Transcript Igneous Rocks - Mr. Ahearn's Earth Science

Igneous Rocks
Mr. Ahearn
Earth Science 2010
What are Igneous Rocks?
• Rocks that cooled and crystallized
directly from molten rock, either at the
surface or deep underground.
• “Ignis” Latin for fire.
• >95% of Earth’s outer 30 miles consist
of igneous rocks.
• 1st type to form on Earth
What is Magma?
• Molten rock that flows within the Earth.
Could be completely liquid or a mixture
of liquid solid crystals and dissolved
gasses.
• Called lava when at surface.
• Different minerals crystallize at different
temperatures changing the composition
of magma.
Types of Magma
• Rhyolitic – 70% SiO2
• Andesitic- 60% SiO2
• Basaltic- 50% SiO2
• Silica (SiO2) is most abundant in
magma and has greatest effect on
characteristics
– Affects melting temp, viscosity
Creation of Magma
• Partial Melting
• Heat
• Pressure- higher
pressure --> higher
M.P.
• Water- lowers M.P.
Classification of Igneous
Rocks
• Texture- size and shape of mineral
crystals and way in which crystals grow
during cooling.
–
–
–
–
–
Glassy
Fine-grained
Coarse-grained
Very coarse-grained
Porphyritic
• Composition- minerals rock contains
– Felsic, Intermediate, Mafic, & Ultramafic
Igneous Textures
• Extrusive- minerals crystallize slowly
underground for thousands of years -->
crystals large, easily seen.(phaneritic
– Slow cooling occurs when magma intrudes
preexisting solid rocks
– Pegmatites- rocks with exceptionally large crystals
(could be several meters long).
Pegmatite
Igneous Textures
• Extrusive- solidify from lava
quickly--> small or no
crystals. (aphanitic)
– Called volcanic
– Basalt 
• Porphyritic- large crystals
surrounded by regions w/
smaller crystals or invisible
grains.
– Form by slow cooling followed
by rapid cooling.
Glassy Texture
• Instantaneous cooling of lava.
– Obsidian- forms when molten rock material cools so rapidly that atoms
are unable to arrange themselves into a crystalline structure. The result is a
volcanic glass with a smooth uniform texture that breaks with a conchoidal
fracture
– Pumice- bubbly, highly gaseous lava.
Igneous Composition
• Ultramafic- (mg, Fe)
–
–
–
–
Dark, very dense.
Crystallize slowly deep in Earth
Mostly olivine and pyroxene (very little silica)
peridotite
• Mafic–
–
–
–
Silica content 40-50%
Mostly pyroxene, calcium feldspar, little olivine.
Most abundant in Earth’s crust.
Basalt, Gabbro.
Peridotite
a generic name used for coarse-grained,
dark-colored, ultramafic igneous rocks.
Peridotites usually contain olivine as their
primary mineral, frequently with other mafic
minerals such as pyroxenes and amphiboles.
Their silica content is low compared to other
igneous rocks and they contain very little
quartz and feldspar.
Newfoundland
Gabbro
a coarse-grained, dark-colored, intrusive
igneous rock. It is usually black or dark green in
color and composed mainly of the minerals
plagioclase and augite. It is the most abundant
rock in the deep oceanic crust.
Basalt
a dark-colored, fine-grained, igneous rock
composed mainly of plagioclase and pyroxene
minerals. It most commonly forms as an
extrusive rock, such as a lava flow, but can also
form in small intrusive bodies, such as an
igneous dike or a thin sill.
Igneous Composition
• Intermediate
– 55-65% silica (lighter in color)
– Some pyroxene, amphibole, sodium feldspar,
mica and little quartz.
– Andesite, Diorite
• Felsic (feldspar and silica)
– >65% silica (lighter in color)
– K feldspar, muscovite mica, quartz.
– Granite, Rhyolite
Andesite
a fine-grained, extrusive igneous rock composed
mainly of plagioclase with other minerals such as
hornblende, pyroxene and biotite.
Diorite
Diorite is a coarse-grained, intrusive igneous
rock that is intermediate in composition
between granite and gabbro. It’s composition is
very similar to the fine-grained extrusive rock
known as andesite.
Granite
a light-colored igneous rock with grains large enough
to be visible with the unaided eye. It forms from the
slow crystallization of magma below Earth’s
surface. Granite is composed mainly of quartz
and feldspar with minor amounts of mica, amphiboles
and other minerals. This mineral composition usually
gives granite a red, pink, gray or white color with dark
mineral grains visible throughout the rock
Rhyolite
a light-colored, fine-grained, extrusive igneous rock
that typically contains quartz and feldspar minerals.