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Types of Igneous Rock
There are four main kinds of igneous rock.
1) Felsic rocks (also sometimes called silicic rocks) contain 65% or more of
the mineral silica quartz (SiO2). They also contain feldspar, a mineral made
of potassium, sodium, and aluminum. The “fel” in felsic comes from feldspar.
2) Intermediate rocks have a chemical composition falling somewhere
between silicic and mafic (their silica content is from 65% down to 50%).
3) Mafic rocks contain from 50% down to 45% silica. These rocks also contain
magnesium and iron. The “ma” in mafic comes from magnesium.
4) Ultramafic rocks have even more magnesium – and less silica – than
mafic rocks (less than 45% of their mass comes from silica). Most of these
rocks come from the mantle.
Types of Intrusive Igneous Bodies
-Intrusive igneous bodies are magma structures which have spread through the
surrounding country rock and then solidified (cooled). They may be part of the
“plumbing system” of a volcano.
 Volcanic necks are formed from magma that solidified inside the
“throat” of a volcano. They might be exposed by weathering and erosion.
 Dikes are igneous structures running perpendicular to the layering of
country rock. They are usually vertical in orientation.
 Sills are igneous structures running parallel to the layering of country
rock. They are usual horizontal in orientation.
 Plutons are bodies of magma that solidify deep inside the crust. Before
they cool and become solid they are called diapirs of magma.
 A stock is an exposed body of igneous rock no more than 100km2 (square
kilometres) in area on the surface.
 A batholith is an exposed body of igneous rock which has an area of
more than 100km2 on the surface. It might be made of many diapirs that
have solidified into plutons.