Understanding how the earth came to be
structured as it is -- a spherical metallic core surrounded by a series of
concentric layers -- requires an appreciation of the important role played by
heat in the planet's formation. Having
accumulated rapidly through three separate processes, all of which were most
intense during the first few hundred thousand years of the earth's history, the
internal heat energy of the planet was much greater in the early stages of its
formation than it is today. It is this heat energy that ultimately led to the
division of the earth's internal structure into the three main zones - the
core, the mantle, and the crust — that we know exist. This happened through the
migration of material of different densities to different depths within the
molten proto-planet.
Most scientists believe the planetary residents
of the solar system evolved from the accretion of solid material
derived from a large nebular cloud — the so-called Nebular Hypothesis. Under
this scenario, a barrage of impacting extraterrestrial material caused the
molten mass that was eventually to become the earth to grow in fits and spurts,
increasing in mass with each successive bombardment. As the proto-planet grew
in size, its increased gravitation field would have attracted even more
material from the surrounding cloud. These would have included metal-rich
objects(iron meteorites), rocky objects (stony meteorites),
and icy objects (comets). Such objects typically travel through the solar
system at great velocities — speeds measured in kilometers per second. Most of
the very large amount of kinetic energy they carry is instantly converted to
heat energy upon impact with an object as massive as a planet, thus providing
one important component of the nascent earth's internal heat supply.
In the early stages of planetary accretion, as
the process is referred to, the earth's mass was lower and its gravitational
field was much weaker. Therefore the material it was composed of was much less
compact than it is today. By adding mass, the accretion process led to a
correspondingly more intense gravitational attraction, forcing the earth to
compact to a higher and higher density. This increasing density resulted in the
conversion of gravitational energy into heat energy and further added to the total
amount of heat contained within the developing planet. Heat conducts very slowly
through rock, so the rapid build up of heat within the earth from these two
sources was not accompanied by an equally rapid dissipation of heat outward
through its surface.
And there was yet another source of energy; one
that, unlike the two already mentioned, is still operative today, though to a
lesser degree than it once was. Radioactive elements are inherently unstable,
breaking down over time to more stable forms. All such radioactive decay
processes release heat as a by-product. In its early stages of formation, the
young earth had a greater proportion of radioactive elements, many of which
were short-lived and have since decayed to near extinction. Those with
lengthier rates of decay are still undergoing this radioactive transformation
and are still generating heat energy in the process. Radioactive decay
accounts for much of the energy that keeps the inner layers of the planet
molten 4.5 billion years after its formation. The greater
complement of unstable elements in the early earth would have generated a
proportionately greater amount of heat energy in its initial stages of
formation.
The cumulative effect of all of the heat contributed from these three main
sources was the melting of the earth’s interior. Once liquefied, the various
components began to separate according to their relative density, with the
heavier(metallic) materials such as iron and nickel sinking toward the center
to form the core, and the lighter(silicate) materials floating upward toward
the surface to form the mental and crust. The lightest and most volatilematerials, which were derived from water and other types of ice delivered by
comets, were easily vaporized. These then rose beyond the earth’s rocky surface
to form the atmosphere, held in place by gravity until temperatures dropped to
the point where large amounts could precipitate back to the surface to form
the oceans.
