Download Igneous Rocks

Igneous Rocks
Although all igneous rocks begin as magma, they have a variety of
compositions and textures. Their differences are the result of variations in
the composition of the magma and in the formation process.
Igneous Rock Formation
As with other rocks, igneous rocks are classified by their mineral
composition and texture. Some igneous rocks form from volcanic ash. Most
form directly from magma. The location of the magma determines the rate at
which it cools, which determines the texture of the resulting rocks. Igneous
rocks formed from underground magma are called intrusive igneous rocks.
Those formed at Earth’s surface are called extrusive igneous rocks.
6.2
KEY IDEA
Igneous rocks form from magma
in distinct ways and can be
classified based on mineral
composition and texture.
KEY VOCABULARY
• felsic
• mafic
• pluton
• batholith
The Starting Material
Magma may be classified as felsic, mafic, or an intermediate form. Felsic
(FEHL-sihk) magma is thick and slow-moving. It contains large amounts of
silica (SiO2) and smaller amounts of the elements calcium, iron, and
magnesium. This magma typically hardens into rocks of light-colored
silicate minerals such as quartz and orthoclase feldspar.
Compared with felsic magma, mafic (MAF-ihk) magma is hotter,
thinner, and more fluid, containing large amounts of iron and magnesium
and much lower amounts of silica. Rocks formed from mafic magma
usually contain large amounts of dark silicate minerals such as
hornblende, augite, and biotite.
Underground Magma
Magma may harden slowly or quickly. The rate of cooling and the texture of
the rock that forms depend on where the cooling occurs. Magma trapped
deep in Earth’s crust hardens very slowly to form intrusive igneous rocks.
Evidence suggests that massive bodies of intrusive rock may take
thousands of years to cool underground. Intrusive rocks appear at Earth’s
surface when they are uplifted and the overlying rock is worn away.
Image not available.
Please refer to the
image in the textbook
or in the eEdition CD.
LAVA This lava is magma that poured
onto Earth’s surface during a volcanic
eruption. It will harden to form
extrusive igneous rock. Magma that
hardens below the surface forms
intrusive igneous rocks.
Chapter 6 Rocks
121