- ROCKS:
Rocks are the solid materials of which the Earth's land formations (mountains, plateaus, etc.) are made.
Each type of rock contains one or more characteristic minerals, but their proportions vary. For example,
granite always contains quartz, orthoclase feldspar, and biotite mica as its principal constituents,
but their proportions vary from one granite specimen to another.
- Geologists classify rocks into three types: Igneous, Sedimentary, and Metamorphic.
- Igneous rocks are those that were melted during their formation and then crystallized from molten material.
(Molten rock material is called magma if it remains below the surface, or lava if it spreads out along the Earth's surface.)
Different types of igneous rocks are distinguished by chemical composition and by the size of crystals.
Composition varies: "SiAl-ic" (more acidic, rich in silicon & aluminum, lighter color) to
"mafic" (more basic, rich in magnesium and iron, darker color).
Intrusive rocks (formed from magma that solidified below the surface):
- Granite: Sialic, with visible crystals (1-10 mm), predominantly containing quartz, potassium feldspar (orthoclase), and biotite mica.
- Pegmatite: Similar in composition to granite, but with much larger crystals.
- Gabbro: Black, similar in composition to basalt, but with visible crystals (1-10 mm).
- Diorite: Intermediate between granite and gabbro, hardly any quartz or pyroxene.
- Peridotite and Dunite: Black or very dark green, rich in olivine.
Extrusive rocks (formed at or near the surface by volcanic action or by lava flows):
- Basalt: The most common extrive rock, formed from solidified lava. Black in color, with crystals usually microscopic in size;
rich in very dark, iron-rich minerals (pyroxine, amphiboles, olivine).
- Rhyolite: A light-colored extrusive rock, similar in composition to granite but similar to basalt in texture, with microscopic crystals.
- Pumice (white or light gray) and Scoria (darker colors): Light-weight rocks filled with lots of tiny holes ("vesicles") that were made by
carbon dioxide or other volcanic gases. Pumice breaks apart very easily and is often used as an industrial abrasive for polishing surfaces.
- Obsidian (volcanic glass): Glass-like texture, close to black in large lumps but greenish and transparent in thin pieces; breaks like glass.
More details and more rock types are on pages 6 & 7
of the
ESRT Reference Chart
- Sedimentary rocks are those that are formed from the consolidation of loose material called sediment.
Different types of sedimentary rocks are distinguished by grain size, by chemical composition, and by the processes involved in their formation.
"Clastic" (formed from fragments or ground-up material):
- Conglomerate: formed from rounded pebbles (larger than 0.25 cm across)
- Breccia: formed from angular pebbles
- Sandstone: formed from sand, with particles between .006 cm and .025 cm across; predominantly made of quartz (SiO2)
- Siltstone: intermediate between sandstone and shale
- Shale: formed from extremely small particles (.0004 cm across or smaller), mostly of clay minerals derived from mica.
Shale usually splits along flat (generally hozizontal) surfaces.
Crystalline or chemical precipitates:
- Halite or rock salt: Salt (NaCl), formed from the evaporation of salt water ("evaporite" deposits).
- Gypsum: Evaporite deposits of calcium sulfate.
- Dolomite: Deposits of magnesium carbonate.
- Limestone: The most common chemically or biolgically formed rock type, made of calcium carbonate (CaCO3),
either as a chemical precipitate or as ground-up animal shells.
- Soft (bituminous) coal: Compacted plant fossils, made mostly of carbon.
- Metamorphic rocks are those that were derived from igneous or sedimentary rocks by high temperatures and/or high pressures
that caused minerals to recrystallize but not melt. Different metamorphic rocks are distinguished principally by the parent rock
from which they are derived, and sometimes by the degree of alteration.
Many unusual minerals (including gems) occur only in metamorphic rocks.
Foliated, with bands or streaks or layers of different colors or textures:
- Slate: Metamorphic rock derived from shale by low-grade metamorphism.
- Phyllite: Intermediate between slate and schist.
- Schist: A higher degree of metamorphism (higher pressure and temperature) produces more mica flakes, which form layers.
- Gneiss: (Pronounced "nice") Still higher pressure and temperature produces a rock that looks almost like granite except that it has bands.
Non-foliated:
- Anthracite coal ("hard coal"): Derived from metamorphosis of bituminous coal.
- Hornfels: Rocks of varying composition (often rich in feldspars) formed mostly by heat from nearby lava or magma.
- Quartzite: Metamorphosed sandstone, close to pure quartz.
- Marble: Metamorphosed limestone.
- Metaconglomerate: Metamorphosed conglomerate, with remains of distorted pebbles.
|