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 Earth —The Earth’s crust is hard and brittle, and cracks up into giant slabs called plates.
near the top it is almost liquid. It, too, is heated from the core and moves slowly by convection. This convection is responsible for the movement of the crust over the Earth’s surface—a process called plate tectonics. The liquid at the top of the mantle is the source of volcanoes.
The crust
The crust is the solid rock on which we live. It is very thin compared with the layers of rock below—something like 25km thick on the ocean floors and 200km thick under the centers of the continents. It is also hard and brittle, which is why it cracks up into giant slabs called plates.
The crust is made from the materials ejected from volcanoes over thousands of millions of years. The ocean floors are made of lava. The lava that was pushed out on land is quickly changed into new materials by weather, rivers, ice, and waves.
When the rocks of the mantle move, they drag the thin crust with them, and in time this causes earthquakes and creates mountains and oceans.
(See also: Earth history and Earth orbit.)
Earth history
Earth history
As far as we can tell, the Earth is unique, an accidental mixture of materials that came together in a very special way some 4.6 billion years ago. At first it was nothing more than a dense mass of gas,
but the force of gravity gradually caused the Earth’s materials to collapse inward, heating the core as they did so. The hot core still drives our world. (See also: Accretion.)
After about one billion years the surface had cooled and developed a crust of solid rocks. At the same time, the surface was bombarded with meteorites that made huge craters just like those seen on
the Moon today. Hot rock inside
1 2
3
4
the Earth was continually turned over, dragging the crust apart and allowing vast sheets of molten rock (lava) to flow out and cool to make more crust. This process is still happening today.
As the crust thickened, and less lava flowed to the surface, there was an opportunity for gases to condense and form the water in the oceans and the atmosphere.
Life forms probably developed first in the hot liquids near volcanic eruptions. Slowly, the developing plant life absorbed carbon dioxide and released oxygen, changing the composition of the air and allowing oxygen-breathing life to colonize the land.
 Earth history—The earliest Earth collected as a mass of dust and gas that collapsed under the force of gravity. This caused a release of heat that made the planet’s rocks melt (1). It took a billion years for the surface to cool
into a hard crust (2). The early Earth
was also bombarded by meteorites. In time volcanoes developed and then an atmosphere and oceans (3). Life probably developed in the hot liquids near volcanic eruptions. Slowly, the developing plant life helped absorb carbon dioxide and release oxygen, allowing life to colonize the land.
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