INTERNAL STRUCTURE OF THE EARTH
Based on a tremendous number of analyses of earthquakes, carried out
globally for nearly a century, the following basic structure of the interior
of the earth has emerged. Below a thin crust up to 35 miles thick, lies
the mantle, some 1800 miles thick; whereas the deepest zone, the core constitutes
the last 2200 miles to the center of the earth.
The crust
The uppermost of these zones, the crust, ranges in thickness from 3 to
35 miles. It is made of two types of material: one type which makes up
continents
is
granitic
in composition (composed of materials rich in silicon, oxygen, aluminum,
sodium and potassium), and has a specific gravity of about 2.7.
Below the granitic crust and underneath oceans where there is no
granitic crust, the crust is basaltic, richer in iron, magnesium
and calcium, and has a specific gravity of about 3.0. The crust
is the thinnest of the three basic layers of the earth, and only represents
some 1.5% of the earth's volume and less than 1% of its mass. Of the two
types, continental crust is the thicker, up to 35 miles. Oceanic crust
is usually thinner than 5 miles.
The mantle
Separating the crust and the mantle is a boundary called the Mohorovicic
discontinuity. Below the Moho or M discontinuity (as it is often abbreviated)
lies the mantle. It comprises over 80% of the earth's volume and nearly
70% of its mass. The uppermost portion of the mantle is solid and rigid,
part of the lithosphere, with a density near 3.5 g/cm3,
and is primarily composed of iron and magnesium silicates (silicates are
a group of materials rich in the elements silicon and oxygen). As the temperatures
rise with depth, there is a zone in the upper mantle where materials
begin to melt, the asthenosphere. Beyond this partially molten zone,
the mantle is mostly solid because the higher pressures prevent melting.
The lower mantle is composed of oxides (combinations rich in oxygen)
rather than silicates, and densities approach 6 g/cm3. The upper
and lower mantles are separated by a transition zone. Another major discontinuity,
the Gutenberg-Weicher (G/W) discontinuity, separates the mantle
from the core.
The core
It is generally accepted that the core is composed of two zones, both of
nickel/iron composition. The high temperatures of the outer core
(estimated to be near 2500C) keep it liquid. Again, as was the case
for the mantle, even though the inner core is hotter than the outer
core, the pressure also rises and prevents the inner core from melting.
Therefore, the inner core is solid. The circulation of the outer
core coupled with the earth's rotation generates the major part of the
earth's magnetic field. Some 16% of the earth in volume, the core accounts
for over 30% (32.4) of its mass because of its high density which ranges
from 10 to 13 g/cm3 and averages near 11 g/cm3.
In sum, based on seismic information, the overall structure of the earth
consists of three basic layers, the crust, mantle and core, greatly differing
in composition and density. These layers represent relict features of an
early mobile earth that density zoned itself. The lighter materials rose
to the top to form the crust, and the heavier iron settled to the center
to form the core, while intermediate materials formed the mantle. |