The Story of the Earth – The Principle of Superposition!

Theory says…

The world was originally a large ball of molten rock which cooled.

As it cooled…

Gasses developed making the atmosphere we have today. This atmosphere holds in the air and water around us making Earth livable.

Rain came causing oceans, lakes and rivers

Where/how did rain come from?

But where did the dirt and soil come from?

Why isn’t the world covered with just rock and water?

THE ROCK CYCLE!

But the rock cycle has done more than just give us dirt and soil, it is a book that tells us about our past.

How? Fossils!

UniformitarianismA fancy word meaning that the changes we see

in the world today has not changed. The same processes of melting, solidifying, erosion, weathering, heat and pressure that we see today are the same changes that have been in place since the Earth first cooled.

Uniform means the same

The Principle of Uniformitarianism

is the idea that the key to the past

is the present

2014

Paleontology

The study of past life through fossils.

Fossils

Are the remains or proof of existence of organisms (plants and animals) preserved by geologic processes.

There are two ways to tell the age of a fossil – absolute age and relative age

Absolute age means we know the age of the fossil, exactly.

Like your age, we know when your life began.

Index fossils help!

An index fossil is a fossil that we know has only existed during a certain period of time. So we know that layer’s age because that animal or plant was alive then!

Trilobites were common during the Paleozoic Era (540 to 245 mya); about half of the Paleozoic fossils are trilobites. They evolved at the beginning of the Paleozoic Era and went extinct during the late Permian period (248 million years ago).

Have Trilobites?

Then the trilobites and any of the sedimentary rocks and fossils located in that area were around during the Paleozoic Era (540 to 245 mya – million years ago)

How can we figure out….

….the age of an organism that is now dead? It isn’t like we know its birthday!

We compare it to plants that we know are older or younger. We call this relative age. We don’t know the exact age but we know how it compares to others.

An analogy….We may not know the age of a student in

elementary school but we know they are younger than most of the middle school students.

Layers of the Earth are like clothes in a basket

The oldest dirty clothes are on the bottom and the newer dirty clothes are on top!

Law of Superposition

We can tell the relative age of items by where they are located in layers of sedimentary rock and any index fossils.

Fossils in the same layers are usually about the same age. With the oldest on the bottom.

Original Horizontality

Why do we say usually and nearly the same age?

Because sometimes things happen:1) Earth can break, bend, push and pull gradually over time or

2) Tidal waves, tornadoes and earthquakes can change them quickly!

So how do we know the relative age of an item.

It’s like making layered brownies….

http://www.youtube.com/watch?v=ZKcSU80qvTE

Review Questions..

1) What do we use to help us determine the approximate age of rocks?

Fossils

2) How does uniformitarianism help us understand what rocks can tell us? Rocks have weathered, eroded, deposited and cemented over all time so we can make assumptions based on this.

3) How does the Law of Superposition help us understand the relative ages of rocks? Older rocks are on the bottom while younger rocks are on the top.

4) How does the Law of Original Horizontality help us understand the relative ages of rocks and fossils? Rock sediment (which forms sedimentary rock) settle initially into horizontal layers .

What should we remember?

1. Oldest layers are on the bottom because they were first.

2. Youngest layers are on the top.3. Fossils are found in layers which were

forming during their life.4. INDEX FOSSILS identify definite time.5. Intrusions in rocks happened after those

layers it penetrates

What is an index fossil?

An index fossil is an organism that we can use to determine the age of a layer because we know when it was alive!

Activity

You have eight index cards on your desk. 1. Each card stands for a layer of the Earth found in a sedimentary rock layer. 2. Each letter stands for a fossil found in that layer.

3. The letters are in order left to right from youngest fossil to oldest based on its location in the layer. Ex: NBU (N is the newest, U is the oldest.

Activity

Put these in order from newest to oldest (just like it would be on the sedimentary rock layer).

MDXONBUAGCT C T

AGC

UA

NBUNB

ONDXO

MD

This is one possible way to arrange the cards.

Now, answer the questions in your notes.

1) Which “fossil” (letter) is oldest?2) Which “fossil” is youngest?3) What fossil shows up the most?4) Which fossils were only shown once?5) Which could be index fossils?6) How can you tell if M or X is older?

Which “fossil” (letter) is oldest?

Which “fossil” (letter) is oldest?

T, because it is on the bottom layer to the right

Which “fossil” is youngest?

Which “fossil” is youngest?

M, because it is on the top layer to the left

What fossil shows up the most?

What fossil shows up the most?

N

Which fossils were only shown once?

Which fossils were only shown once?

M, X and G

Which fossils could be index fossils?

Which fossils could be index fossils?

M, X and G

How can you tell if M or X is older?

X is on a layer below M so it was deposited earlier and is the older of the two.