4. Igneous rocks form from
cooled molten material
If magma crystallizes
underground, then the
rock is INTRUSIVE
If lava cools above
ground, then the rock is
EXTRUSIVE
At one point in Earth’s
history, all rock was
igneous
5. GRANITE (intrusive)
Large, interlocking crystals
Forms from slow
crystallization of magma
Composition:
Quartz
Feldspar
Mica
“Dimension Stone”
Hard (resists scratching)
Strong (bears much weight)
Inert (resists weathering)
Polishes brilliantly
6. GABBRO (intrusive)
Course grained
Usually dark green
“Black Granite”
Composition
Plagioclase
Augite
Very little quartz
Uses
Cemetery markers
Counter tops
Floor tiles
7. OBSIDIAN (extrusive)
Molten lava cools
quickly (extrusive)
“Volcanic glass”
Composition
No minerals
Most is at least a few
million years old
Not found east of the
Mississippi
Used as a cutting tool
8. PUMICE (extrusive)
Lava cools during
flight, forming pores
During an explosive
volcanic eruption, may
rain down from the sky
High silica content
Uses:
Abrasives for cleaners
Scouring agents
Insulation
9. SCORIA (extrusive)
Often forms as the top
of a lava flow cools
Many of the same uses
as pumice
Porous, light
Higher basalt content
than pumice, giving it a
red to black color
10. These form from
existing rocks that
are chemically
changed by high heat
and/or pressure
Parent rocks can be
igneous, metamorph
ic, or sedimentary
11. SCHIST
Intermediate; between
phyllite and gneiss
Foliated (low-grade)
Contains lots of
mica, which allows it to
split into pieces
Garnet mica schist
contains small garnet
crystals (dark red)
12. PHYLLITE
Well defined foliation
Made of mostly fine-
grained mica
Intermediate; between
slate and schist
Formed from morphed
shales or mudstones
Sedimentary layers have
been preserved
Orange tint indicates
presence of iron
13. TALC SCHIST
(SOAPSTONE)
Extremely soft
Soapy or greasy texture
Talc
Softest mineral on Earth
Used in baby powder
Some areas of the San
Andreas Fault contain
soapstone
Limits severity and
frequency of earthquakes
Alabaster, serpentine
14. MARBLE
Recrystallized limestone
or dolemite, hence the
white color
Composed entirely of one
mineral: calcite
Often found interbedded
with
schists, phyllites, gneisses,
and granulites
If the marble has other
colors, it is because of
impurities in the original
limestone deposit
Uses:
Buildings, monuments, tab
letops
15. AMPHIBOLITE
Parent rocks:
Basalt, gabbro, dolemite
(sedimentary)
Medium to coarse-
grained crystals
Hornblende and
plagioclaste (ex.)
16. Sedimentary rocks form
from pieces and parts of
older
rocks, plants, skeletons, s
hells, or other debris
Most of the rock in our
area is sedimentary, like
sandstone and slate
Sedimentary rocks are
the only rocks that we
find fossils in
17. LIMESTONE
Composed primarily of
CaCO3 (calcium
carbonate), or calcite
Forms underwater, from
shells and skeletons of
dead sea creatures
Fossil rich
Common in PA
Limestone blocks were
used to construct the
Great Pyramid of Egypt
18. SANDSTONE
Clastic rock made up of
fine grains of sand
“glued” together by
minerals (usually
quartz)
Common to
beaches, floodplains, de
ltas, and deserts
Siltstone, mudstone
A red tint indicates the
presence of clay
19. OIL SHALE
Shale is extremely
common in
riverbeds, under, and
around lakes
Forms from the
compaction of silt and
clay sized particles
Black shales contain
organic materials that
produce oil, natural
gas, and coal
20. ROCK GYPSUM
Gypsum is a common
mineral that is used in
drywall, plaster, and
concrete
Sulfate mineral (SO4)
Forms in caves
Satin spar gypsum and
alabaster are used in
decorative pieces
21. ROCK ON!
And thank you geology.com for all your
useful info and pictures!
