Earth

Facts about Earth

A. The Earth formed in the inner solar nebula. It passed through the four stages of planetary development that other solid planets also experience to varying degrees. These four stages are:

1. Differentiation: The earth was originally molten, at which time matter separated according to density: the heavier iron sank to the core, while the lighter silicate minerals "floated” to the surface, thus forming the crust.
2. Cratering: The solid surface is bombarded with debris from the solar system.
3. Flooding: Decay of radioactive elements heats planet's interior, causing lava to well up through fissures in the crust and flood deeper basins. As the planet cools, water falls as rain and floods basins to form oceans.
4. Surface Evolution: Plate tectonics and erosion change surface features slowly.

B. The Earth's interior is differentiated. Because the density of the entire earth is 5.52 g/cm(3), and the crust is much less dense, the interior is made of very heavy elements. It is divided into four areas.

1. The crust is very thin relative to the radius of the Earth, only 35-60 Km deep. In fact, with respect to the size of the Earth, it is proportionally thinner than the skin on an apple.
2. The mantle is a layer of dense rock, which is very hot, and under a lot of pressure. The heat and pressure make the rock plastic, or malleable. The mantle is denser than the crust, which floats on it. As the mantle moves, the crust floating on it also moves, causing earthquakes.
3. The core has two regions, a liquid core and a solid core. The interior of the planet can be explored by monitoring shock waves from earthquakes. As the surface crust is dislocated, the shock of the motion spreads through the Earth. There are two types of waves that result. S-waves are shaking waves, like the shaking of jello. P-waves are pressure waves like sound waves. P-waves will travel through all materials, but S-waves only travel through solids. When an earthquake strikes one part of the Earth, S and P waves are felt nearby, but P-waves are also felt on the other side of the Earth. Therefore, the center of the Earth must be liquid. This liquid core is made of molten iron and nickel and has a density of about 14 g/cm(3), compared to 3.0 g/cm(3) for the crust and 4.4 g/cm(3) for the mantle. In the inner core, the pressure is so great that the iron and nickel become solid again.

C. Plate tectonics is the process by which the crust of the planet changes. The plastic mantle, which is always moving due to convection currents, causes the plates to move and rub against one another. A region where one plate is forcing another downward is called a subduction zone.

D. The Earth's atmosphere is called a secondary atmosphere. That is, the Earth did not have the atmosphere it has now when it was originally formed. The primeval atmosphere was rich in carbon dioxide, nitrogen and water vapor. During the flooding stage of planetary development, volcanoes bellowed up large amounts of gasses. The carbon dioxide was dissolved into the Earth's oceans and turned into carbonate rocks, thus removing it largely from the atmosphere. The atmosphere received its oxygen from green plants.

E. The Earth wobbles a tiny amount in its rotation about its axis. This wobbling is called precession, and takes about 26,000 years to complete one circle.

F. The Earth's moon is unique in the solar system in that it is so large relative to the Earth.

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