PART 2: Rock Type Samples
IGNEOUS ROCK (Click here to view reference photos)
EXTRUSIVE (Volcanic)
1. fine (small) or absent crystals; crystal sizes vary
2. vesicles (gas bubble cavities & veins) and flow structures present
3. sometimes layered (lava, ash, ignimbrite) rapid cooling
4. occasional phenocrysts (mineral "supercrystals")
5. may have xenoliths (embedded non-igneous inclusions)
INTRUSIVE (Plutonic)
1. coarse (large) crystals; crystal sizes somewhat uniform
2. may contain xenoliths (host rock fragments) slow cooling
3. Often occur as branching veins in host rock seams
Very High Silica ("Acid", "felsic") |
High Silica |
Low Silica |
Very
Low Silica ("Basic", "mafic") |
|
| small crystals | Obsidian |
Rhyolite |
Andesite |
Basalt |
| large crystals | Granite |
Diorite | Gabbro |
SEDIMENTARY ROCK (Click here to view reference photos)
CLASTIC - compaction, cementation, desiccation
1. consolidation of individual fragments ("clasts")
2. visible angular or rounded grains; size uniformity varies
3. grains often sorted by size or shape into layers
4. often contain depositional structures or fossils
CHEMICAL - precipitation, mineralization
1. grains and/or crystals usually invisible or absent
2. often contain marine fossils sorted into layers
3. often have solution cavities and/or crenulations ("channels")
4. often react with various acids (e.g., CaCO3 with HCl)
| Smallest grain/atom | Largest grain/atom | |||
| Clastic | Shale | Mudstone | Sandstone |
Conglomerate |
| Chemical | Chert/Agate | Limestone | Gypsum | Halite |
METAMORPHIC ROCK (Click here to view reference photos)
FOLIATED
1. contorted narrow bands from compressive force
2. banded coloration/texture due to mineral separation
3. usually more dense ("solid") than original rock
4. contact (in igneous buffer zone) or regional (orogenic)
NONFOLIATED
1. larger crystals than in original rock
2. unbanded coloration/texture from single mineral composition
3. usually more dense ("solid") than original rock
ORIGINAL ROCK |
Lowest
Temperature/ Pressure Stresses |
Highest
Temperature/ Pressure Stresses |
||||
| BASALT | Schist | Amphibolite |
||||
| GRANITE | Granite | Gneiss | ||||
| SANDSTONE | Quartzite | |||||
| SHALE | Slate | Phyllite | Schist |
Gneiss | ||
| LIMESTONE | Marble | |||||
| VEGETATION | Peat |
Lignite |
Bituminous Coal | Anthracite Coal | ||
Mineral Content
|
Texture
|
Other
Characteristics
|
Parent Material
|
Rock Type &
Subtype
|
Rock Name
|
reddish-FeOx
black-MgOx
|
very coarse
|
HCl reactive
|
delta alluvium
|
1. conglomerate
|
|
olivine, ferromags
|
aphanitic
|
no HCl reaction
|
lava
|
2. basalt
|
|
calcite (CaCO3)
|
very fine/
grainless
|
HCl reactive,
fossiliferous
|
marine
precipitates
|
3. limestone
|
|
quartz (SiO2), ferromags
|
glassy/
no crystals
|
no HCl reaction
|
tephra or lava
|
4. obsidian
|
|
quartz, FeOx
|
coarse
|
no HCl reaction
|
sandstone
|
5. quartzite
|
|
calcite (CaCO3)
|
coarse/
granular
|
HCl reactive
|
limestone
|
6. marble
|
|
quartz, feldspar,
mica
|
phanitic
|
no HCl reaction
|
magma
|
7. granite
|
|
quartz, feldspar
|
aphaneritic
|
no HCl reaction
|
tephra or lava
|
8. rhyolite
|
|
quartz, FeOx
|
fine/medium
|
no HCl reaction
|
sand
|
9. sandstone
|
|
feldspar, quartz,
ferromags
|
medium
|
no HCl reaction
|
granite or gabbro
|
10. gneiss
|
2. In what part (N,S,E,W) of Wisconsin do we find mainly:
a. sedimentary rocks? SOUTH & EAST
b. igneous rocks? NORTHWEST AND NORTH-CENTRAL
c. metamorphic rocks? NORTH-CENTRAL AND NORTHEAST
3. What specific types of bedrock are found in Portage County?
GRANITE-g, METAVOLCANIC-vo, METAGABBRO-ga, GNEISS-gr, AMPHIBOLITE-gn, SANDSTONE-C;
i.e., IGNEOUS, SEDIMENTARY, METAMORPHIC
4. a. What is the oldest rock type in Wisconsin?
AMPHIBOLITE-gn (FOLIATEDMETAMORPHIC)
b. In what part of Wisconsin are the oldest rocks located?
NORTH-CENTRAL
c. What is the youngest rock unit in Wisconsin?
DEVONIAN DOLOMITE-D
d. In what part of Wisconsin are the youngest rocks located?
SOUTHEAST (around Milwaukee)
e. Generally speaking, older rocks are located beneath younger rocks. Assuming this is true for Wisconsin, how do you suppose the older rocks became exposed? How were the overlying younger rocks removed?
EROSION (FLUVIAL AND GLACIAL)
5. Rib Mountain, located almost exactly in the center of Marathon County, creates a clearly visible feature interrupting the generally flat landscape in central Wisconsin.
a. Using the geologic map, what type of rock is Rib Mountain made of?
NONFOLIATEDMETAMORPHIC (QUARTZITE-q)
b. Wherever this rock unit is exposed at the surface, mountains or hills can be found. What does this indicate about the resistance of this type of rock to erosion?
VERY HARD, THUS HIGHLY RESISTANT TO EROSION
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