Igneous Rocks - physgeoltools

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Transcript Igneous Rocks - physgeoltools

 Intrusive
igneous rocks form when magma
cools within existing rocks in Earth’s crust.
 Extrusive
igneous rocks form when magma
cools on Earth’s surface, where they have
been “extruded.”
The two most obvious textural features of an
igneous rock are the size of its mineral grains
and how the mineral grains are packed
together.
• Intrusive rocks are coarse-grained.
 Magma that solidifies in the crust cools slowly and has sufficient time to
form large mineral grains.
 Extrusive rocks are fine-grained.
 Magma that solidifies on the surface usually cools rapidly, allowing
insufficient time for large crystals to grow.
Coarse-grained igneous rock is called a phanerite (from the Greek
word meaning visible).
Igneous rock that contains unusually large mineral grains (2cm or
larger) is called a pegmatite.
Fine-grained igneous rock is called an aphanite (from the Greek
word meaning invisible).
- Factors: cooling rate and viscosity
Figure 4.7 A
Granite
Rhyolite
Diorite
Andesite
Gabbro
Figure 4.7 C
Basalt
• The isolated large grains are phenocrysts.
Glassy rocks.
Extrusive igneous rocks that are largely or wholly
glassy are called obsidian.They display a distinctive
conchoidal fracture (smooth, curved surface).
Once the texture of an igneous rock is
determined, its name will depend on its
mineral assemblage. All common igneous
rocks consist largely of:
Quartz.
Feldspar
Mica
Amphibole.
Pyroxene.
Olivine.
The overall lightness or darkness of a rock is
a valuable indicator of its makeup.
FELSIC Light-colored rocks are:
Quartz.
Feldspar.
Muscovite.
MAFIC Dark-colored rocks are:
Biotite.
Amphibole.
Pyroxene.
Intrusive
Extrusive
Felsic
Intermediate
Granite
Rhyolite
Diorite
Andesite
(Porphyritic)
Mafic
Gabbro
Basalt
Figure 5.7
 All
bodies of intrusive igneous rock, regardless
of shape or size, are called plutons, after Pluto,
the Greek god of the underworld.
 Plutons are given special names depending on
their shapes and sizes.
Figure 4.11
Tephra can be converted into pyroclastic rock
in two ways:
Through the addition of a cementing agent, such as
quartz or calcite, introduced by groundwater.
Through the welding of hot, glassy, ash particles.
Welded tuff.
 A magma
of a given composition can
crystallize into many different kinds of
igneous rock.
 Solidifying magma forms several different
minerals which start to crystallize from the
cooling magma at different temperatures.
 Crystal-melt
separation can occur in a number
of ways:
• Compression can squeeze melt out of a crystal-melt
mixture.
• Dense, early crystallized minerals may sink to the
bottom of a magma chamber, thereby forming a solid
mineral layer covered by melt.
 However
a separation occurs, the
compositional changes it causes are called
magmatic differentiation by fractional
crystallization.
 Canadian-born
scientist N. L. Bowen
(1887-1956) first recognized the
importance of magmatic differentiation by
fractional crystallization.
 Bowen argued that a single magma could
crystallize into both basalt and rhyolite
because of fractional crystallization.
Isolated
Two series of minerals formed during crystallization of
magma
Temperature of
Low Silica Magma
1200oC
1000o
Intrus. Extrus.
Gabbro Basalt
Diorite Andesite
750o
Framework
Sheet
Double
Chain
Single
Chain
Crystallization
Granite Rhyolite
High Silica Magma
Figure 4.19 A
Figure 4.19 B
 The
solid olivine reacts with silica in the melt
to form a more silica-rich mineral, pyroxene.
 The pyroxene in turn can react to form
amphibole.
 Amphibole can react to form biotite.
 Such a series of reactions is called a
discontinuous reaction series.
Figure 4.20
Figure 4.21