Download Internal Structure of the Earth File

Seismic records have led geologist to believe that the Earth has a layered
structure, and can be thought of as being a bit like a cracked egg.
The solid crust is broken up into
smaller bits, called plates, which float
on a dense mantle. Parts of the
mantle are molten liquid and
movements in this liquid cause the
plates to drift into one another. It is
the meeting of the plates that causes
the earthquakes.
The density of the material forming
the Earth increases with depth.
Nuclear reactions within the solid
inner core maintain the high
temperatures of the body of the
Earth.
The outer core is liquid and large
convection currents within this layer
give rise to the Earth’s magnetic field.
P-Waves
The speed of both types of waves
increase as they travel through the
Earth’s interior.
However, the speed rapidly changes
when they cross between layers of
material.
S-Waves
The speed of the p waves
decrease considerably when
encountering the outer core, but S
waves are stopped altogether.
A
B
Direct
P-wave
shadow
zone
The diagram above shows the paths taken by P and S waves as they travel through the
interior of the Earth.
 Point A - Both P and S waves are gradually refracted as they travel and therefore
follow curved paths as they spread out from the epicentre. This indicates that the
density of the material within a layer gradually increases with depth.
P and S waves are detected up to angles of 1030 from the epicentre.

Point B - At certain points the refraction becomes more pronounced as the
waves meet a definite boundary between layers.
No S waves can travel through angles greater than 1030 hence the boundary at B
must separate solid from liquid. This is the evidence that the outer core is made of
a dense liquid.

P waves entering the liquid outer core slow down and are focused (like waves
travelling through a lens) into the region at the bottom of the diagram (angles greater
than 1400). A shadow zones for direct travelling P waves exists between 1030 and
1400, however, weak P waves are detected in this region due to the refraction of the
inner core.
1 Show P + S waves
curving through the mantle.
(Not interacting with the
outer core)
2. Show S-waves forming a
shadow zone.
3. Show P-waves refracting
through core.