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small grains. The grain size indicates the cooling rate of the rock. Fine
grained (dense) igneous rocks form when rapid cooling occurs. Conversely,
coarse-grained rocks cooled slowly and crystals grew large. Fundamentally,
igneous rocks are classified as either intrusive or extrusive. Intrusive rocks
originate from magmas (molten rock materials combined with gases) at depth
in the earth. Intrusive rocks occur as massive structure or as in "injection"
structure. This latter structure forms when the hot, liquid or plastic rock is
injected into fractures in the surrounding solid rock. Intrusive action leads to
the formation of batholiths, laccoliths, stocks, dikes, and sills. Weathering
and erosion later expose these structures on the earth's surface. Extrusive
rocks are formed by volcanic activity at the surface of the earth. These rocks
cool rapidly. Examples of some common igneous rocks are: rhyolite,
andesite, basalt, granite, diorite, gabbro.
The formation of sedimentary rocks begins with the breaking down of other
rocks into fragments. The forces of weathering and erosion, such as running
water or freezing and thawing, accomplish this mechanical and chemical
breakdown. Fragments are transported to and deposited in lake and ocean
bottoms. Later, spaces between the fragments are filled with a cementing
material or are eliminated by pressure. After some time passes, a massive
rock layer results. Sedimentary rocks are classified based on the size of the
particle of the sediment, or fragment. Shale (dense, fine particles), sandstone
(particles distinguishable to the naked eye) and conglomerate (pebbles and
gravel cemented together) are examples of sedimentary rocks.
Metamorphic rocks are formed from previously existing igneous,
sedimentary or possibly other metamorphic rocks. Great heat and pressure,
yet not enough to completely melt the rock, alter the rocks original physical
composition. Sometimes the process of metamorphism aligns the grains in
parallel layers or bands. This layering is called foliation. When broken, a
metamorphic rock usually breaks along the plane of foliation. Metamorphic
rocks are classified based on their grain size and degree of foliation. Some
examples of metamorphic rocks are: slate, schist, gneiss.
(The above information was adapted from Leo Mark Anthony and Michael Mark Anthony,
Introductory Prospecting and Mining
, pp. 69-93. Mining and Petroleum Training Service,
University of Alaska, Soldotna, AK, 1997.)