Earth’s Interior
How were the layers of Earth formed?
• 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.
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.
Granite
Basalt
Oceanic bedrock crust is made of basalt (an igneous rock) and is found under
the oceans. Oceanic crust is more dense than continental crust.
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
seismograph
Seismic waves are shock waves produced by
earthquakes and are recorded by a seismograph.
So how do seismic waves tell us about the layers
of the earth?
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
- “twisting “ waves called Love waves
- “rolling” waves called Rayleigh 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 it
The inner core of the earth is solid because the P waves sped up again.