Chapter 5-Igneous Rocks
Lecture notes- to be put in notebook
What are igneous rocks?
• Temperature generally
increases with depth in
Earth’s crust.
• Pressure also increases with
depth, a result of the weight
of overlying rock.
What are igneous rocks?
• As pressure on rock
increases, its melting point
also increases.
• Different minerals have
different melting points,
not all parts of a rock melt
at the same time.
• Partial melting -Some
minerals melt at low
temperatures while other
minerals remain solid.
• As each group of minerals
melts, different elements are
added to the magma “stew”
• Fractional crystallizationWhen magma cools, it
crystallizes in the reverse
order of partial melting.
• As each group of minerals
crystallizes, it removes
elements from the remaining
magma instead of adding new
elements.
Mineral composition-
• Felsic rocks such as
granite are light-colored,
have high silica contents,
–contain quartz and the
feldspars orthoclase and
plagioclase.
Granite
Mineral composition-
• Mafic rocks such as
gabbro are dark-colored,
have lower silica contents,
and are rich in iron and
magnesium.
– contain plagiocide, biotite,
amphibole, pyroxene, and olivine.
Gabbro
• Diorite is a good example of
an intermediate rock with
moderate amounts of biotite,
amphibole, and pyroxene.
Diorite
• Ultramafic rocks-have low
silica contents and very high
levels of iron and magnesium
–Peridotite and dunite
–Formed by the crystallization
of olivine and pyroxene
Peridotite & Dunite
• Cooling rates–Extrusive igneous rocks such as
obsidian have no visible
mineral grains (cooled quickly)
–Intrusive igneous rocks such as
gabbro may have crystals larger
than 1 cm (cooled slowly)
• Cooling rates- Continued
• Geologists make thin sections of
minerals so that they can see the
shapes of the grains
• Interlocking edges form when the
grains crystallize and grow together
• During fractional crystallization, the
minerals that form early in the
process float in a liquid and have
space to grow distinct shapes
• Porphyritic rocks have grains
of two different sizes
–Caused by slowly cooling
magma which suddenly
begins to cool rapidly
• The interlocking grain textures
of igneous rocks help to give
them strength.
• Many of the minerals found in
igneous rocks are resistant to
weathering.
• Granite is among the most
durable of igneous rocks.
Ore Deposits
• Valuable ore deposits are
often associated with
igneous intrusions.
• These ore deposits are
found within igneous rock,
such as the layered
intrusions.
Ore Deposits
• Veins-Metallic elements
are released at the end of
magma crystallization.
–Veins of extremely largegrained minerals are
called pegmatites.
• Diamonds are found in ultramafic
rocks known as kimberlites.
• Diamond can only form under very
high pressures.