Earth’s Interior

• Scientists believe the Earth formed 4.6 billion years ago.

• When it first formed , it was a spinning mass of rocks and dust that was loosely held together.

• Over time, many comets and asteroids crashed into its surface and added to its mass.

• Impacts, radioactive decay and gravity produced intense heat.

• It was a young planet, a glowing ball of melted rock.

• In time, the denser materials such as iron and nickel sank towards the center.

• Less dense materials rose to the surface, while other materials settled in between.

• Earth’s main layers formed.

How were the layers of Earth formed?

There are two different types of layers of the earth:

1. Compositional layers

These are the layers of the earth based on what the layers are made of.

2. Structural layers

These are based on how the layers behave.

Layers of the Earth (Compositionally)

The crust is composed of many elements, but the top three are oxygen (47%), silicon (28 %), and aluminum (8 %).

The mantle is composed of many elements, but the top three are oxygen (45 %), silicon (22 %), and magnesium (23%).

The outer and inner core are composed of iron and nickel.

Crust

• Thin, outermost layer

• Contains all three types of rock

• Thickness: 8-32 kilometers

• Thicker under the continents; thinner under the oceans

The crust of the earth is composed of bedrock continental crust and oceanic crust.

Continental bedrock crust is made of granite (an igneous rock) and is found under the continents and the continental shelf. Continental crust is less dense than oceanic crust.

Oceanic bedrock crust is made of basalt (an igneous rock) and is found under the oceans. Oceanic crust is more dense than continental crust.

Granite Basalt

Mantle

• Layer of the earth right under the crust

• 80% of earth’s volume is the mantle

• The boundary between the crust and the mantle is called the Moho or Mohorovičić discontinuity

• Because of the heat and pressure in the mantle, the solid rock tends to flow like a liquid (This is called plasticity.)

Outer Core

• Found under the mantle

• Liquid iron and nickel

USE THIS SLIDE FOR THE COMPOSITIONAL DRAWING.

Inner Core

• Beneath the outer core

• Solid (very dense) iron and nickel

• May be responsible for the earth’s magnetic field called the magnetosphere

How Hot is the Inside of the Earth ?

Layers of the Earth (Structurally)

The lithosphere is the crust and upper mantle and is very rigid. The tectonic plates are composed of the lithosphere.

The asthenosphere is the middle portion of the mantle. It is the plastic layer that the tectonic plates move on.

The mesosphere is the lower part of the mantle and is rigid.

The outer and inner core are the same structurally and compositionally. The outer core is liquid; the inner is solid.

USE THIS SLIDE FOR THE STRUCTURALLY DRAWING.

How do we know about the different layers of the earth?

The answer simply:

Seismic waves

So how do seismic waves tell us about the layers of the earth?

Seismic waves are shock waves produced by earthquakes and are recorded by a seismograph.

seismograph

seismogram

There are three types of seismic waves:

1. P (primary waves)

Scientists know:

- they move straight down from the point (focus) of the earthquake

- that P waves move quickly through a solid, and slowly through a liquid

- they are compression waves

2. S (secondary) waves

Scientists know:

- they move straight down from the point (focus) of the earthquake

- that S waves move quickly through a solid, and CANNOT move through a liquid

- they are transverse waves

3. L (surface) waves

Scientists know:

- they move along the surface of the earth

- “rolling” waves called Rayleigh waves

- “twisting “ waves called Love waves

So scientists study the speed of waves and the fact that S waves cannot move through a liquid to make determinations about the layers of the earth.

And this is what scientists found …..

When P and S seismic waves reached certain points in the earth, some of them stopped and others slowed down.

And then when the P and S waves reached other points in the earth, they would speed up again.

So scientists concluded:The outer core of the earth is liquid because S waves could not move through itThe inner core of the earth is solid because the P waves sped up again.