Page 6, Review of our Solar System [Previous Page] [First Page] [Class Home Page] Text Pages 124-125

The earth is divided into four layers: the inner core, outer core, mantle, and crust. The core is composed mostly of iron (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.

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.



Earth's Magnetic Field
Paleomagnetic records indicate that the geomagnetic field has existed for at least three billion yrs. Based on the size and electrical conductivity of the Earth's core, the field would decay away in only about 20,000 years. Also, the dipole polarity has reversed many times in the past. A mechanism within the Earth's interior must continually generate the geomagnetic field. This mechanism is a convective dynamo operating in the Earth's fluid outer core.