Previous: Lecture Fourteen
(Chp. 8 in Christiansen and Hamblin; Chp. 6 in Beatty and Chaikin)
- The Earth is differentiated
- 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.
- Seismology tells us that there are discontinuous changes
in the rigidity and sound speed of the material in the Earth's
- 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.
- 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.
- 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
Barringer (Arizona, USA),
Wolf (Astralia), and
- The surface is active
- 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.
- 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.
The USGS provides a nice introduction to plate tectonics at
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