Download Characteristics of Igneous Rocks

Characteristics of Igneous Rocks
Earth is made of thousands of types of rocks,
which have been made by a number of different of
processes and are used for countless purposes.
The world of rocks is comprised of an amazing
variety of textures, colors, and patterns. Some
rocks are smooth and have a uniform color, while
others are rough, speckled, and multi-colored.
All rock is made of different combinations of
minerals, inorganic materials found in nature. The
mineral composition of a rock determines many of
its properties. The process of rock formation also
influences the rock’s physical properties. During
the rock cycle, different conditions determine how
the rock forms. Sedimentary rock is formed when
Rocks come in an amazing
sediment is layered and pressed together over
variety of textures and colors.
time. Metamorphic rock is formed when rock is
subjected to intense heat and pressure under Earth’s surface.
Igneous rock is formed when molten rock, magma, or lava cools
and hardens.
Intrusive and Extrusive Igneous Rock
There are two main types of igneous
rock. Intrusive igneous rock, also
known as plutonic igneous rock,
forms beneath Earth’s surface. As
magma deep inside Earth moves
closer to the surface, it cools. The
cooling process occurs very slowly
because the temperatures under
Earth’s surface are still very high. The
magma eventually solidifies. As this
happens, the minerals that make up
the magma crystallize. Since under
Granite’s texture is coarse because its crystals
most conditions magma takes a long
developed slowly, over a long period of time.
time to solidify, the crystals have a
long time to grow. This process gives intrusive igneous rock a
coarse texture with large, visible crystals. Granite, with its
speckled appearance, is a common example of intrusive igneous
rock.
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Characteristics of Igneous Rocks
Extrusive igneous rock, also known as
volcanic rock, forms when magma rises slowly
or erupts through Earth’s crust. Once above
Earth’s surface, the molten rock is called lava.
Since temperatures are much lower on
Earth’s surface than deep beneath it, lava
cools and hardens very quickly. There is not
much time for the minerals in the lava to form
large crystals. As a result, the texture of
extrusive igneous rock has a very fine grain.
Extrusive igneous rock may be rough like
pumice or smooth and shiny like obsidian.
Obsidian owes its smooth texture to
the short time in which it cooled.
Igneous Rock Shapes Earth’s Surface
Extrusive igneous rock plays a large part in shaping and
changing Earth’s surface. For example, a large percentage of the
land making up the Hawaiian Islands is volcanic rock. Repeated
volcanic eruptions have caused ocean-floor mountains to rise
above the surface of the ocean, forming the chain of islands.
Evidence of Hawaii’s volcanic beginnings can be observed at its
many beaches. Eroded volcanic rock gives Hawaii’s beaches
various colors. Some beaches are covered by black or green sand
made from weathered and eroded volcanic rock.
Other dramatic landforms are made of
intrusive igneous rock. Devil’s Tower
National Monument in Wyoming is made of
a type of igneous rock called phonolite
porphyry. The term "porphyry" describes
the texture of the rock in Devil’s Tower.
Igneous rocks with a porphyritic texture
have both larger and smaller crystals.
Geologists believe that this variation has
occurred because the magma that solidified
into Devil’s Tower began to cool and
Devil’s Tower National Monument in
crystallize deep under Earth’s surface. The
Wyoming is made of igneous rock.
larger crystals formed during this stage. As
the partially solidified magma moved closer
to Earth’s surface, it cooled quickly. This quick cooling caused
the smaller crystals to form.
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