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Rocks are classified by their mode of formation. There
are three major rock forming processes on Earth,
producing three kinds of rocks.
Igneous Rocks
Formed when magma
(molten rocks) solidifies
Environment: Hot
enough to melt rock,
pressure varies
Sedimentary Rocks
All non-igneous rocks
formed by processes
acting on the surface of
the Earth
Environment: normal for
Earth’s surface
Metamorphic Rocks
Formed by chemically
and physically altering
rocks under heat and
pressure deep within the
Earth’s crust.
Environment: High
pressure, not hot enough
to melt rock
The Rock
Cycle
Igneous Rocks
Igneous rocks form from magma (a
complex mix of molten rock with
dissolved gases and other
materials)
Magmas that cool slowly inside the
Earth form plutonic igneous rocks.
The mineral crystals in these rocks
are usually large because they had
lots of time to grow.
Magmas that erupt onto Earth’s surface are called lavas and very quickly. As they
cool, volcanic igneous rocks form. If mineral crystals form, they tend to be small
because they had little time to form. If cooling is fast enough, no minerals form and
the lava solidifies to form a solid glass, with no crystalline structure.
When the temperature is high enough (and the
pressure* is low enough), some solid Earth materials
will begin to melt, forming magma.
Magma - complex mixture of high temperature solid,
liquid, and gaseous material.
Dissolved gasses - mostly H2O, SO2, CO2
*If the pressure is too high, melting will not occur – which is why magma is more
commonly formed in the crust than the mantle, even though mantle temperatures are
much higher than crust temperatures.
Properties of Magma
Viscosity – resistance of a fluid to flow (low
viscosity=water; high viscosity=syrup)
http://www.earth.northwestern.edu/people/seth/107/Rocks/magviscosity.htm
Intermediate igneous rocks
contain minerals with
intermediate melting points
Mafic igneous rocks
contain minerals with high
melting points
felsic
Felsic igneous rocks contain
minerals with low melting
points
Bowen’s Reaction Series
felsic
Crystalline Igneous Rock Texture
Texture - refers to the size of mineral crystals. The more slowly a magma
cools, the more time crystals have to grow.
Magma that cooled slowly produces large
crystals, and the resulting rock is coarsegrained (phaneritic).
In magmas that cooled quickly only small
crystals form, and the resulting rock is
fine-grained (aphanitic).
Crystalline Igneous Rock Texture
Texture - refers to the size of mineral crystals. The more slowly a magma
cools, the more time crystals have to grow.
Porphyritic Texture
Sometimes a magma will have a complex
cooling history, and may stay at a mineral’s
crystallization temperature for a long time,
allowing those mineral crystals to grow large.
If the magma is later cooled more quickly,
only smaller crystals of the other minerals
will form.
Felsic
Volcanic Igneous Rocks with no Minerals
bombs!
glassy – obsidian and
other volcanic glass
scoria
obsidian
Some magmas cooled too fast for minerals to form. The rocks are made of glass and contain no
minerals, so are NOT described using the felsic-intermediate-mafic system.
vesicular – full of holes formed
by bubbles in the magma.
fragmental or pyroclastic –
formed from debris spewed
out by the volcano.