Scientists think that the Earth was created about 5 billion years ago.
 
That is quite late in the history of the Universe, which -- as we told you in our last Instruction -- is thought to have begun about 15 million years ago with an almost unimaginable explosion called The Big Bang.
 
At first, the Earth was just a ball of molten rock and gasses. As it began to cool,
dense materials like iron sank down into its core. Lighter materials like compounds of oxygen and water rose toward the surface.
 
That's why the Earth is made up of different layers -- as you can see in this cross section:

There are three layers in the Earth: the Crust, the Mantle and the Core.
 
For another way to look at them, click: http://mediatheek.thinkquest.nl/~ll125/en/fullstruct.htm 
 
 
The Crust
 
The outer layer of the Earth 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. These plates drift slowly across the Earth's surface (tectonic means moving).
 
The movement of these plates creates mountains and valleys.

At weak points in the crust, it causes volcanic eruptions.
 
And when plates bump into each other, earthquakes occur -- emitting
shock waves or vibrations called seismic waves.
 
The crust is the Earth's coldest layer. There are two parts to it -- the Oceanic Crust and the Continental Crust.
 
 
The Oceanic Crust
 
The Oceanic Crust lies beneath the oceans. It is between 4 to 7 miles (6 - 11 km) thick. Its rocks are heavy and young, not more than 200 million years old. They are mostly basalt, which has a gritty volcanic structure.
 
 
The Continental Crust
 
71% of the surface of the Earth is water. The remaining 29% is land. This land is called the Continental Crust.
 
It is divided into six parts. These parts are called continents.
 
At the present time, the continents are Eurasia (Europe & Asia), Africa, North America, South America, Antarctica and Australia. But since the Earth is always changing, the continents are too.
 
The Earth's crust is thicker beneath the continents than it is beneath the oceans. The Continental Crust is between 20 to 25 miles (30 to 40 km) thick -- with a maximum of 45 miles (70 km) thick.
 
It is older, too. Some of its rocks are 3.8 billion years old. They are mostly igneous rocks, which means they were formed from molten magma or lava pushed up from beneath the Earth's surface.
 
The Continental Crust is divided into two layers. The upper layer is mostly granite, while the lower layer is primarily basalt and diorite (a kind of rock formed from the impurities in granite).
 
 
The Mantle
 
The layer of rock just below the Earth's crust is called the Mantle.
 
It contains most of the Earth's mass -- 80% of it.
 
It is divided into two parts: the Inner Mantle and the Outer Mantle.
 
 
The Inner Mantle
 
The Inner Mantle is nearest the Earth's Core. It is sometimes also called the Lower Mantle.
 
It is between 190 miles (300 km) and 1,800 miles (2,890 km) below the Earth's surface.
 
Scientists think it is made up of sulfides and oxides of silicon and magnesium. Its temperature is 5,400 degrees Fahrenheit (3,000 degrees Celsius) -- but even at that temperature the rock is solid because it is under such tremendous pressure.
 
 
The Outer Mantle
 
The Outer Mantle is between 7 and 190 miles (10 - 300 km) beneath the Earth's crust.
 
Its temperature is between 2, 520 and 5,400 degrees Fahrenheit (1,400 to 3,000 degrees Celsius).
 
It is much thinner than the Inner Mantle.
 
Like other things we are talking about today, the Outer Mantle is made up of two layers.
 
The bottom layer is tough liquid rock that is probably made up of silicates of iron and magnesium. Scientists sometimes call it the Asthenosphere.
 
The upper layer is made of the same stuff, but it is firmer because it is cooler.
Scientists often refer to this layer and the Earth's crust together as the Lithosphere -- especially when describing the way it moves.
 
The Lithosphere floats on the Asthenosphere like an iceberg floats on the ocean or a marshmallow floats hot chocolate.
 
 
The Core
 
The innermost part of the Earth is the Core. It is about 1,800 miles (2,900 km) beneath the Earth's surface.
 
It is a dense ball of iron and nickel and is divided into two layers: the Inner Core and the Outer Core:
 
 
The Inner Core
 
The Inner Core -- the center of the Earth -- is solid and about 780 miles (1,250 km) thick. It is from 3,200 to 3,960 miles (5,150 - 6,370 km) below the Earth's surface.
 
It is primarily made of iron and nickel. But scientists think it contains sulfur, carbon, oxygen, silicon and potassium too.
 
It is very very hot -- from about 9,032 to 10,832 degrees Fahrenheit (5,000 - 6,000 degrees Celsius). But the pressure is so great that even at those temperatures it remains solid.
 
Because the Earth rotates, The Outer Core spins around this Inner Core. This creates a dynamo effect, which is what causes the Earth's magnetism.
 
 
The Outer Core
 
The Outer Core is about 1,800 to 3,200 miles (2,890 -- 5,150 km) below the Earth's surface. It is made up of iron, some nickel and about 10% sulfur and oxygen.
 
The temperature in the Outer Core is about 7,200 - 9,032 degrees Fahrenheit
(5,000 degrees Celsius) so it remains liquid.
 
As we said, it is the relationship between the Inner and Outer Cores that causes the Earth's magnetism.
 
 
Convection Currents
 
Because the Core of the Earth is so hot, it radiates currents of heat upward toward the upper layers.
 
These currents of heat are called convection currents. They cool down as they get nearer the surface and move into a horizontal direction along the bottom of the crust.
 
These convection currents are what cause the movement in the tectonic plates that we told you about -- the movement that results in volcanic eruptions and earthquakes.
 
We'll tell you more about tectonic plates -- and plate tectonics -- in an upcoming Instruction.

Experiments for Home and Classroom

Designed to be led by a teacher, this "Plate Tectonics Convection Activity" gives students an excellent understanding of Earth's structure and how it creates tectonic plate movement. Although created for grades 5 through 8, it is equally appropriate for older students. The actual activity begins half way down the page, but it's a good idea to read the introductory material first. Click: http://www.carolina.com/earth/plates.asp

This activity describes how to build a model of the outer 300 km (180 miles) of the Earth. This model can be used to give students a better understanding of the principal features of plate tectonics, including sea-floor spreading, magnetic stripes on the sea floor, transform faulting, thrust faulting, subduction and volcanism. In addition to a printout of the instructions, materials required include a cardboard shoebox, glue, scissors, a straight edge and a safety razor blade. Click:
http://pubs.usgs.gov/of/1999/ofr-99-0132

Although primarily intended for younger students (Grades 6 through 8), there are some interesting experiments listed at the end of eTAP's Earth Science Instructions 1-1 and 1-2 for 6th Grade. Some are good enough to eat!

for Students, Parents and Teachers

Now let's do Practice Exercise 3-1 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.