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GEOLOGYMACC
Bill Palmer
Lecture 4
Rocks
What are Rocks?
Rocks are solid materials that comprise nearly all of the earth (and moon and planets).
Rocks are almost always aggregates of minerals.
(This is why we study minerals first).
What is the Rock Cycle?
The rock cycle explains how all rocks can be formed, deformed transformed, destroyed, and reformed as a result of natural processes.
Rock Cycle
Materials for forming rocks comes from:1) the Earth’s mantle (molten rock is magma when in the
ground and lava when it erupts on the surface)
2) Space (meteorites)
3) Organisms (plants and animals)
4) Fragmentation and decay of other rocks and minerals
Rock Cycle
There are three main rock groups:1) IGNEOUS ROCKS –Form when magma or lava
cool.2) SEDIMENTARY ROCKS- Form when chemical
residues and fragments of plants, animals, crystals, or rocks, or sand are compressed and cemented together. Also form from chemical precipitation.
3) METAMORPHIC ROCKS- Form when other rocks are changed by heat, pressure, or hot fluids.
The Rock Cycle
Rock Properties
1. General form-smooth, rough, irregular 2. Color-light, dark, 3. Grains-sand, gravel, crystals, fossils, fragments 4. Shape-teardrop, icicle, smooth, sphere
Rock Properties
5. Texture is very important-GlassyFine-grained , <1mmCoarse-grained, >1mmVesicular-bubblesFoliated-layersClastic-”clasts” of plants, animals, minerals
Igneous Rocks
Intrusive Igneous Rocks-Magma is the source and the rocks cool slowly beneath the ground and form large crystals.
Extrusive Igneous Rocks-Lava is the source and the rocks cool quickly above the ground and form small crystals.
INTRUSIVE STRUCTURES
1. Batholith-Massive, no “bottom” 2. Sills-Sheet-like between layers of other rocks3. Laccolith- “Blister-like”4. Pipe-vertical tube5. Dikes-Layers vertical to rock
1. Radial2. Sheet3. Ring
EXTRUSIVE
Lava Flows Pyroclastic deposits (fire fragments)
How to Analyze Igneous Rocks
STEP 1 and 2-Study the Color and Mineral CompositionLight- Felsic Minerals (feldspar)
Quartz (gray, white)Plagioclase Feldspar (white)Potassium Feldspar (pink)Muscovite Mica (clear, brownish)
How to Analyze Igneous Rocks
STEP 1 and 2-Study the Color and Mineral CompositionDark-Mafic Minerals (no feldspar)
Biotite Mica (black)Amphibole (dark gray)Pyroxene (dark green)Olivine (green)
How to Analyze Igneous Rocks
STEP 3-Study the Texture Intrusive
Pegmatitic (crystals >1 cm)Phaneritic (crystals 1-10 mm)Porphyritic (large and small crystals indicate that it cooled at
TWO different rates)
How to Analyze Igneous Rocks
STEP 3-Study the TextureExtrusive
Porphyritic (large and small crystals indicate that it cooled at TWO different rates)
Aphanitic (crystals < 1mm)Glassy (like glass, crystals too small to see)Vesicular (small or large bubbles, like pie meringue or froth)Pyroclastic- particles emitted from volcano
Now, put it all together!
Now, put it all together!
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained?
What is it?
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained?
What is this Igneous Rock? Granite Yes, both are granite-one
gray and one red!
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained?
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained?
What is this Igneous Rock? Rhyolite
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained?
Be careful with this one, it is really very fine grained with some larger white minerals.
What is this Igneous Rock? Rhyolite Porphyry
What is this Igneous Rock? Light or Dark? Coarse or Fine Grained? What is the unique
texture?
What is this Igneous Rock? Pumice
What is this Igneous Rock? Light or Dark? Coarse or Fine-grained? What is the unique
texture?
What is this Igneous Rock? Light or Dark? Coarse or Fine grained?
What is this Igneous Rock? Basalt
Some Igneous Rocks to Know
Granite Gabbro Rhyolite Basalt Scoria Pumice Rhyolite Porphyry
Wrap-ups
1. What are rocks?
2. What is the Rock Cycle?
3. What is the difference between magma and lava?
4. What are the three types of rocks?
5. What is a batholith? Sill? Laccolith?
Wrap-ups
Name the rock described:
6. Light colored, coarse grained.
7. Dark colored, coarse grained.
8. Light colored, very fine grained
9. Spongy looking and light in color.
10. Dark and glassy looking.
