Structure of the Earth

The structure of Earth is divided into three main layers: the lithosphere, mantle and core. The relative thickness of the thin, brittle lithosphere, the flexible mantle and the outer and inner core is comparable to the shell, white, and yolk of an egg. The characteristics of Earth's different layers are responsible for earthquakes, volcanoes and other natural phenomena.

Lithosphere

The outermost layer of Earth is known as the lithosphere (lithos means stone). The rigid, brittle lithosphere includes the crust and the upper portion of the mantle. It is the thinnest layer, comprising less than 1% of Earth's total mass.

There are two types of crust: continental crust and oceanic crust. Oceanic crust under the seas is thinner, averaging 7 km (5 miles) thick. It is mainly made up of basalt, a dense igneous rock. Continental crust is thicker, ranging from 35-70 km ( 22- 44 miles). It is less dense than the oceanic crust, and composed primarily of granite.

Because it is so thin and brittle, the lithosphere has broken up into many pieces like a jigsaw puzzle. These sections, or plates, move about on the mantle by a process called plate tectonics. The movement of plates relative to one another causes earthquakes, pushes up mountain ranges, and changes the shape of the surface in many other ways.

Asthenosphere and Mantle

Below the lithosphere lies the upper part of the mantle, known as the asthenosphere (asthene means weak). Temperature increases with depth below the surface, and the average temperature of the asthenosphere is about 1600 degrees C (3000 degrees F). The heat causes the rock to be softened and flexible and it flows very slowly, carrying the brittle lithospheric plates with it. The asthenosphere extends down to a depth of 660 km (400 miles).

The lower mantle begins at the base of the asthenosphere, and is 2300 km (1400 miles) thick. Its average temperature is higher than that of the asthenosphere, but since it is deeper, it is under greater pressure and is less flexible. The mantle as a whole makes up 82% of Earth's total volume.

Core

At Earth's center is the core, which is comprised of two parts: the liquid outer core and the solid inner core. The core is primarily composed of iron and nickel.

The intense heat of Earth's interior is generated by radioactive decay of naturally occurring isotopes, combined with residual heat left over from when the planet was first formed. The temperature of the core is estimated to be in the range of 4000-5000 degrees C (7000-9000 degrees F.)

The liquid outer core is 2300 km (1400 miles) thick, and the solid inner core has a radius of 1200 km (750 miles). While the temperature of the core is hot enough to melt iron, the tremendous pressure at the center compresses the inner core into a solid sphere. The outer core is not subjected to quite as much pressure, and is therefore liquid.

How do Scientists Know what is Inside the Earth?

People often ask, how do scientists know what it is like inside the Earth? It may be impossible to travel there, but there are indirect methods of measuring and observing Earth's interior.

Scientists have been able to map the different layers inside the Earth by studying the speed and direction of seismic waves generated by earthquakes. Certain types of seismic waves cannot pass through liquids, and analyzing the path these waves follow through Earth's interior provides information about the location and composition of the different layers. Laboratory experiments studying iron at extremely high temperature and pressure have provided estimates of the temperature of the core.

References:

Tarbuck, Edward J. and Lutgens, Frederick K. (2006). Earth Science. 11^th edition. New Jersey: Pearson Prentice Hall.

http://www.scientificamerican.com/article.cfm?id=why-is-the-earths-core-so

http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.html