Earth’s Interior

The best way to find out what’s inside Earth might be to dig a tunnel to its center—but that isn’t possible.

Scientists use indirect observations to gather clues about Earth’s interior.

1. Seismic waves

Depending on the speed of the waves and the paths they take, geologists uncover clues about the inside of the Earth.

Waves can: speed up, slow down, bend, or stop

2. Rock Clues

Since many rocks form far below Earth’s surface, scientists can study what the rocks are made of and make inferences about Earth’s interior.

Earth’s Layers

Scientists theorize that the earth is made up of 4 layers…

1. The Inner Core

It is solid, dense, and made of iron. There is great pressure in the inner core.

5,000º C, hottest, most pressure

2. The Outer Core

It is made of molten (liquid) metal.

3. The Mantle

It is the largest layer; it is solid, but flows slowly, like putty.

4. The Crust

It is the thinnest layer, but its thickness varies…

It is thinnest under the oceans. This is oceanic crust.

It is thickest under the continents. This is continental crust.

All features on Earth’s surface are part of the crust.

Pressure and temperature increase as depth increases.

EARTH’S PLATES

The lithosphere is made up of the crust and the topmost part of the mantle.

The lithosphere is broken into about 30 sections, or plates.

These plates “float” on the asthenosphere, also part of Earth’s mantle.

Earth’s plates vary greatly in size and shape.

The plates move slowly, often taking more than a year to creep a few centimeters.

Lasers and satellites are used to measure these small plate movements.

The places where the edges of different plates meet are called plate boundaries.

The constant movement of Earth’s plates creates forces that affect Earth’s surface at those boundaries.

PLATES THAT MOVE APART

Plates move apart because of tension (pulling forces).

When plates move apart, gaps form between the plates.

The gaps are filled by magma that is pushed up and which cools to form new crust.

The places where this new crust is formed are called mid-ocean ridges.

PLATES THAT COLLIDE

When plates collide, the outcome depends on the density of the plates.

Oceanic crust is more dense than continental crust.

If two continental plates collide, there is a pile up of crust; this is called compression.

If two oceanic plates collide, or if an oceanic and a continental plate collide, the denser one goes to the bottom.

This is called subduction.

PLATES THAT SLIDE PAST

Shearing: When a force pushes something in two different directions.

A transform boundary is a place where two plates that are moving past one another meet.

Shearing at a transform boundary causes faults and earthquakes to occur.

WHY DO PLATES MOVE?

There are only theories about why plates move.

The theory of convection currents is that movements are caused by differences in density.