Earth’s Earth’s HistoryHistory

But…there is no book!But…there is no book!

Oh…rats…Oh…rats…

Why do we want to know Earth’s Why do we want to know Earth’s History?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…

How does Earth “write” its history?How does Earth “write” its history?Erosion = Eraser

Deposition = Deposits = Writing History

Igneous Intrusion = Writing

All sediment begin somewhere else from

where we usually find it

Eroded sediments end Up somewhere else Deposited in layers

Q: How do we see Earth’s Q: How do we see Earth’s history?history?

A: StrataStacked layers of rock and

soil.

Each layer is made of slightly different stuff which make

them look just a bit different:

There are at least three different layers in this picture:

Normally, the layers are found flatAnd horizontal Like these

But sometimes they can be tilted like these

or even have their order totally mixed up as we will see!

They

go

like

this

Main Concept: We can use layers of sediment to determine what the area used to be like and

even figure out what has happenedin the area over periods of time.

Horizontal Layers in a Depositional Horizontal Layers in a Depositional EnvironmentEnvironment

Body of Water

Sedimentary Rock Layers

Layers of Sediment are deposited one upon the other. Clearly The layers on the bottom were deposited first.

More examples of strata:More examples of strata:

Sandstone with folded layers made of the exact same

material

Colorful strata near Capitol Reef National Park

You can see that there are many different layers

in this picture

Can you see the fold?

Originally these layers were flat and horizontal but now theyOriginally these layers were flat and horizontal but now theyAre folded and tilted…but they did start out flatAre folded and tilted…but they did start out flat

Horizontal Layers can get squished (they Horizontal Layers can get squished (they won’t look horizontal anymore):won’t look horizontal anymore):

Body of Water

Mountain / Erosion

squeezing squeezing

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”

Q: How to tell the age of rock:Q: How to tell the age of rock: A: A: SuperpositionSuperposition

• 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

The youngest (age) layer is on top!

How to tell the age of rock:How to tell the age of rock:

Relative DatingRelative 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 livedonly in a certain time periodthen you know that whatever layer you find its fossil, that layer is the same age as the fossil

• Notice that what you see on one side of the stream, you also see on the other side of the stream.

Comparing stratalocations:

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!

Originally the layers continued all the Originally the layers continued all the

way through: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 depositedIn a shallow sea, but the sea Dried up and a river cut part

Of the layers awayRiver

Index FossilsIndex 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…

Where is the index fossil?Where is the index fossil?

Find the layer with fossils that only

appears once

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!

The layers of strata are numbered along this side

The types of fossilsAre indicated here

Can you match up the layers?Can you match up the layers?The soil and rock between these 3 locations has been eroded away.

Fill in the empty spaces:

There are two index fossils indicated here as well. They help Us match the layers together!

Notice that layer D is totally missing in location 2

Something has caused this layer to disappear between location 2 and 4

Why are layers sometimes “messed up”:Why are layers sometimes “messed up”:

Why are layers sometimes “messed up”:Why are layers sometimes “messed up”: Volcanic “Intrusion” Example:Volcanic “Intrusion” Example:

“Intrusion” means that magma has cut it’s way through other layers that

were already there! Now it has become igneous rock:

This diagonal layer of volcanic rock cut it’s way

through As magma after the sandstone was already there (young).

You can see the sandstone layersThat were already present (old)

Why are layers sometimes “messed up”:Why are layers sometimes “messed up”: Faulting and FoldingFaulting and Folding

• Earthquake events can mix layers up and put them totally out of order.

Top

Bottom

The fold has caused what should be the bottom

To become the top

The layers are getting separated

Faul

t

Imperfections in the StrataImperfections in the Strata

Sometimes the strata is difficult to read!

Q: Can you follow the order?Q: Can you follow the order? There are 15 events/featuresThere are 15 events/featuresTry to predict which layer isTry to predict which layer isNext in the sequence before Next in the sequence before

You click the button!You click the button!

1

2

3

4

5Tilting=6

7

8

9

10

1112? Hard to be sure…

13

1415 don’t forget the tree!