Before discussing the formation of the major geological units of the solid Earth, we should review the internal structure of our planet as described by seismic wave studies (Fig. 11.1).

The most important discontinuities observed by seismologists are:

• The base of the crust (called the Mohorovičić discontinuity or Moho), 40 km below the continents, but only 5–7 km beneath the oceans.

• The base of the lithosphere, on average 80 km below the oceans, and deeper still beneath the continents. Rather than a discontinuity, this is a rather diffuse transition. This is the lower boundary of the rigid tectonic plates. The softer part of the upper mantle underneath the lithosphere is called the asthenosphere. The 410 km discontinuity corresponding to a change in olivine structure (spinel or ringwoodite phase). The 660 km discontinuity corresponding to the transformation of all minerals into perovskite and minor Fe–Mg oxide (magnesio-wüstite). This is the base of the upper mantle.

• The mantle–core boundary at about 2900 km. Above this boundary is a seismically abnormal layer some 200 km thick, known as the D”layer.

• The outer core–inner core boundary at about 5150 km. The core is composed mostly of metallic iron and nickel. The motion of the fluid outer core generates the Earth's magnetic field.

Plate tectonics is a powerful theory that unifies the geological expression of crustal and upper-mantle geodynamics (Fig. 11.2). The Earth's surface is covered with rigid lithospheric plates that may or may not carry continents.