upon the
other. Clearly The layers on the bottom were
deposited first.
Body of Water
Sedimentary Rock Layers
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More examples of strata
Colorful strata near Capitol Reef National Park
Can you see the fold?
You can see that there are many different layers
in this picture
Sandstone with folded layers made of the exact
same material
Originally these layers were flat and horizontal
but now they Are folded and tiltedbut they did
start out flat
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Horizontal Layers can get squished (they wont
look horizontal anymore)
Mountain / Erosion
Body of Water
squeezing
squeezing
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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
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Q How to tell the age of rock A Superposition
The youngest (age) layer is on top!

• 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
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How to tell the age of rock Relative Dating

• This is not an exact method.
• It does not give an exact year!
• Used to find the age of a fossil or rock
  compared to others!

A) Comparing Strata Locations
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
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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!
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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
  its 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
River
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Index Fossils

• 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!

Any layer of strata we find this particular
fossil in we know must have been deposited in
the Jurassic Period which was years ago
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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
Here it is!!!
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!
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Can you match up the layers?
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!
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Why are layers sometimes messed up Volcanic
Intrusion Example
Intrusion means that magma has cut its 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 its
way through As magma after the sandstone was
already there (young).
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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
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Imperfections in the Strata
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15 dont forget the tree!
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Q Can you follow the order? There are 15
events/features Try to predict which layer
is Next in the sequence before You click the
button!

• Sometimes the strata is difficult to read!

10
9
7
2
Tilting6
5
3
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12? Hard to be sure
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1
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