http://www.c3.lanl.gov/~cjhamil/SolarSystem/earthint.htm (Einblicke ins Internet, 10/1995)
Earth's Interior & Plate Tectonics
by Rosanna L. Hamilton, Copyright © 1995.
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 the package 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 which are produced by earthquakes, the impact
of meteorites, or artificial means
such as an explosion. On these
occasions a seismograph may be used to measure and record the
actual movements and vibrations within the Earth and of the
ground.
Types of seismic waves (GIF, 15K)
Seismic movements have been categorized into four types of
diagnostic waves which travel at speeds ranging from 3 to 15 km
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, 26K)
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 have also 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 km.
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 km.
The outer core is a hot electrically conducting liquid
within which convective
motion occurs. This
combined with the Earth as a rotating body creates a
dynamo effect which maintains the system of
electrical currents known as 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 about 10% of sulfur
and/or oxygen because of their abundance in the
cosmos and due to the fact that they would dissolve
readily in molten iron.
- D": 3% of Earth's mass; depth of 2,700-2,890.
This layer is 200-300 km 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 may differ
chemically from the lower mantle lying above it.
Theories suggest the material either dissolved in the
core at some point or because of its density, was
able to sink through the mantle but not into the core.
- Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 km.
The lower mantle contains 72.9% of the mantle-crust
mass and by deduction contains mainly silicon,
magnesium, and oxygen. It probably also contains
some iron, calcium, and aluminum. These
deductions are made by assuming the Earth has a
similar abundance of cosmic elements as found in
the Sun and primitive meteorites (including by
inference other planets) and according to the
proportions found thereon. It is amazing what
scientists can learn through deduction, inference,
elimination and assumption.
- Transition region: 7.5% of Earth's mass; depth of 400-650 km.
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 and 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 km.
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 may be
partially molten.
- Oceanic crust: 0.099% of Earth's mass; depth of 0-10 km.
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-km-long network of volcanoes,
generates new oceanic crust at the rate of 17 km^3
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 km.
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 and as such is the coldest part of
our planet. Since 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 midocean
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 as on a conveyor
belt traveling across the seafloor to subduction zones, 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 100km. 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 km 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 Pangea, a
supercontinent that eventually broke into todays continents.
Several hundred million years prior to that the southern
continents: Africa, South America, Australia, and Antarctica, as
well as 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. Today Earth's lithosphere 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 cm 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.
Earth