Minerals make up rocks. Rocks formed in igneous, metamorphic, and
sedimentary environments. The minerals found within each rock help to
identify and name the rock, but the rock textures, such grain size, are also
used in naming and identification.
Igneous rocks are formed when rocks are melted and then cool. Magma can
cool slowly inside the crust and upper mantle of the Earth, forming rocks
like granite. Granite has large minerals that can be seen with the naked
eye. These are called plutonic rocks. Quick cooling magmas are generally
erupted onto the Earth’s surface, called volcanic rocks. Igneous rocks are
found where plates diverge, as lava rises and fills the gap between the
plates. Igneous rocks also form where plates converge. The subducting plate
melts as it sinks into the crust of the Earth, and the melt rises into the
overriding plate forming volcanoes.
Metamorphic rocks are formed mainly in the lithosphere, wherever there is
high pressure and high temperature. If the pressure and temperature are too
high, metamorphic rock will melt and become magma. Metamorphic rocks are
most abundant at convergent plate boundaries, but can occur in other areas
where there are increased pressures and/or temperatures.
Sedimentary rocks form only on the surface of the Earth. Sedimentary
rocks form in two main ways: (1) from clastic material (pieces of other
rocks or fragments of skeletons) become cemented together, and (2) by
chemical mechanisms including precipitation and evaporation. There are many
environments associated with sedimentary rock formation, including oceans,
lakes, deserts, rivers, beaches, and glaciers. They may form at all types of
plate boundaries, but the thickest sedimentary rock accumulations occur at
convergent plate boundaries. Fossils are associated with sedimentary rocks.
- Review rocks and rock forming environments. Use the illustration in
the electronic presentation or draw the following diagram on the board
and have the students predict where each type of rock formed.