Week IIA. The Rock Cycle: The Layers of Earth
(student reading)
Most of the EarthÕs rocky surface
–the crust -- is just a thin layer on the EarthÕs
surface. It makes up the continents and the ocean floors. It is thinnest under
the oceans (6–11 km) and thickest under the largest mountain ranges (up
to 70 km).
You can get a sense of how thin
the rocky crust is by drawing a large circle; the EarthÕs crust is
proportionately even thinner than the width of the circleÕs chalk line. You can
also peel an apple – in this case the thin peel represents the crust.
Continental crust and oceanic
crust differ. The continental crust is 33
km thick and light, so it floats on the lower layers. It is made up largely of
granite and sedimentary layers. It is much deeper under mountains. The oceanic
crust is thinner (10 km), more dense, and made of basalt, a darker volcanic
mineral.
From USGS
The crust plates slide slowly
about on a much thicker thick layer of rock called the mantle. The mantle is about 2,900 km thick and makes up
about 80% of the planetÕs entire volume. How do scientists know when they have
reached the mantle? The rock in the mantle is so hot that it is often partially
molten.
In a volcanic explosion, or in a
situation in which rocks crack, material comes from the mantle through the crust
and cools on the surface.
Under the mantle in the EarthÕs core is a dense ball of elements such
as nickel and iron. Temperatures within the core reach 3,700¼C.
lithos is Greek for
ÒrockÓ or Òstone,Ó and sphairais
Greek for ÒglobeÓ
How Do Scientists Know?
Listening to Waves as They Pass Through Earth Materials
When there is an earthquake,
scientists have an opportunity to listen to the way the energy waves of the
earthquake travel through different parts of the Earth's crust, mantle, and
core. Adding waves together from many earthquakes, they generate a
three-dimensional model of the Earth.
One type of seismic wave can only
move through solid rock and not through liquid. It moves particles up and down
or side to side.
Another type of wave generated by
an earthquake moves in the direction it is being "pushed". This wave goes through both liquids and
solids.
Sonar can generate waves through
the Earth, too, for nearby materials
http://www.geo.mtu.edu/UPSeis/making.html