Earth's Interior & Plate Tectonics
Copyright © 1995,1996 by Rosanna L. Hamilton. All rights reserved.
A theory is a tool - not a creed. -J. J. Thomson
Table of Contents
Just as a child may shake an unopened present in an attempt to
discover the contents of a gift, so man must listen to the ring and
vibration of our Earth
in an attempt to discover its
content. This is accomplished through seismology, which has
become the principle method used in studying Earth's interior.
Seismos is a Greek word meaning shock; akin to earthquake,
shake, or violently moved. Seismology on Earth deals with the
study of vibrations that are produced by earthquakes, the impact
of meteorites, or artificial means
such as an explosion. On these
occasions, a seismograph is used to measure and record the
actual movements and vibrations within the Earth and of the
ground.
Types of seismic waves (GIF, local 15K)
(Adapted from, Beatty, 1990.)
Scientists categorize seismic movements into four types of
diagnostic waves that travel at speeds ranging from 3 to 15 kilometers
(1.9 to 9.4 miles) per second. Two of the waves travel around the surface of the
Earth in rolling swells. The other two, Primary (P) or compression
waves and Secondary (S) or shear waves, penetrate the interior of
the Earth. Primary waves compress and dilate the matter they
travel through (either rock or liquid) similar to sound waves.
They also have the ability to move twice as fast as S waves.
Secondary waves propagate through rock but are not able to travel
through liquid. Both P and S waves refract or reflect at points
where layers of differing physical properties meet. They also
reduce speed when moving through hotter material. These
changes in direction and velocity are the means of locating
discontinuities.
Divisions in the Earth's Interior (GIF, local 26K)
(Adapted from, Beatty, 1990.)
Seismic discontinuities aid in distinguishing divisions of the Earth
into inner core, outer core, D", lower mantle, transition region,
upper mantle, and crust (oceanic and continental). Lateral
discontinuities also have been distinguished and mapped through
seismic tomography but shall not be discussed here.
- Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 kilometers (3,219 - 3,981 miles)
The inner core is solid and unattached to the mantle, suspended in the
molten outer core. It is believed to have solidified as a result of
pressure-freezing which occurs to most liquids when temperature decreases
or pressure increases.
- Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 kilometers (1,806 - 3,219 miles)
The outer core is a hot, electrically conducting liquid within which
convective motion occurs. This
conductive layer combines with Earth's rotation to create a dynamo
effect that maintains a system of electrical currents known as the
Earth's magnetic field. It is also responsible for the subtle jerking
of Earth's rotation. This layer is not as dense as pure molten iron,
which indicates the presence of lighter elements. Scientists suspect
that about 10% of the layer is composed of sulfur and/or oxygen because
these elements are abundant in the cosmos and dissolve readily in
molten iron.
- D": 3% of Earth's mass; depth of 2,700-2,890 kilometers (1,688 - 1,806 miles)
This layer is 200 to 300 kilometers (125 to 188 miles) thick and
represents about 4% of the mantle-crust mass. Although it is often
identified as part of the lower mantle, seismic discontinuities suggest
the D" layer might differ chemically from the lower mantle lying above
it. Scientists theorize that the material either dissolved in the core,
or was able to sink through the mantle but not into the core because of
its density.
- Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 kilometers (406 -1,806 miles)
The lower mantle contains 72.9% of the mantle-crust
mass and is probably composed mainly of silicon,
magnesium, and oxygen. It probably also contains
some iron, calcium, and aluminum. Scientists make these
deductions by assuming the Earth has a
similar abundance and proportion of cosmic elements as found in
the Sun and primitive meteorites.
- Transition region: 7.5% of Earth's mass; depth of 400-650 kilometers (250-406 miles)
The transition region or mesosphere (for middle
mantle), sometimes called the fertile layer,
contains 11.1% of the mantle-crust mass and is the
source of basaltic
magmas. It also contains calcium,
aluminum, and garnet, which is a complex
aluminum-bearing silicate mineral. This layer is
dense when cold because of the garnet. It is buoyant
when hot because these minerals melt easily to form
basalt which can then rise through the upper layers
as magma.
