Geology Reference
In-Depth Information
Similar to the core below it, the mantle has layers that respond differently to the move-
ment of earthquake waves:
Mesosphere:
In the lower mantle, surrounding the outer core, temperatures are
high enough to melt rock, but the intense pressures found so deep in the earth
keep mantle materials solid. This deep, solid part of the mantle is called the
meso-
sphere.
This layer of the mantle begins about 660 kilometers (410 miles) below
earth's surface and continues to where it meets the outer core. Temperatures in
the mesosphere range from 3,000 degrees F to almost 8,000 degrees F near the out-
er core.
Asthenosphere:
The upper part of the mantle is called the
asthenosphere
and exhibits a special physical property. Mantle rocks are solid, but they flow in a
way that you may associate with a thick liquid. When solid materials flow in this
way, it's called
plastic flow.
I describe plastic flow further in Chapter 13, where I
discuss glaciers (which also move in this way).
The asthenosphere is found starting at about 200 kilometers (124 miles) from the
earth's surface and extends down to the mesosphere. The heat and pressure in
this layer can lead to some melting. This zone of the mantle is considered the
weak zone; thus its name
astheno,
which means “weak” or “soft.”
Lithosphere:
The uppermost portion of the mantle is attached to the underside of
the earth's crust, and together they make up the
lithosphere.
This portion of the
mantle is approximately 100 kilometers (62 miles) thick, very rigid, and
brittle
— it
breaks and cracks rather than bends or flows. Physically, this part of the mantle is
just like the rocks of the crust it is attached to (a brittle solid). But this layer is
still mantle because of its mineral composition. In other words, when scientists
classify the earth's layers based on their composition, they separate the upper
portion of the mantle from the crust because they are composed of different min-
erals. But when scientists classify the earth's layers based on physical properties,
both the uppermost part of the mantle and the earth's crust are part of the brittle
lithosphere.
Collecting moon rocks
The moon is not made of cheese after all. When NASA astronauts visited the moon during the Apollo
missions of 1969 to 1972, they collected samples of moon rock material for scientists to test. It turns out
the rocks of the moon have a composition similar to the earth's mantle. The moon rocks — even those
from deep within the moon — are similar to basalt (see Chapter 7) with almost no heavy metals. (The
astronauts collected surface rocks and also drilled core samples.)
