The Earth is a sphere of radius 6371km which is stratified or
layered. Compositional layers differ in chemical composition. The Earth has three
- The crust: low density silicate rock, 5-70 km thick. There are two distinct types
- Continental crust is variable in thickness and composition. Thickness
ranges from 5-70 km. The composition ranges from mafic to felsic.
- Oceanic crust is uniform in thickness and composition. It is 5-6 km
thick and is mafic in composition.
- The differences in thickness and density between continental and oceanic are responsible
for the existence of ocean basins due to isostatic balance as the crust
floats on the more dense mantle.
- The mantle: high density, ultramafic silicate rock which can
flow when subjected to long duration stresses. The mantle is over 2900 km thick and makes
up over 80% of the volume of the Earth. The mantle is not molten!
- The core: iron and nickel, liquid outer region with a solid center. The core is
just over half the diameter of the Earth.
These compositional layers have sharp or abrupt boundaries between them.
Whole earth composition is estimated from unbiased samples of meteorites. Earth
structure is obtained by combining this with seismic data.
Motion of liquid iron and nickel in the outer core gives the Earth a dipole
magnetic field, nearly aligned with the rotational axis. The magnetic field of the Earth
reverses spontaneously at random times. Over the last several million years, the average
time between reversals has been about 200,000 years. The last reversal was 730,000 years
ago. Reversals probably take less that 5,000 years. Reversals of the field probably
involve a period of time where the field weakens substantially and becomes disorganized
(non-dipole), then reorganizes in the opposite polarity. People should wear lead underwear
during a reversal, as the Earth's surface will be bombarded with a higher than normal
amount of cosmic radiation!
In addition to the compositional layers, the Earth has mechanical layers. Mechanical
layers differ in their strength or rigidity. These layers do not correspond on a
one-to-one basis with the compositional layers. The Earth has five mechanic layers:
- The lithosphere is the outermost mechanical layer and is the most rigid
layer of the Earth. The lithosphere consists of the crust, and some of the uppermost
mantle. The lithosphere averages about 100 km thick. It is somewhat thicker beneath
continents, and dramatically thinner under mid-ocean ridges.
- The asthenosphere lies beneath the lithosphere. It is a part of the mantle,
approximately 100 km thick, with very little strength. The asthenosphere flows relatively
easily and accomodates the movement of the overlying lithosphere. The upper and lower
boundaries of the asthenosphere are diffuse as they involve gradual changes in the
rigidity of the mantle, not a change in composition.
- The lower mantle or mesosphere consists of most of the
mantle. This part of the mantle flows, but at much slower rates than the asthenosphere.
- The outer core is liquid iron (with some nickel and other elements).
This is the only internal layer of the Earth that is a true liquid. The core-mantle
boundary is the one mechanical boundary that is also a compositional boundary. Movement of
the electically conductive fluid in the outer core generates the Earth's magnetic field.
- The inner core is solid. It has the same composition as the outer core,
and is about half the diameter of the core.