BACKGROUND:
Igneous rocks come in many varieties. However, all igneous rocks began as
molten rock (magma) which cooled and crystallized into minerals. Igneous
rocks may look different because of two factors: (1) they may have cooled at
different rates and (2) 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. Magma that
cools slowly creates rocks like granite which have large minerals that can
be seen with the naked eye. Geologists classify igneous rocks based on both
their crystal size and composition. The Rock Cycle has its origin in Igneous
Rocks.
Sedimentary rocks form at the Earth’s surface in two main ways: (1)
from clastic material (pieces of other rocks or fragments of skeletons)
which have become cemented together, and (2) by chemical mechanisms
including precipitation and evaporation. 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."
All three types of rock make up the Earth’s lithosphere, the outermost
layer. The lithosphere averages about 100 kilometers in thickness. It is
like an eggshell compared to the Earth’s total radius (the distance from
the Earth’s core to the surface). The lithosphere is solid rock.
Sedimentary rocks are the most abundant rock only on the surface of the
Earth, but igneous and metamorphic are abundant deeper into the mantle.
PROCEDURE:
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Sing "Ricky the Rapping Rock." Go over with
students the rock cycle. Ricky travels through Igneous,
Sedimentary and Metamorphic Rocks. You may want students to sing
along.
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Introduce the students to the lithosphere. Ask them where most of the
rocks that we see on the surface of the Earth were created. Explain why
the lithosphere is the correct answer. Show them a cross-section of the
Earth by showing them the physiographic relief globe. They will be
amazed at how thin the lithosphere is compared to the rest of the Earth.
Explain that igneous, sedimentary, and metamorphic rocks are created in
the lithosphere. Briefly distinguish between the lithosphere,
hydrosphere, and the atmosphere. Lithosphere is sphere of rocks;
hydrosphere is sphere of water; and atmosphere is Earth’s envelope of
gases.
