Earth Materials II

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) .