Page 30 - Curriculum Visions Dynamic Book

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The Earth’s crust
The earliest Earth collected as a mass of dust and gas that collapsed under the force of gravity. That 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 arose in the hot liquids near volcanic eruptions. Slowly the emerging
1 plant life helped absorb carbon dioxide and release oxygen, allowing life to colonize the land and for the present atmosphere
to form (4).
Plates
The Earth’s “crust” is a general term for its upper, rigid rocks. Scientists call it the lithosphere.
But the crust is not, as it is on many other planets, a continuous shell of similar material. On the Earth the crust is broken into about a dozen large pieces, which have come to be called plates. Cracking in the plates is the result of movements of materials in the mantle below the crust. Again, as far as we know, this is a unique feature of our planet.
 The early history of the crust. Numbers are explained in the text.
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