The Structure of the Earth
Geophysics, which studies the
physics of the Earth, has led to many significant discoveries about the Earth
and its make-up. Seismologic studies of the Earth have uncovered new
information about the interior of the Earth that has helped to give credence to
plate tectonic theory.
Geophysical studies have revealed that the Earth has several distinct layers.
Each of these layers has its own properties. The outermost layer of the Earth
is the crust. This comprises the continents and ocean basins. The crust has a
variable thickness, being 35-70 km thick in the continents and 5-10 km thick in
the ocean basins. The crust is composed mainly of alumino-silicates.
The next
layer is the mantle, which is composed mainly of
ferro-magnesium silicates. It is about 2900 km thick, and is separated into the
upper and lower mantle. This is where most of the internal heat of the Earth is
located. Large convective cells in the mantle circulate heat and may drive
plate tectonic processes.
The last layer is the core, which is separated into the liquid outer core and the solid inner core. The outer core is 2300 km thick and the inner core is 1200 km thick. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost entirely composed of iron. Earth's magnetic field is believed to be controlled by the liquid outer core.
The Earth
is separated into layers based on mechanical properties in addition to
composition. The topmost layer is the lithosphere, which is comprised of the crust
and solid portion of the upper mantle. The lithosphere is divided into many
plates that move in relation to each other due to tectonic forces. The
lithosphere essentially floats atop a semi-liquid layer known as the asthenosphere. This layer allows the solid
lithosphere to move around since the asthenosphere is much weaker than the
lithosphere.
The Crust
Because
the crust is accessible to us, its geology has been extensively studied, and
therefore much more information is known about its structure and composition
than about the structure and composition of the mantle and core. Within the
crust, intricate patterns are created when rocks are redistributed and
deposited in layers through the geologic processes of eruption and intrusion of
lava, erosion, and consolidation of rock particles, and solidification and
recrystallization of porous rock.
By the
large-scale process of plate tectonics, about twelve plates, which contain
combinations of continents and ocean basins, have moved around on the Earth's
surface through much of geologic time. The edges of the plates are marked by
concentrations of earthquakes and volcanoes. Collisions of plates can produce
mountains like the Himalayas, the tallest range in the world. The plates
include the crust and part of the upper mantle, and they move over a hot,
yielding upper mantle zone at very slow rates of a few centimeters per year,
slower than the rate at which fingernails grow. The crust is much thinner under
the oceans than under continents (see figure above).
The
boundary between the crust and mantle is called the Mohorovicic discontinuity
(or Moho); it is named in honor of the man who discovered it, the Croatian
scientist Andrija Mohorovicic. No one has ever seen this boundary, but it can
be detected by a sharp increase downward in the speed of earthquake waves
there. The explanation for the increase at the Moho is presumed to be a change
in rock types. Drill holes to penetrate the Moho have been proposed, and a
Soviet hole on the Kola Peninsula has been drilled to a depth of 12 kilometers,
but drilling expense increases enormously with depth, and Moho penetration is
not likely very soon.
The Mantle
Our
knowledge of the upper mantle, including the tectonic plates, is derived from
analyses of earthquake waves (see figure for paths); heat flow, magnetic, and
gravity studies; and laboratory experiments on rocks and minerals. Between 100
and 200 kilometers below the Earth's surface, the temperature of the rock is
near the melting point; molten rock erupted by some volcanoes originates in
this region of the mantle. This zone of extremely yielding rock has a slightly
lower velocity of earthquake waves and is presumed to be the layer on which the
tectonic plates ride. Below this low-velocity zone is a transition zone in the
upper mantle; it contains two discontinuities caused by changes from less dense
to more dense minerals. The chemical composition and crystal forms of these
minerals have been identified by laboratory experiments at high pressure and
temperature. The lower mantle, below the transition zone, is made up of
relatively simple iron and magnesium silicate minerals, which change gradually
with depth to very dense forms. Going from mantle to core, there is a marked
decrease (about 30 percent) in earthquake wave velocity and a marked increase
(about 30 percent) in density.
The Structure of the Moon
The Moon,
our fellow-traveler in space, has a diameter half that of the Earth's core, and
it revolves around the Earth, as all the planets revolve around the Sun, under
the force of gravity. Moonquakes of very low energy are caused by land tides
produced by the pull of Earth's gravity, and, from analysis of moonquake data,
scientists believe the Moon has two layers: a crust, from the surface to 65
kilometers depth, and an inner, more dense mantle from the crust to the center
at 3,700 kilometers. The crust is presumed to be com- posed primarily of rocks
containing feldspar, calcium aluminum silicate, and lesser pyrox- ene, iron and
magnesium silicate; the crust also contains basalt in the mares, which con-
tains less iron and more titanium than earth basalt. The mantle is thought to
be made up of calcic peridotite, containing both pyroxene and feldspar.