Scientists can use the speed of the waves
generated by earthquakes to determine what the
interior of the Earth is made of. The speed of
an earthquake wave (also called seismic
waves) depends on the density of the rock it
is traveling through. Higher density means the
waves will travel slower, lower density means the
waves will travel faster. So, if you know the
speed of a seismic wave, you could use that to
find the density of the rock and then draw a
conclusion about the composition of the rock.
Seismic waves were also used to determine
the state (i.e. solid or liquid) of the
interior layers of the Earth. Earthquakes
generate four different types of waves; there are
waves that travel through the Earth, called
P-waves and S-waves, and waves that travel along
the surface, called Love waves and Rayleigh waves.
P-waves can travel through any kind of material,
solid or liquid. S-waves however can only travel
through solids, they cannot travel through liquid.
This is how we discovered that the outer core of
the Earth is actually liquid- there were no
S-waves traveling through it. The other layers,
the crust, the mantle and the inner core, are all
So, seismic waves can be used to find both
the composition of the rock (what kind of rock it
is) and the state of the material (is it solid or
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