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Rock Types
IGNEOUS ROCKS(5.1-5.2)
Igneous Rocks form
underground or near the
surface, from the crystallization
of magma.
Magma – molten rock
underground
Lava – magma reaches
Earth’s surface
What is Porphyritic
Texture?
Porphyry – Where large crystals are
surrounded by the rest of the rock. Usually
represents 2 or more stages of eruption or
cooling. “CRYSTAL CHUNKS”
HIGH PORPHYRY tells you. .
• The rock cooled
underground
• The rock took a
long time to cool
• The rock could
have melted and
cooled several
different times
Igneous rocks are very strong
because of interlocking crystal
grains. . .
When there is not enough “room” for crystals
to grow large, they fuse with each other.
“Vesicular” texture means
having bubbles. . .
• These normally
depend on the
thickness of the
lava, and whether
gases can get
trapped.
There are two types of Igneous
Rock, based on the elements/
minerals they’re made of . . .
• MAFIC --
“DARK” . Rich in Iron and
Magnesium. Are dark colored, grainy
textured, with low luster. Low porphyry.
• FELSIC – “LIGHT” .
Rich in Silica
(glass) . Contain colors such as tan, pink,
cream, light grey, etc. High luster, or large
crystals present. Greater porphyry.
MAFIC
FELSIC
The other classification we use,
is the location where rocks
cooled off.
• Intrusive – Cooled slowly beneath earth’s
surface. These will contain a variety of
crystals, because they had a longer time
to form.
• Extrusive -- Cooled quickly at or just
below the surface. Very little / no
porphyry.
• INTRUSIONS --
A long “tube” of igneous
rock that came in through a vent/crack
underground , and cuts across older rock. (Also
known as veins / dikes – “INTRUDER”)
– Pegmatites: Felsic veins with very large,
chunky crystals.
– Kimberlites: Mafic veins. (DIAMONDS are
found in these)
Local Evidence of Extrusive
Igneous Rocks
“Lava Caps” down Latrobe Road,
And Scoria in the fireplaces
In older homes in EDH!
Local Evidence of Intrusive
Igneous Rocks
Sierra Nevada Granite
formations up highway 50
and 80 . . .
When we learn about rock types
this time, we will classify them in
this manner . . .
IGNEOUS ROCK
Intrusive
Mafic
Extrusive
Felsic
Mafic
Felsic
EXTRUSIVE MAFIC
SCORIA: Large bubbles, concrete-like
texture, brittle, dark in color.
From thick, pasty lava.
BASALT: Dark, grainy, dense rock.
Comprises the crust of the ocean floor
INTRUSIVE MAFIC
GABBRO: Dark background with large, dark
crystals present.
EXTRUSIVE FELSIC
PUMICE: Can float, very
Small bubbles (vesicular), from
thin lava. Texture of spun glass.
RHYOLITE: Salmon to cream color, may
have very small splinterLike crystals.
OBSIDIAN: Volcanic glass
ANDESITE :Light color,
low porphyry
INTRUSIVE FELSIC
DIORITE (Granite):
High porphyry.
What most of the
Sierra Nevadas are
made of!