Formation of Rocks: Alberta's Foundation - p 10
Rocks make up the earth's crust and are the most fundamental unit in geology. Each rock tells a story of
how and where it was formed. They provide clues to the history of our planet; they record changes in the environment that occurred
millions of years ago; and rocks containing fossils supply evidence of life forms over the last 3.5 billion years. As well, their
economic importance is obvious: mining rocks gives us raw materials, such as iron, petroleum, coal, and gravel, that are essential
in our everyday lives.
Geologists define a rock as a natural aggregation of one or more
minerals. Some rocks do not have minerals but are made up of glasses
(obsidian) or organic material (coal). There are three groups of rocks
that are classified according to how they were formed:
igneous,
metamorphic, and
sedimentary. Each type can be altered from one kind into another -
this is part of the "rock cycle."
Metamorphic rocks form when preexisting rocks, either igneous,
sedimentary, or metamorphic, are altered by high temperature, high
pressure, or both. Sedimentary rocks form when metamorphic, sedimentary,
or igneous rocks are uplifted to the earth's surface where erosion then
breaks them down into fragments, called
sediments. When these sediments are transported, deposited and
buried, they are compacted and cemented together to form sedimentary
rocks. When sedimentary, metamorphic, or igneous rocks are buried deep
beneath the earth's surface, they melt when the temperature gets hot
enough. Igneous rocks are formed when molten rock, called magma, cools
and hardens. |