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1. The Earth is made of many different and
distinct layers. The deeper layers are
composed of heavier materials; they are
hotter, denser and under much greater
pressure than the outer layers.
Core: The Earth has a iron-nickel core that
is about 2,100 miles in radius. The inner
core may have a temperature up to about
13,000°F (7,200°C = 7,500 K), which is hotter than the surface of the Sun.
The inner core (which has a radius of
about 750 miles (1,228 km) is solid. The
outer core is in a liquid state and is about
1,400 miles (2,260 km) thick.
Mantle: Under the crust is the rocky mantle, which is composed of
silicon, oxygen, magnesium, iron, aluminum, and calcium. The upper
mantle is rigid and is part of thelithosphere (together with the crust). The
lower mantle flows slowly, at a rate of a few centimeters per year.
The asthenosphere is a part of the upper mantle that exhibits plastic
properties. It is located below the lithosphere (the crust and upper mantle),
between about 100 and 250 kilometers deep.
Convection (heat) currents carry heat from the hot inner mantle to the
cooler outer mantle. The mantle is about 1,700 miles (2,750 km) thick.
The mantle gets warmer with depth; the top of the mantle is about 1,600°
F (870° C); towards the bottom of the mantle, the temperature is about
4,000-6,700° F (2,200-3,700° C). The mantle contains most of the mass of
the Earth. The Gutenberg discontinuity separates the outer core and the
mantle.
Surface and crust: The Earth's surface is composed mostly of water,
basalt and granite. Oceans cover about 70% of Earth's surface. These
oceans are up to 3.7 km deep. The Earth's thin, rocky crust is composed of
silicon, aluminum, calcium, sodium and potassium. For a page on soil,
click here.

2. The crust is divided into continental plates which drift slowly (only a few
centimeters each year) atop the less rigid mantle. The crust is thinner
under the oceans (6-11 km thick); this is where new crust is formed.
Continental crust is about 25-90 km thick. The lithosphere is defined as
the crust and the upper mantle, a rigid layer about 100-200 km thick. The
Mohorovicic discontinuity is the separation between the crust and the
upper mantle.
The crust is the outermost layer of the Earth.
Earth has three layers. Beneath the crust is the mantle, which is made of magma and other semi-solid rocks andminerals.
At the center of the Earth is the extremely hot,metal core. The lack of air, water, and moderate temperature prevents
organisms from living in the mantle or core.
Earths crust is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic,
Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates. Tectonic plates
actually slide around on the mantle, causingearthquakes, mountain formation, continental drift, volcanoes, and other
geologic activity on the crust.
Billions of years ago, the Earth started out as a hot, gooey ball of rock. The heaviest material, mostly iron and nickel, sank
to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. These surface
rocks became the crust.
The crust is divided into two types: oceanic crust andcontinental crust. Oceanic crust, found under the ocean floor, is
made of dense rocks such as basalt. It is about 7 kilometers (4 miles) thick. Continental crust, found under land masses,
is made of less dense rocks such as granite. Its thickness varies between 10 and 75 kilometers (6 to 47 miles).
Continental crust is almost always much older than oceanic crust. Some of the oldest rocks in the world can be found in
the Nuvvuagittuq greenstone belt in Quebec, Canada. This continental crust formation has rocks that are about 4 billion
years old. Unlike continental crust, oceanic crust is still being formed in places called mid-ocean ridges. Here, magma
from the mantle erupts through cracks in the ocean floor, creating crust as it cools.
Oceanic crust is heavier than continental crust. The heavy oceanic crust is constantly sinking, very slowly, underneath the
lighter continental crust. This important process is calledsubduction. A chain of volcanoes formed at a subduction zone is
called a volcanic arc. One such volcanic arc exists where the oceanic crust of the Australian plate subducts under the
continental crust of the Eurasian plate. The Indonesian Island Arc, which includes the islands of Sumatra and Java in
Indonesia, has some of the most powerful volcanoes in the world.
Eventually, oceanic crust sinks low enough to enter the mantle. Once this happens, the crust melts, then rises up again as
magma in the mid-ocean ridges. In this way, the Earth enjoys a brand-new oceanic crust once every 200 million years or
so.

3. 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 -- theInner 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.

4. 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
Earth is unique among the known planets: it has an abundance of water. Other worlds — including a
few moons — have atmospheres, ice, and even oceans, but only Earth has the right combination to
sustain life.
Earth's oceans cover about 70 percent of the planet's surface with an average depth of 2.5 miles (4
kilometers). Fresh water exists in liquid form in lakes and rivers and as water vapor in
the atmosphere, which causes much of Earth's weather.
Earth has multiple layers. The ocean basins and the continents compose the crust, the outermost
layer. Earth's crust is between three and 46 miles (five and 75 km) deep. The thickest parts are under
the continents and the thinnest parts are under the oceans.
Crust
Earth's crust is made up of several elements: iron, 32 percent; oxygen, 30 percent; silicon, 15
percent; magnesium, 14 percent; sulfur, 3 percent; nickel, 2 percent; and trace amounts of calcium,
aluminum and other elements.
The crust is divided into huge plates that float on the mantle, the next layer. The plates are constantly
in motion; they move at about the same rate as fingernails grow. Earthquakes occur when these
plates grind against each other. Mountains form when the plates collide and deep trenches form
when one plate slides under another plate. Plate tectonics is the theory explaining the motion of these
plates.
Mantle
The mantle under the crust is about 1,800 miles deep (2,890 km). It is composed mostly of silicate
rocks rich in magnesium and iron. Intense heat causes the rocks to rise. They then cool and sink
back down to the core. This convection — like a lava lamp — is believed to be what causes the
tectonic plates to move. When the mantle pushes through the crust, volcanoes erupt.
Core
At the center of the Earth is the core, which has two parts. The solid, inner core of iron has a radius of
about 760 miles (about 1,220 km). It is surrounded by a liquid, outer core composed of a nickel-iron
alloy. It is about 1,355 miles (2,180 km) thick. The inner core spins at a different speed than the rest
of the planet. This is thought to cause Earth's magnetic field. When charged particles from the solar
wind collide with air molecules above Earth's magnetic poles, it causes the air molecules to glow,
causing the auroras — the northern and southern lights.