BACKGROUND:
Minerals make up rocks. Rocks are formed in many environments upon and
within the Earth's crust. There are three types of rock, each formed in a
different way. Igneous rock , formed by the cooling of magma (molten rock)
inside the Earth or on the surface. Sedimentary rocks, formed from the
products of weathering by cementation or precipitation on the Earth’s
surface. Metamorphic rocks, formed by temperature and pressure changes
inside the Earth. All three types of rock make up the Earth’s lithosphere,
the outermost layer. The lithosphere averages about 100 kilometers in
thickness.
All igneous rocks began as magma (molten rock) which cooled and
crystallized into minerals. Geologists classify igneous rocks based on both
their crystal size and composition. Igneous rocks may look different because
they may have cooled at different rates and the "mother" magma
(original melted rock) was of a different composition. Variations in these
two factors have created many different types of igneous rocks. When the
magma cools at different rates, it creates different sized minerals. Quick
cooling magmas have small minerals (with the exception of obsidian, which is
actually composed of silica, but has no crystalline structure). Basalt, for
example, has small minerals, most of which can only be seen under a
microscope. Quick cooling lavas are called volcanic rocks. Magma that cools
slowly creates rocks like granite, which have large minerals that can be
seen with the naked eye. These igneous rocks cool inside the lithosphere,
and are called plutonic rocks.
Sedimentary rocks form at the Earth’s surface in two main ways. Clastic
material (pieces of other rocks or fragments of skeletons) may become
cemented together and chemical precipitation and evaporation can form
sedimentary rocks. Sedimentary rocks are usually associated with liquid
water (which facilitates erosion, transportation, deposition, and
cementation). However, sedimentary rocks may also form in dry, desert
environments or in association with glaciers.
Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic
rocks that have been changed by great pressures and temperatures within the
crust and upper mantle of the Earth. The temperatures were not enough to
melt the rock, otherwise, an igneous rock would have formed. The pressures
were much greater than those required to simply break the rocks into
pieces. They were high enough to change the chemical make up of the rock by
forcing the elements in it to "exchange partners." Different
grades of temperature and pressure will cause the same original rock to form
very different metamorphic rocks. Slate, which forms from the sedimentary
rock shale, is very dense, smooth and does not contain visible minerals.
However, if more pressure and temperature are applied to a slate, it could
turn into schist, which has visible layers of minerals. If yet higher
temperature and pressure are applied, the schist could turn into gneiss,
which shows visible bands of minerals.
PROCEDURE:
- Review with students where igneous, sedimentary, and metamorphic rocks
are formed using the diagram below.
