How Volcanoes Work - Earth's Internal Heat, Energy, and Interior Structure
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Alexa Internet
has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the
Wayback Machine
after an embargo period.
The Wayback Machine - https://web.archive.org/web/20110303023207/http://www.geology.sdsu.edu:80/how_volcanoes_work/Heat.html
THE EARTH'S INTERNAL
HEAT ENERGY
AND INTERIOR STRUCTURE
THE EARTH'S HEAT FURNACE
The Earth's internal heat source
provides the energy for our dynamic planet, supplying it with
the driving force for plate-tectonic motion, and for on-going
catastrophic events such as earthquakes and volcanic eruptions.
This internal heat energy was much greater in the early stages
of the Earth than it is today, having accumulated rapidly by heat
conversion associated with three separate processes, all of which
were most intense during the first few hundred thousand years
of the Earth's history: (1) extraterrestrial impacts, (2) gravitational
contraction of the Earth's interior, and (3) the radioactive decay
of unstable isotopes.
EXTRATERRESTRIAL IMPACTS
Most
scientists believe that our solar system evolved from the accretion
of solid particles derived from a large nebular cloud - the so-called
Nebular Hypothesis. Under this scenario, proto-planet Earth would
have grown over time from a barrage of extraterrestrial impacts,
increasing its mass with each bombardment. As the proto-planet
grew in size its increased gravitational field would have attracted
even more objects its surface. The composition of these colliding
bodies would have included metal-rich fragments (i.e..,
iron
meteorites
), rocky fragments (i.e.,
stony meteorites
),
and icy fragments (i.e.,
comets
). Although accretion was
much more prevalent in the early stages of the Earth's history,
these extraterrestrial collisions are still occurring today, exemplified
by shooting stars and fireballs in the night sky, and by the occasional
impact of larger bodies on the Earth's surface.
Such particles travel at great
velocities, typically ~30,000--50,000 km/hr, similar to that of
the Earth as it rotates around the Sun. The very large amount
of kinetic energy inherent in these moving bodies is instantly
converted to heat energy upon impact, thus providing a component
to the Earth's internal heat source.
GRAVITATIONAL CONTRACTION
In the early stages of planetary
accretion, the earth was much less compact than it is today. The
accretionary process led to an increasingly greater gravitational
attraction, forcing the Earth to contract into a smaller volume.
Increased compaction resulted in the conversion of gravitational
energy into heat energy, much like a bicycle pump heats up due
to the compression of air inside it. Heat conducts very slowly
through rock, so that the rapid build up of this heat source within
the Earth was not accommodated by an equally rapid loss of heat
through the surface.
DECAY OF RADIOACTIVE ELEMENTS
Radioactive
elements are inherently unstable, breaking down over time to more
stable forms. The unstable isotope Uranium-238, for example, will
slowly decay to Lead-206. All such radioactive decay processes
release heat as a by product of the on-going reaction. In its
early stages of formation, the young earth had a greater complement
of radioactive elements, but many of these (e.g., aluminum-26)
are short-lived and have decayed to near extinction. Others with
a more lengthy rate of decay and are still undergoing this radioactive
process, thus still releasing heat energy. The greater complement
of unstable elements in the early Earth thus generated a greater
amount of heat energy in its initial stages of formation.
MELTING AND COMPOSITIONAL
DIFFERENTIATION OF THE EARLY EARTH
The heat buildup inside earth
reached a maxim early in the Earth's history and has declined
significantly since. The greater heat content of the early Earth
was the product of (1) a greater abundance of radioactive elements,
(2) a greater number of impacts, and (3) the early gravitational
crowding. The initial accretion of particles resulted in a rather
homogeneous sphere composed of a loose amalgam of metallic fragments
(iron meteorites), rocky fragments (stony meteorites), and icy
fragments (comets). However, the increased heat content of the
early Earth resulted in melting of the Earth's interior, so that
the young planet became density stratified with the heavier (metallic)
materials sinking to the center of the earth, and the lighter
(rocky) materials floating upward toward the surface of the earth.
The very lightest volatile materials (derived from comets) were
easily melted or vaporized, rising beyond the earth's rocky surface
to form the early oceans and the atmosphere. We now have a differentiated
earth due to melting and mobilization of materials driven by the
earth's internal heat engine. This has resulted in the development
of a series of concentric layers that are both density and compositionally
stratified. This demonstrated in the diagram below,
courtesy
of the USGS.
|
These layers include (1) the dense
inner core
composed largely of solid Fe and subordinate
Ni, with radius of about 1200 km, (2) the molten
outer core
composed largely of liquid Fe, with subordinate sulfur, with
a radius of about 2250 km, (3) the
mantle
, composed of
relatively dense rocky materials, with radius of about 2800 km
thick, and (4) the
crust
which comprises the thin relatively
light outer skin of the earth, is divisible into two types: the
oceanic crust
(~7 km thick) and the
continental crust
(about 35 km thick). Whereas oceanic crust is composed of
basaltic rock, the less dense continental crust is composed of
a great variety of rock types having an overall average composition
akin to granite.
|
Within the mantle exists the
asthenosphere
(Grk. asthenos = weak), between about 100 km and 350 km, which
is a special zone composed of hot, weak material that is capable
of gradual flow. The layer above the asthenosphere is the
lithosphere
(Grk. lithos = rock), the rigid and relatively cool outer layer
of the earth, composed of both crust and a portion of the upper
mantle.
Lying above the lithosphere is
(1) the liquid
hydrosphere
, comprising 71% of the Earth's
surface, and (2) that the still lighter gaseous
atmosphere
,
both of which were ultimately derived from the accretion of comets.
The occurrence of these volatile components along the outermost
portion of the Earth is a product of volcanic outgassing during
the differentiation event.