- Upper mantle: 10.3% of Earth's mass; depth of 10-400 kilometers (6 - 250 miles)
The upper mantle contains 15.3% of the mantle-crust
mass. Fragments have been excavated for our
observation by eroded mountain belts and volcanic
eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene
(Mg,Fe)SiO3 have been the primary minerals found
in this way. These and other minerals are refractory
and crystalline at high temperatures; therefore, most
settle out of rising magma, either forming new
crustal material or never leaving the mantle. Part of
the upper mantle called the asthenosphere might be
partially molten.
- Oceanic crust: 0.099% of Earth's mass; depth of 0-10 kilometers (0 - 6 miles)
The oceanic crust contains 0.147% of the mantle-crust
mass. The majority of the Earth's crust was made
through volcanic activity. The oceanic ridge
system, a 40,000-kilometer (25,000 mile) network of volcanoes,
generates new oceanic crust at the rate of 17 km3
per year, covering the ocean floor with basalt.
Hawaii and Iceland are two examples of the
accumulation of basalt piles.
- Continental crust: 0.374% of Earth's mass; depth of 0-50 kilometers (0 - 31 miles).
The continental crust contains 0.554% of the
mantle-crust mass. This is the outer part of the Earth
composed essentially of crystalline rocks. These are
low-density buoyant minerals dominated mostly by
quartz (SiO2) and feldspars (metal-poor silicates).
The crust (both oceanic and continental) is the
surface of the Earth; as such, it is the coldest part of
our planet. Because cold rocks deform slowly, we
refer to this rigid outer shell as the lithosphere (the
rocky or strong layer).
The rigid, outermost layer of the Earth comprising the crust and
upper mantle is called the lithosphere. New oceanic lithosphere
forms through volcanism in the form of fissures at mid-ocean
ridges which are cracks that encircle the globe. Heat escapes the
interior as this new lithosphere emerges from below. It gradually
cools, contracts and moves away from the ridge,
traveling across the seafloor to subduction zones in a process
called seafloor spreading. In time, older lithosphere will thicken
and eventually become more dense than the mantle below, causing
it to descend (subduct) back into the Earth at a steep angle,
cooling the interior. Subduction is the main method of cooling the
mantle below 100 kilometers (62.5 miles). If the lithosphere is young and thus hotter
at a subduction zone, it will be forced back into the interior at a
lesser angle.
The continental lithosphere is about 150 kilometers (93 miles) thick with a low-density
crust and upper-mantle that are permanently buoyant.
Continents drift laterally along the convecting system of the mantle
away from hot mantle zones toward cooler ones, a process
known as continental drift. Most of the continents are now sitting
on or moving toward cooler parts of the mantle, with the
exception of Africa. Africa was once the core of Pangaea, a
supercontinent that eventually broke into todays continents.
Several hundred million years prior to the formation of Pangaea,
the southern continents - Africa, South America, Australia,
Antarctica, and India - were assembled together in what is called
Gondwana.
Plate tectonics involves the formation, lateral movement,
interaction, and destruction of the lithospheric plates. Much of
Earth's internal heat is relieved through this process and many of
Earth's large structural and topographic features are consequently
formed. Continental rift valleys
and vast plateaus of basalt are
created at plate break up when magma
ascends from the mantle to
the ocean floor, forming new crust and separating midocean ridges.
Plates collide and are destroyed as they descend at subduction
zones to produce deep ocean trenches, strings of volcanoes,
extensive transform faults, broad
linear rises, and folded mountain
belts. Earth's lithosphere presently is divided into eight large plates
with about two dozen smaller ones that are drifting above the
mantle at the rate of 5 to 10 centimeters (2 to 4 inches) per year. The eight large plates
are the African, Antarctic, Eurasian, Indian-Australian, Nazca,
North American, Pacific, and South American plates. A few of
the smaller plates are the Anatolian, Arabian, Caribbean, Cocos,
Philippine, and Somali plates.
References
Beatty, J. K. and A. Chaikin, eds. The New Solar System.
Massachusetts: Sky Publishing, 3rd Edition, 1990.
Press, Frank and Raymond Siever. Earth. New York: W. H. Freeman
and Company, 1986.
Seeds, Michael A. Horizons. Belmont, California: Wadsworth, 1995.
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