~The Heat History of the Earth~
The earth is a heat engine. It remains geologically and biologically active, and evolves, because there are two great sources of energy. One source of energy is from the earth's molten core (that drives the geology), and the second is from the sun (that drives life and the atmosphere.)
Origin of the Concentric Layering
The earth grew from the accumulation of planetismals (meteorites and asteroids), over a period of 1-200 million years approximately 4.3 to about 4.5 billion years ago. A large mini-planet collided with the earth sometime after the earth had formed and because it hit the earth at an oblique angle, it spun off into an orbit around the earth to become the moon.
If the earth had grown from random accumulation of planetismals, it would be homogeneous - more or less made of the same material throughout, and the earth's cross section to would have no structure. However, the earth is stratified into layers (see right) by density (heaviest at the center) indicating that early in its history, the earth went through a molten stage that led to separation of heavy materials, which settled inward to form the core, and lighter materials, which floated toward the surface to form the crust. The heat for this melting came from meteorite impacts, the moon's impact, and the decay of radioactive elements.
All geological activity on the earth today is driven from this initial source of heat, aided by continued radioactive decay of elements in the earth's interior. The earth's heat engine worked approximately 3 times faster at the beginning of formation than it does now, indicating that the earth is cooling off. In the future, escaping heat with decrease with time until there the earth internal activity stops.
Cooling History of Planetary Bodies
By 4 billion years ago, the earth had cooled enough for the outer layers to have solidified and for oceans to form. A vast ocean covered the earth from pole to pole at this time, with volcanoes scattered, however, there were no continents. The figure to the right shows what the earth may have looked like 3.8-4.0 billion years ago. The oldest rocks found on the earth are 3.96 billion years old and contain evidence of sedimentary rocks that require water.
The earth cooled from the outside in, and the still molten iron-nickle core is all that remains from this early stage of melting and cooling. This heat is what keeps the earth geologically active, and without it, continents, volcanoes,mountains, oceans, or life would not exist. The earth would be similar to the moon or mars. There are several mechanisms by which a planet can lose heat:
1) The moon, for example,began losing heat immediately after formation and was geologically dead within a few hundred thousand years. The heat escaped two ways; one was through volcanic activity induced by meteorite impacts. The meteorites penetrated through the crust and into the core, and provided a conduit for magma to reach the surface and form the mare (smooth areas) we see from earth. This magma brought much heat to the surface to radiate to space. The second way heat escaped was through conduction. Rock is a poor conductor of heat, however, so in larger planetary bodies (like Earth) most of the heat must escape by other means.
2) Mars provides another example of a different way internal heat can be lost. Olympus mons, a large Martian volcano, is a hot spot, and it probably tapped directly into the core of the planet, allowing heat in the form of magma to flow directly to the surface. Hot spots are typically huge volcanoes, and Olympus mons is much bigger than any on earth. Mars is also less than half the size of the earth, which lent to an early demise.
3) A third way a planet can lose its heat is presence of convection cells in the mantle. That is, hot, low density material from the lower mantle, heated by the core, flows upward towards the surface where the heat escapes through volcanic activity. The cooled, now denser material then sinks back toward the core to be heated again. This creates a cycle of movement, but it is very slow, only centimeters per year, taking on the order of a billion years of so for a complete cycle.Typically many convection cells exist simultaneously all over the planet, and they lead to widely scattered volcanic activity, and plate tectonic processes.
Contributed by Lynn Fichter