Earth Materials- Rocks
Three types of rocks:
1) Igneous : formed from liquid-crystal mush- magma
2) Metamorphic: formed at high Temp. and Pressure
3) Sedimentary: formed at surface of earth
Igneous rocks
Classified according to 1) mineral composition and 2) texture or grain size.
Two main types: 1) Plutonic or intrusive and 2) volcanic or extrusive.
Plutonic (intrusive) rocks are coarse grained, volcanic (extrusive) rocks are
fine grained- different rates of cooling
Examples of intrusive (plutonic) igneous rocks: Granite, Diorite, Gabbro
Examples of extrusive (volcanic) rocks: Rhyolite, andesite, basalt.
The rock pairs granite- rhyolite, diorite-andesite, gabbro-basalt have the
same chemical composition, but different texture (i.e. grain size).
When a magma cools, low silica (SiO2 or quartz), and high Fe and Mg
silicate minerals crystallize first: olivine and pyroxene (also called mafic
minerals).
Last silicate minerals to crystallize are high silica (SiO2 or quartz) and high
K minerals: Quartz and feldspar and mica (also called felsic minerals).
Volcanic rocks include: lava flows and pyroclastic rocks.
Sedimentary rocks
3 types: clastic, chemical and biogenic
1) clastic: composed of fragments of preexisting rocks. Classified according
to grain size: clay < silt < sand < gravel.
Clay lithifies to shale
Silt lithifies to siltstone
Sand lithifies to sandstone
Gravel lithifies to conglomerate
2) Chemical: chemically precipitated from water:
e.g. limestone (CaCO3), rock salt (NaCl), gypsum (CaSO4).
3) Biogenic: formed from fossilized living material:
Coal from dead plant material (trees etc),
Chert from dead SiO2-rich shells,
Chalk form CaCO3-rich shells
In general all sedimentary rocks characterized by layering or stratification.
Cross-bedding – formed by wind or flowing water.
Mud-cracks – drying out of sediment by sun.
Metamorphic rocks
Produced by high temperatures and pressures in Earth
Characterized by Foliation: parallel alignment of minerals.
Foliation is a plane of weakness in metamorphic rocks
Metamorphic equivalents
Shale = Slate, Schist
Limestone = marble
Quartz sandstone = quartzite
Granite = Gneiss (pronounced nice)
Gneiss is a coarse grained foliated metamorphic rock
Rock cycle: all three rock types can be converted to each other by common
geologic processes (see Fig. 2,12 text) .