The internal structure of the Earth has already been explained in my previous pages as the collisions of very massive asteroids
and planetesimals which accreted to form one large terrestrial planet. The internal structure of the Earth is also explained
through this process. As the collisions occur, partial melting results, leading to planet differentiation. The planet differentitation
lead to a highly silicate rich crust and mantle, and an iron rich outer and inner core.
The Mantle is a tricky concept for most people. Its not one large section of the Earth, but rather divided into several
different layers based on chemical and mechanical composition, as determined through geophysical seismic refractometry and
tomography. The Mantle, most simply, is made up of an Upper and Lower Mantle. The Upper Mantle, which ranges from approximately
70km - 670km in depth is further divided into Cold Upper Mantle, Low-Velocity Zone, and two transitional zone.
The Cold Upper Mantle is part of the lithosphere, which also includes the crust, and is almost welded onto the bottom
of the crust. The Low-Velocity Zone is a region where seismic P- and S-waves decrease in velocity because of its semi-molten
state and density. This is the only layer in the Mantle that is semi-molten, the rest is completely solid rock. The Low-Velocity
Zone is concurrent with the asthenosphere. A common misnomer is that velocity of waves increase as a result of a density increase.
Is is true and false. In fact its not the density that results in an increase of velocity, but rather an increase in elastic
moduli. This factor is a quantitative measure of the efficiently of the materials ability to transfer energy (waves) through
The transition zone, which occurs between the depths of 230km - 410km and 410km - 670km. The one that occurs between 230km
- 410km is dubbed the "Peridotite Layer" because of the high concentration of peridotite ultramafic igneous rocks.
The one that occurs from 410km - 670km is known as the "Spinel Layer" because of its transition between Beta-spinel
into Gamma-spinel. These divisions are all determined by a jump in seismic velocity.
The Lower Mantle ranges from 670km - 2880km in depth. It consists of Perovksite and Magnesioscuvstite minerals. The entire
Mantle, excluding the Low-Velocity Zone, is solid rock. However, due to its large mass and very hot temperatures, approximately
500 - 900 degrees Celsius at the upper boundary and 4000 degrees Celsius at its lower boundary, it is able to act like a puddy/wax
or is able to plastically deform. The differentials in pressure and temperature allow the Mantle to convect. Hot substances
are typically less dense then colder ones, which is why hot air rises. So the hotter material at the Mantle-Core Boundary
rises through the Mantle, until it reaches the surface, which is a spreading zone. It cools and begins to fall, pushing the
slab with it. This is the general model for Plate Tectonic Theory, albeit very simplified.
|Artist rendition on Mantle Convection, simplified.
Next: The Core: