Compositional Layers

The Earth is a sphere of radius 6371km which is stratified or layered. Compositional layers differ in chemical composition. The Earth has three compositional layers:

  1. The crust: low density silicate rock, 5-70 km thick. There are two distinct types of crust.
    1. Continental crust is variable in thickness and composition. Thickness ranges from 5-70 km. The composition ranges from mafic to felsic.
    2. Oceanic crust is uniform in thickness and composition. It is 5-6 km thick and is mafic in composition.
    3. 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.
  2. 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!
  3. 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!

Mechanical Layers

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. The inner core is solid. It has the same composition as the outer core, and is about half the diameter of the core.