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New
Mexico Master
Gardeners |
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Geology
of the Rio Grande Valley How Rocks are
Formed There
are three major ways that rocks are formed. The first of these is from heat.
They are called igneous rocks. They are the volcanic rocks, among which are
obsidian, and granite. Both obsidian and granite contain the same minerals but
their different appearances are caused by differences in the speed of their
cooling. Obsidian cooled very rapidly and has a glassy texture. In fact, it is a
glass and amorphous or non-crystalline. Granite cooled very slowly and the
separate minerals crystallized into larger crystal. The three major minerals of
granite and obsidian are quartz, feldspar, and mica.
The second method of rock formation is from deposition. These rocks are
sedimentary. They are formed when other rocks are decomposed by weathering and
other factors and are carried by wind, water or gravity to a location where they
accumulate and become hardened together. Sandstone and limestone are such rocks. The third method of rock formation is called metamorphous. The name means
changed form. Three factors can cause the changing form. They are Heat,
Compression or Pressure, and Infiltration of solutions containing other minerals
such as Quartz, Calcite, and other minerals. Examples of metamorphic rocks are
marble (changed by heat and pressure from limestone) and petrified wood (changed
by infiltration of quartz). All soils are formed from rock at some time or another. The factors that
cause soil formation are as shown in the chart below.
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