The Earth's crust is made up of rocks that are heterogeneous
aggregates of minerals. They may crop out at the surface or be covered
by a layer of unconsolidated materials (like sand, gravel, silt
and clay).
Most rocks are mixtures of two or more minerals, but some, such
as pure sandstones (made up entirely of quartz) and pure limestones
(consisting of calcite), contain only one. Other rocks, such as
coal, are made up of organic materials and contain no minerals.
Igneous Rocks
Igneous rocks, which are also referred to as magmatic rocks, are
formed from materials that come from the depths of the Earth. There
are two categories of igneous rocks: intrusive (or plutonic) rocks
and volcanic rocks.
Plutonic rocks are consolidated at depth. Because they crystallize
slowly, they have a coarse-grained texture and their crystals can
be distinguished with the naked eye. Granite is a good example of
a plutonic rock.
Volcanic rocks are born of volcanic eruptions and consolidated
at the surface. The molten materials cool more rapidly at the surface
than they would in the depths of the Earth, which gives volcanic
rocks their fine-grained crystals and glassy appearance. Rhyolite,
basalt and obsidian are examples of igneous rocks that have formed
at the surface of the Earth.
Volcanic and plutonic rocks are similar in composition since they
form from the same materials. They differ mainly in terms of crystal
size, which is a function of the environments in which they formed.
[D] Click for larger version, 156 KB Columnar joints in rhyolite flow, MacDougalls Island, New Brunswick
Sedimentary Volcanic Rocks
Some rocks are partly volcanic and partly sedimentary in origin. They are produced by volcanic eruption when materials that have already crystallized in the chimney of a volcano are broken into large angular blocks and small fragments that are transformed into ash and projected into the air. As this material is deposited, it lands in successive layers on the ground or the sea floor and is later lithified. Rock that contains larger fragments is called volcanic breccia or agglomerate; finer-grained rock is called tuff. These rocks are classified according to their composition.
Sedimentary Rocks
Sedimentary rocks are the product of the consolidation of loose
sediment that has accumulated in beds. Such sediment may be produced
by the disintegration of previously existing rock or the precipitation
of dissolved minerals, or it may consist of plant and animal remains.
Regardless of origin, these deposits settle gradually under the
weight of overlying beds and are transformed into solid sedimentary
rock by cementation.
Sedimentary rocks are classified according to grain size and composition.
The largest grain sizes are seen in conglomerates, tillites and
sedimentary breccias. Sandstones are formed by the consolidation
of sand grains. If the grains are very small, the rock is referred
to as a siltstone. Still finer sediments, such as clays, produce
shales.
Both fresh and salt water often contain chemical compounds, such
as calcium carbonate, in solution. Under the right chemical conditions,
these compounds can precipitate, forming deposits that harden into
rock. The most common sedimentary rocks in this category are limestone,
which consists of calcium carbonate, and dolomite, which contains
significant amounts of magnesium carbonate.
[D] Click for larger version, 37 KB Calcite crystals. Calcite is the principal constituent of limestone.
Metamorphic Rocks
A metamorphic rock is derived from a pre-existing igneous or sedimentary
rock that is transformed, to a greater or lesser degree, by the
action of physical factors such as temperature and pressure, or
chemical factors that include the addition of new minerals. Such
transformations take place at depth in the Earth’s crust.
Metamorphism reorganizes the minerals in the rock, changes mineral
grains sizes to produce a somewhat coarser-grained texture and creates
new structures, primarily by aligning minerals in foliated layers
or parallel bands. If rocks are sufficiently metamorphosed, it can
be difficult, if not impossible, to identify the original rock.
Examples of metamorphic rocks include: marble, serpentinite, quartzite,
argillite, slate and gneiss.
[D] Click for larger version, 52 KB Tourmaline is widely distributed in acid igneous rock, and metamorphic rock such as gneisses.
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