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Rock Types
The Rocks in the exhibit are grouped in accordance with the composition, environment and processes by which they were formed.
Igneous-plutonic rocks are formed directly from molten rock which cooled slowly at considerable depth below the earth's surface. These rocks have visible crystals and some of the crystals may be very large. Examples include: granite, granite-pegmatite, syenite, diorite and gabbro.
Igneous-Volcanic Rock
Igneous-volcanic rocks are formed directly from molten rocks which cooled quickly at or near the surface of the earth. The textures of these rocks vary from glassy to dense; these rocks usually lack visible crystals. Examples include: rhyolite, pumice, basalt, scoria, obsidian, porphyritic andesite, tuff and volcanic breccia.
Sedimentary Rocks
Sedimentary Rocks may be divided into clastic and non-clastic types. Clastic rocks (conglomerate, sandstone, shale) are composed of broken fragments of pre-existing rocks which have been subjected to mechanical weathering. The farther that fragments are carried by glaciers, streams or wind, the smaller and more rounded the fragments become. Non-clastic rocks are composed of materials deposited by chemical precipitation or by the agency of organisms. Examples of these are: salt, gypsum, some deposits of limestone and dolomite and diatomaceous earth. Sediments cover most of the sea floor and perhaps three-quarters of the land surface.
Metamorphic Rocks
Metamorphic rocks are formed under conditions of intense heat and pressure at considerable depth in the earth's crust from older parent rocks. Commonly the minerals of the parent rock are altered and the metamorphosed rock becomes more coarsely crystalline. In many metamorphic rocks, the minerals tend to become layered and these layers may be bent or folded. Examples include: marble, schist, gneiss, and slate.