Earth, Page 25: [Previous Page] [Next Page] [Class Home Page] Textbook pages 116 - 118

Earth's Interior

The earth is divided into four layers: the inner core, outer core, mantle, and crust, brought about by differentiation. The core is composed mostly of nickel-iron (Ni, Fe). The inner core remains solid under extreme pressure. Most of the mass is in the mantle, composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. The mantle is solid at 1000C but can deform slowly in a plastic manner. The crust is thin and composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. The crust is cold, rocky and brittle, and fractures in earthquakes. The world's deepest hole


Exploring the Earth's Core
How was the Earth's core discovered? Seismic waves from earthquakes gave the first clue.

Seismic waves will bend and reflect at the interfaces between different materials. The two types of seismic wave behave differently, depending on the material. Compressional P waves will travel and refract through both fluid and solid materials. Shear S waves, however, cannot travel through fluids like air or water. Fluids cannot support the side-to-side particle motion that makes S waves. P stands for "Primary" because P waves arrive first (fastest), S stands for "Secondary" because they arrive second.

What is Differentiation?
Different earth materials have different densities. In a fluid medium, heavier material sink while lighter materials rise, thus sorting themselves by density. Thus the Earth's interior is zoned by density, the heaviest materials at the center (the core), and the lightest stuff at the top (the crust). This process continues until either all materials are sorted appropriately, OR the Earth becomes solid and all floating and sinking must stop.

At first, the Earth was entirely 'liquid', giving us the initial separation of Crust, Mantle and Core. However today, only parts of the mantle and the outer core is still liquid enough to allow true differentiation.