EQ:
How do geologists classify rocks?
How are rocks alike and how are they different?
I.
Rock Classification
To study a Rock
sample:
— All rocks are mixtures
Observe the rock’s…
of minerals and other
materials.
Ø Color
Ø Texture
Ø Mineral composition
— mixture - two or
more substances
mixed together but not
chemically combined
—by their Origin …
—where/how the rock formed
—Geologist try to identify a rock by
its texture (grain size) and mineral
composition
Texture of a Rock is
determined by…
A. Size, shape, and pattern of the rock’s GRAINS
(p.75-76)
1. COARSE-GRAIN
~large & easy to see b/c they’ve had a chance to cool slowly; ex.
diorite
2. FINE GRAIN
~very small-microscopic; ex. slate
3. GRAIN SHAPE: varies
4. GRAIN PATTERN: flat layers
Swirling
Multicolored beads (gneiss)
Random
5. NO VISIBLE GRAIN – cools very quickly so it looks smooth &
— observing the shape & size of crystals
— identify the minerals
— test using acid to determine mineral
composition
— test using a magnet to detect
elements iron (Fe) or nickel (Ni)
A. 3-Major Rock Groups
1.
Igneous
a. Formed first (according to our text); any rock formed from
lava or magma…“ignis” is Latin for fire
b. 2 major Groups (based on how they’re formed)
1. Extrusive igneous rock – formed from lava;
ex. basalt
2. Intrusive igneous rock – formed when magma hardens
beneath the Earth’s crust; cools slowly…ex. Granite –
2. Metamorphic
a. Forms when existing rock is changed by extreme
heat, pressure, or chemical reactions; any rock can be
changed into a metamorphic rock even other
metamorphic rocks *Ex. granite can be changed to
gneiss
b. Classified by their grains as…
Foliated - grains arranged in parallel layers or bands (ex.
slate, schist & gneiss)
Unfoliated - grains arranged randomly (ex. marble &
3. Sedimentary
a. Forms from small pieces of other rocks or the
remains of plants and animals deposited by wind,
water & ice (sediments); pressed and cemented
together; forms in layers below the surface
b. Sedimentary rock formation: erosion, deposition,
compaction, and cementation
c. 3-major groups: clastic, organic & chemical
rocks