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The Three Classes of Rocks
Class and Origin   Common Types

These rocks are formed from the hot, molten magma which either comes to the surface as lava, or is cooled and solidified within the crust. The rate of cooling and the mineral content of the magma determine the kind of igneous rock. Rocks with visible mineral crystals result from slow cooling. The faster the magma cools, the smaller are the crystals.

pegmatite PEGMATITE originates deep in the crust, forming very large crystals due to slow cooling.
granite GRANITE results from slow cooling and consists of crystals of quartz, mica, feldspar, and hornblende which are easily visible.
porphyry PORPHYRY is formed from magma that cooled deep in the crust first and then moved nearer the surface. Large crystals are set in a background of very small crystals.
basalt BASALT is a dark, dense rock. Crystals are microscopic in size, which tells us that this rock once flowed near or on the surface or the earth.
obsidian OBSIDIAN is commonly called volcanic glass. It cooled so quickly that it has no crystals.
pumice PUMICE is very light, full of holes, and floats in water.

Rocks are broken up into small particles by weathering and erosion. These materials, or sediments, are carried by streams and deposited in the oceans in layers. These layers are eventually buried by more sediments and compressed into rock. Some sedimentary rock is made of tiny particles that settle out of sea water onto the ocean floor, and some is made from the actual remains of plants and animals.

conglomerate CONGLOMERATE consists of pebbles and finer material. It resembles concrete.
sandstone SANDSTONE is made of sand particles, principally quartz.
shale SHALE, the most common sedimentary rock, is made of visible layers of silt and clay.
limestone LIMESTONE can be made up of tiny particles of calcium carbonate that settled to the ocean floor, or it can be entirely made of shells or coral.
peat PEAT is the compressed remains of plants. Compressed further, it forms lignite, then bituminous coal.
gypsum GYPSUM is the result of the evaporation of water from ancient seas.

Metamorphic means change of form. When rocks are subjected to sufficient heat, pressure, or movement (like faulting), there is a "recooking" of minerals within the rocks and the minerals are rearranged, in some cases in distinct bands.

gneiss GNEISS was once granite or conglomerate. It has dark and light minerals in wide bands.
schist SCHIST may have originally been shale, slate, granite, basalt, sandstone, or limestone. It is usually rich in mica.
slate SLATE, once shale, breaks easily into layers.
quartzite QUARTZITE was originally sandstone.
anthracite ANTHRACITE, or hard coal, was originally bituminous coal.
marble MARBLE was once limestone.

Our Wonderful World, An Encyclopedic Anthology for the Entire Family. NewYork: Grolier Incorporated, 1963

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This page was last updated on 12/15/2017.