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 The major Rock Types and how they form 

Igneous Rocks

Igneous rocks solidify from a liquid magma as it cools. They are described on two axies: 1) Rocks that are quartz rich (felsic) to magnesium rich (mafic) and 2) fast cooling (small crystals) to slow cooling (large crystals). When a magma cools rapidly, mineral crystals do not have time to grow very large. When a magma cools slowly crystals grow to several millimeters or more in size. Granite and basalt are opposites on these axies for the description of igneous rocks - granite is a slow cooled quartz rich rock, basalt a rapidly cooled mafic rock.

an igenous rock: basaltic lava flow with ropy textured pahoyhoy on surface

Basaltic lava flow, Idaho

Basalt is the stuff of the ocean floor. It comes from fresh upwellings from the mantle. Basalts are extrusive igneous rocks rich in minerals containing magnesium and other similar metallic atoms. Most of these minerals have a dark color. Basalts are thus dark in color. Basalt cools rapidly as volcanic eruptions dump magma out into the air, water or inject it in little intrusions into cold rocks, crystals never have chance to grow large.

A magma with at basaltic composition that cooled slowly and had a chance to grow large mineral crystals would form an intrusive igneous rock that we would call a gabbro.

an intrusive igneous rock pinkish tan exposures of the Cadillac granite on the top of Mt Cadillac, Mt Desert Island, Maine.

Granite, Acadia National Park, Maine

Granite, on the other hand, is the stuff of continents. At subduction zones, basalt and other rocks are recycled and are fractionated, producing new magmas that concentrating the felsic minerals - quartz and k-feldspar. These magmas cool to produce granitic igneous rocks. Quartz and feldspar are light in color. Since they make up the bulk of a granite, granites are light colored. Granite cools slowly in large bodies of magma that are trapped underground, thus large crystals of quartz, feldspar, mica, and hornblende have a chance to grow. The slower the cooling, the larger the mineral grains grow.

A granitic composition magma that was erupted at the earth's surface and cooled rapidly would not grow large crystals, and would form an extrusive igneous rock that we would call a rhyolite. A similar magma that cooled very rapidly would form volcanic glass - obsidian.

Sedimentary Rocks

Granite is primarily composed of quartz and feldspar. Quartz is very resistant to chemical weathering. Feldspar weathers easily into clay minerals. Thus mountains made of granite weather into sand grains made of quartz and clays. Sand grains are heavier than clays, and clays thus tend to get carried further by water before they sink to the bottom. Thus the edges of continents tend to get accumulations of sand, while areas offshore tend to get accumulations of mud. All through this process, animals that build their skeletons of calcium carbonate die and add their shells to the sediment, but only very far from the source areas of the sand and clay can deposits made up just of animal skeletons form. Such deposits made mostly from calcium carbonate form limestones. Sand is proximal (close to source), clay is distal (far from source), and carbonates (limestones) are so far from the source that not much clay ever gets there. Sedimentary rocks tend to form a gradient of grain size and composition.

a clastic sedimentary rock: sand grains in hawkesbury sandstone, sydney, australia a carbonate sedimentary reock: massive blue grey lowville limestone, ordovician, new york.
Sandstone (2cm field of view)

Metamorphic Rocks

If you take existing rocks, bury them, and expose them to heat an pressure, they get cooked into other rocks. Chemical reactions take place, and mineral grains grow. The simplest stage of this process is the formation of sedimentary rocks: take sediments, bury them and they lithify - soft sediments turn into rocks. The basic proccess involved in the formation of metamorphic rocks is that in rocks exposed to heat and pressure, but not enough heat enough to melt, mineral grains start to grow. The rock that forms depends on two things: the original composition of the rock that is being cooked and the temperature and pressure to which it is exposed.

a metamorphic rock red slate, locality unknown, probably new england.  George Flagg Collection.

Slate (4cm wide)

Start, for example, with a clay. Heat and pressure turn it into a shale. Under more heat and pressure the, rock loses its original fabric as minerals start to grow; it turns into slate. Under more heat and pressure, the mineral grains grow large enough to be visible to the eye, into a medium grained schist, or a coarse grained gneiss. More heat and pressure melt the rock into a liquid magma.

If however, you start with a limestone, no amount of cooking will produce a slate. Under mild increases in temperature and pressure, a limestone will metamorphose into a marble (with growth of calcium carbonate crystals), and under further cooking, will melt.

 Related Pages 
Sedimentary Rocks    Coal Formation
Igneous Rocks    Granite
Geologic Sections
Metaphors for geologic time

Sources: Press and Siever, 1978
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Copyright © 2000 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris
Maintained by Athro Limited
Date Created: 25 Jan 2000
Last Updated: 30 Dec 2000