Download File

What's below our feet? What's way below?
If we could cut Earth open, we'd see the following layers from inside to outside: inner
core, outer core, mantle, and crust. Alternatively, you can think of the brittle lithosphere
riding on the plastic asthenosphere.
Layers by Composition
The layers scientists recognize are pictured below.
A cross section of Earth showing the following layers: (1) crust (2) mantle (3a) outer
core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core.
Core, mantle, and crust are divisions based on composition:
1. The crust is less than 1% of Earth by mass. The two types are oceanic crust and
continental crust.
2. The mantle is hot, dense, dark rock. It represents about 68% of Earth's mass.
3. The core is mostly iron metal. The core makes up about 31% of the Earth. Earth's
metallic core has two layers: a solid inner layer and a liquid outer layer.
Layers by Mechanical Properties
Lithosphere and asthenosphere are divisions based on mechanical properties:
1. The lithosphere is composed of both the crust and the uppermost mantle. The
lithosphere is a brittle, rigid solid. It is easily cracked or broken. The lithosphere is
made up of the crust and the uppermost mantle.
2. The asthenosphere is below the lithosphere. The asthenosphere is also in the
upper mantle. This layer is solid, but it can flow and bend. A solid that can flow is like
silly putty.
Crust
Crust, mantle, and core differ from each other in chemical composition. It's
understandable that scientists know the most about the crust, and less about deeper
layers. Earth’s crust is a thin, brittle outer shell. The crust is made of rock. This layer is
thinner under the oceans and much thicker in mountain ranges.
Oceanic Crust
There are two kinds of crust. Oceanic crust is mostly made of basalt lavas that flow
onto the seafloor. It is relatively thin, between 5 to 12 kilometers thick. Thick layers of
mud cover much of the ocean floor.
Continental Crust
Continental crust is much thicker than oceanic crust. It is 35 kilometers thick on
average, but it varies a lot. Continental crust is made up mostly of granite. On average,
continental crust rises higher above the mantle than oceanic crust.
Mantle
Beneath the crust is the mantle. The mantle is made of hot, mostly solid rock. They
know this because of seismic waves, meteorites and the heat that comes from inside
the planet. Mantle rock is rich in oxygen, silicon, and magnesium.
Heat flows from warmer objects to cooler objects. The lower mantle is heated directly by
conduction from the core. Hot lower mantle material rises upward. As it rises, it cools.
At the top of the mantle it moves horizontally. Over time it becomes cool and dense
enough that it sinks. Back at the bottom of the mantle, it travels horizontally. Eventually
the material gets to the location where warm mantle material is rising. The rising and
sinking of warm and cooler material is called convection.
Outer Core: Is this what the core looks like?
Yes! The outer core, anyway. The outer core is liquid iron and nickel, like in this photo.
Of course, the metal is under an incredible amount of pressure.
Inner Core
The dense, iron and nickel core forms the center of the Earth. Scientists know that the
core is metal. The inner core is solid, and the outer core is molten. Here are some of the
reasons they know this:

Metallic meteorites are thought to be from the same type of material that is found at
the core.

Scientists calculate Earth's density from the planet's rotation. To match the total
density, the inner layers must be denser than the outer layers. They must be as
dense as metal.

Seismic waves show that the outer core is liquid. The inner core is solid.

For there to be a magnetic field, there must be liquid metal. The metal must be
convecting. If the core did not have convecting metal, there would be no magnetic
field.