Rock Types
- Andesite
- Extrusive igneous rock. Intermediate color; fine grain. Rapid-
cooled mineral equivalent of diorite.
- Basalt
- Extrusive igneous rock. Dark color; fine grain. Rapid-cooled
mineral equivalent of gabbro or diabase.
- Breccia
- Clastic sedimentary rock composed of angular fragments in a
binding matrix.
- Chert
- Flint-like (glassy) sedimentary rock composed mostly of silica.
Chert nodules may be deposited by precipitation of silica in voids
left by organic decay.
- Coal
- Black or dark brown carbon-righ combustible rock composed of
compressed plant remains. Usually sedimentary.
- Conglomerate
- Clastic sedimentary rock composed of rounded fragments in a
binding matrix.
- Diabase
- Intrusive igneous rock. Dark color; medium grain. Mineral
equivalent of gabbro or basalt.
- Diorite
- Intrusive igneous rock. Intermediate color; coarse grain. Slow-
cooled mineral equivalent of andesite.
- Dolomite
- Sedimentary rock composed of calcium and magnesium carbonate;
usually limestone altered by magnesium-rich brines. Sometimes
called "dolostone" to distinguish from mineral dolomite.
- Gabbro
- Intrusive igneous rock. Dark color; coarse grain. Slow-cooled
mineral equivalent of basalt or diabase.
- Gneiss
- Foliated metamorphic rock composed of light (feldspar, quartz) and
dark (biotite, hornblende) mineral layers or lenses. Product of
high pressure and temperature.
- Granite
- Intrusive igneous rock. Light color; coarse grain. Slow-cooled
mineral equivalent of rhylolite.
- Graywacke
- Clastic sedimentary rock composed of particles of many sizes,
poorly sorted and bound in a "dirty" matrix. Thought to be
deposited by marine turbidity currents.
- Hornfels
- Non-foliated very-fine-grain metamorphic rock; usually produced by
high-temperature contact metamorphism around intrusive bodies.
- Limestone
- Organic/chemical or clastic sedimentary rock composed of calcium
carbonate; usually shells, skeleta, or secretions of marine
animals.
- Marble
- Non-foliated metamorphic rock composed of interlocking calcite
(and possibly dolomite) crystals. Parent material: limestone or
dolomite (dolostone).
- Migmatite
- Foliated quasi-metamorphic rock; transitional between gneiss and
igneous rock. Product of high pressure and temperatures near
melting point.
- Mudstone
- Clastic sedimentary rock composed of fine particles. Finer than
sandstone; coarser than shale; similar to siltstone
- Pegmatite
- Intrusive quasi-igneous rock. Usually light color; very coarse
grain; large feldspar and quartz crystals with frequent massive
mica and tourmaline. Thought to condense from water-rich residual
melts.
- Phyllite
- Foliated fine-grain metamorphic rock containing microscopic mica
particles in parallel alignment. Silky texture; often wavy
cleavage.
- Quartzite
- Non-foliated metamorphic rock composed of interlocking quartz
crystals. Parent material: sandstone.
- Rhyolite
- Extrusive igneous rock; light color; fine grain. Rapid-cooled
mineral equivalent of granite.
- Sandstone
- Clastic sedimentary rock composed of sand-size particles, usually
quartz.
- Schist
- Foliated medium- to coarse-grain metamorphic rock containing
visible sheet mineral particles (often mica) in quasi-parallel
alignment. Texture: flaky, platy. May contain garnets.
- Shale
- Clastic sedimentary rock composed of very fine (clay-size)
sediment particles.
- Siltstone
- Clastic sedimentary rock composed of fine particles. Finer than
sandstone; coarser than shale; similar to mudstone.
- Slate
- Foliated very-fine-grain metamorphic rock contain microscopic clay
mineral particles aligned at low temperature by shearing
pressure. Splits on flat parallel planes.