The Rock Cycle: Igneous Rocks
What is a rock?
• An aggregate of
minerals
• 3 types of rock
– Igneous
– Sedimentary
– Metamorphic
• Rock cycle
Igneous Rocks
• Rocks created when molten rock cools and
crystallizes
• Two types
– Extrusive (volcanic)
• Erupted and cooled quickly at the surface
– Intrusive (plutonic)
• Cooled slowly beneath the surface of the earth
Extrusive Igneous Rocks
Extrusive Igneous Rocks
• Occur when lava cools
quickly on the surface
• Fine-grained textures
common
Fine grained basalt
– Cools quickly, crystals
have little time to cool
• Porphyry—large crystals
formed at depth prior
to emplacement on
surface
Porphyritic texture
Glassy obsidian
Intrusive Rocks
Granite: a common intrusive rock
• Occur when magma
cools within the crust
• Medium to course
grained texture
– Crystals can generally be
identified without the
aid of a magnifier
• Large crystals are the
result of slow cooling
over thousands/millions
of years
Diorite: another common intrusive rock
“Can I see your I.D., please?”
• Rocks are identified by
• Textures
• Mineral compositions
• Ex: We have a rock that
is composed of feldspar,
quartz and
hornblende—could be
granite or rhyolite
Course grained:
• Texture will help us
granite
differentiate between
the two
Fine grained:
rhyolite
More on classification…
• Felsic
– Light colored—generally
composed of light
colored minerals
• mafic
Rhyolite: felsic
Gabbro: mafic
– Generally composed of
Fe/Mg—dark colored
minerals
• Intermediate rocks—
composition between
felsic and mafic
– “Salt and pepper”
Diorite: intermediate
More on Classification
peridotite
• Ultramafic rocks:
– Very low Si
– Dark colored, and heavy
for their size
– Often contain dark
pyroxene, and large
amounts of olivine—a
granular-looking, green
mineral
dunite
How do magmas form?
• Si rich minerals—
feldspar, quartz, etc..
Have low melting points
• Melting point increases
with decreasing Si
content
Partial Melting
• Melts of different
compositions can be
generated by melting
the same rock several
times at different
temperatures
– This is why not all
igneous rocks look the
same
– Can also occur as
magmas cool
How does rock melt?
• Increase temp
• Temp increases 30°C for
every kilometer of depth
• At depths of 100 to 350
km, the temp is so high
that some areas of melt
begin to form
• There are other
ways; well talk
about them later