The Earth seems solid beneath your feet, except when you come to a geyser or volcano or body of water. But it isn't solid all the way through.
 
The Earth is made up of different layers, as you can see in this graphic.

Another good cut-away graphic of Earth's layers can be found at:
http://ontwikkel.thinkquest.nl/~ll125/en/fullstruct.htm 


The Layers of the Earth

Let's start at the surface and move downward.
 
The Crust
 
The uppermost layer, the layer we walk around on, is called the Crust. It is made up of rock that floated to the surface when the Earth was formed.

It is not a continuous layer, but is made up of large masses called tectonic plates. Tectonic means "moving," and these plates are moving all the time -- although you don't feel it except during earthquakes or volcanic eruptions. We'll tell you more about tectonic plates in our next Instruction.
 
Actually, there are two parts to the Earth's Crust -- a land part and a water part.
 
The land part is called the Continental Crust. It is mostly between 10 to 25 miles thick, although it goes up to 45 miles thick in some places. It is divided into six continents. These continents are Eurasia (Europe and Asia), Africa, North America, South America, Antarctica and Australia.
 
The water part of the Earth's Crust is at the bottom of the oceans. It is called the Oceanic Crust and is between 4 and 7 miles thick.


The Mantle

The next layer down is called the Mantle.
 
The Mantle is also divided into two parts, like the Crust. These parts are the Outer (or Upper) Mantle and the Inner (or Lower) Mantel.
 
The Upper Mantel is made up of silicates of iron and magnesium. It is firm on top with liquid rock on the bottom. Scientists often refer to the top part of the Upper Mantle and the Crust together as the Lithosphere.
 
The Lithosphere is the coldest, most brittle of Earth's layers. It floats on the lower, liquid part of the Upper Mantle (the Asthenosphere) like a marshmallow on hot chocolate.
 
Beneath the Upper Mantle lies the Inner (or Lower) Mantle. It is the layer closest to the Earth's Core.
 
 
The Core

The center of the Earth is called the Core.  It is a dense metallic ball made mostly of iron, with some nickel. It, too, is made up of two parts -- the Inner Core and the Outer Core.

The Outer Core begins about 1,800 to 3,200 miles beneath the Earth's surface. It is made up of iron, nickel, sulfur and oxygen. It is cooler than the Inner
Core -- about 7,200 to 9,032 degrees Fahrenheit -- so it remains liquid.

The Inner Core is about 780 miles thick. It is from 3,200 to 3,960 miles beneath the Earth's surface. It is very hot -- about 10,000 degrees Fahrenheit. But the pressure is so great that it remains solid.

In addition to iron and nickel, scientists think it contains sulfur, carbon, oxygen, silicon and potassium.

Because this Core is so hot, it radiates currents of heat upward toward the upper layers. These currents are called convection currents.

It is these convection currents that cause the tectonic plates on the Earth's surface to move. And it is this movement of tectonic plates that causes volcanic eruptions, earthquakes and mountain building.

Reading List

Title, Author	Description from the California Dept. of Education
Pebble in My Pocket: A History of Our Earth (Hooper, Meredith)	http://www.cde.ca.gov/ci/sc/ll/ap/details.asp?id=403
Shaping the Earth (Patent, Dorothy Hinshaw)	http://www.cde.ca.gov/ci/sc/ll/ap/details.asp?id=1049
Exploring Caves: Journeys into the Earth (Aulenbach, Nancy Holler)	http://www.cde.ca.gov/ci/sc/ll/ap/details.asp?id=1049

Now let's do Practice Exercise 1-1 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.