Global Properties

1. The Earth is differentiated
   1. The bulk density of Earth is 5.52 g/cm³; the density of typical rocks is around 3 g/cm³. Hence the interior has to consist of much denser material than the surface material.
   2. Seismology tells us that there are discontinuous changes in the rigidity and sound speed of the material in the Earth's interior.

2. The surface is young

   Earth is the largest of the terrestrial planets, so we may expect that it should cool more slowly and be geologically active longer than the other terrestrial planets.

   1. Radiometric dating of surface rocks show that the crust of Earth is much younger than that of the moon, the only other body for which we have reliable radiometric dates (we may have a dozen or so Martian rocks in the form of meteorites, which range from 180 million years for shergottites to 1.3 billion years for Nakhlites, up to 4.5 billion years for ALH84001, the rock causing all the fuss about life on Mars). Rocks up to about 4 billion years old have been found on Earth, but most crustal material is around 100 million years old.
   2. There are only about 130 known impact craters on the surface of Earth. From radar mapping of Venus we know of several thousand impact craters on that planet, and photographs of the Moon, Mercury, and to a lesser extent Mars reveal large numbers of impact craters. Three examples of terrestrial impact craters are Barringer (Arizona, USA), Wolf (Astralia), and Manicouagan (Quebec).

3. The surface is active

4. The mantle supports shear stresses but is deformable. A shear stress is a applied tangentially to a body; solid bodies support shear stresses, while liquids and gasses do not. We know that the mantle supports shear stresses because we observe shear waves after earthquakes. A shear wave involves the displacment of part of a solid body relative to the rest; an example is the plucking of a guitar string. The restoring force supplied by the tension in the string causes the string to move back to its unperturbed shape (a straight string). Sound or compression waves are also excited in earthquakes. In a compression wave the restoring force is supplied by the internal pressure. Sound waves can propagate in fluids and in solids, whereas shear waves only propagate in solids.
5. The core is fluid and does not support shear stresses, as shown by the lack of detections of shear waves at the antipodes to large earthquakes, while compression waves are seen at these locations. By mapping out the "shadow" of the core, seismologists have determined that there is a fluid outer core with a radius of about 3,500km. Indirect arguments suggest that the inner part of the core is solid.

Geologic Characterstics

The USGS provides a nice introduction to plate tectonics at this url.

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