Rock Identification
Igneous Rocks
Plutonic
When
magma solidifies, it
becomes igneous rock. The magma
may cool and solidify under the Earth's surface, creating an intrusive
igneous rock. The grains of minerals are large
enough to see
because they cooled slowly underground. Rocks that formed far
underground, very slowly, are also called plutonic. Granite
is
one of the most common igneous rocks. It makes up the
continental
crust and many mountains are made of granite that has been forced
upwards.
Granite
Uses
of Granite
- Inside homes for countertops, sinks, tile walls and flooring
- Outside homes for building stones
- Construction projects for structural or decorative purposes
- A more costly, but beautiful substitute for concrete or asphalt pavers
- Durable curbs along streets
- Memorial stones
- Mount Rushmore
Diorite
contains less quartz than
granite (or none at all)
and is darker in color.
Gabbro is a very dark intrusive igneous rock containing no quartz and forms at the bottom of the magma chamber.
Volcanic Rocks
Extrusive (or volcanic) rocks, form when magma cools above the Earth's surface. The rapid cooling means that the crystals usually cannot be seen with the naked eye. Basalt is probably the most common volcanic rock and is mainly composed of iron and magnesium-rich minerals. It makes up most of the oceanic crust. The magma flows from Kilaeua Volcano in Hawaii cool rapidly into basalt.
Andesite also contains these minerals, but has a higher percentage of feldspars, aluminum silicates.
Rhyolite is light in color with a chemical composition equivalent to granite.
Gabbro is a very dark intrusive igneous rock containing no quartz and forms at the bottom of the magma chamber.
Volcanic Rocks
Extrusive (or volcanic) rocks, form when magma cools above the Earth's surface. The rapid cooling means that the crystals usually cannot be seen with the naked eye. Basalt is probably the most common volcanic rock and is mainly composed of iron and magnesium-rich minerals. It makes up most of the oceanic crust. The magma flows from Kilaeua Volcano in Hawaii cool rapidly into basalt.
Andesite also contains these minerals, but has a higher percentage of feldspars, aluminum silicates.
Rhyolite is light in color with a chemical composition equivalent to granite.
Other
volcanic rocks:
- Obsidian
- Pumice
- Tuff
- Scoria
Sedimentary Rocks
Sediment
comes from igneous rocks
that have been exposed to weathering
and
erosion. If the
sediment becomes lithified (bonded together),
it
forms a sedimentary rock. These are the most common, known as
detrital sedimentary rocks, in order from the most coarse to finest
particle size:
- Conglomerate
- Breccia
- Arkose Sandstone
- Quartz Sandstone
- Graywacke Sandstone
- Siltstone
- Shale
- Mudstone
- Micrite Limestone
- Oolitic Limestone
- Travertine
- Dolostone
- Chert
- Rock Gypsum
- Rock Salt
Rocks
formed from plant
or animal remains are called organic or biochemical sedimentary rocks:
- Fossiliferous Limestone
- Coquina Limestone
- Chalk
- Peat
- Lignite Coal
- Bituminous Coal
Metamorphic Rocks
Some
rocks, either igneous or
sedimentary, may be transformed into
a metamorphic rock. The changes take place under the earth's
surface. The variation in color and chemical composition is
due
to the
conditions in which the rocks formed, including heat, pressure,
tectonic forces, effects from water and time.
Metamorphic
rocks may be either foliated or nonfoliated. The foliated
rocks
have a platy, sheet-like, or parallel appearance.
Foliated
Foliated
- Slate
- Phyllite
- Schist
- Gneiss
Schist
Non-Foliated
Non-Foliated
- Marble
- Quartzite
- Metaconglomerate
- Anthracite
- Hornfels
Back to Geology
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