The Solar System and the Planet Earth 309
since the last ice age terminated only ten thousand years ago. Given the
fact of global warming due to the human consumption of fossil fuels it is
possible there will not be any more ice ages.
The Structure of the Earth
The structure of the Earth is not as easy to study as one might imagine.
The surface of the Moon, almost half a million km away is easier to
study than the interior of the Earth. Man has traveled to the surface of the
Moon to collect samples, but has never penetrated more than a few km
below the surface of the Earth. It is not an easy matter to dig a very deep
hole. Space travel is simpler.
The basic tool for studying the interior structure of the Earth are
earthquakes, particularly the propagatum of the shock waves produced
by these large scale movements of rocky material within the Earth’s
crust. Earthquakes release the tremendous amounts of energy generated
by the stresses that develop within the crust. Thousands of earthquakes
take place every year. Most of these are minor tremors, which release
very little energy and cause virtually no damage. Major earthquakes,
however, are quite frequent and cause quite a lot of damage. The largest
earthquakes release as much energy as a hydrogen bomb and produce in
their wake gigantic tidal waves known as tsunamis, which propagate
from one side of the Pacific Ocean to the other.
Shock waves from the earthquake propagate through the interior of
the Earth. By studying the arrival time of the shock waves at various
seismographic stations across the globe one learns of the internal
structure of the Earth. It was in this way that geologists discovered that
the very center of the Earth is a metallic core surrounded by a rocky
mantle upon which the crust is situated. The radius of the Earth is
approximately 6400 km. The radius of the iron core on the other hand is
only 3500 km, which means the rocky mantle is 2900 km thick. The iron
core consists of an inner and outer core. The outer core is molten and the
inner core due to the immense pressure is solidified. The material
forming the inner and outer cores is probably the same, consisting
basically of iron with small amounts of elements such as nickel, cobalt,
silicon and sulphur. The density of the core ranges for 9.5 to 13.5 grams
per cubic centimeter, the inner core being densest part of the Earth.
The density of the mantle increases with depth from 3 to 6 grams
per cubic centimeter. The mantle consists primarily of oxides of silicon,