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Rock Descriptions
Lab Activity
Igneous Rocks (1-5)
Obsidian is either a black or brown igneous rock that forms when molten rock material
cools so rapidly that atoms are unable to arrange themselves into a crystalline structure.
The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal
fracture.
Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of
trapped gas within the melt at the time of solidification. It often forms as a frothy crust on
the top of a lava flow or as material ejected from a volcanic vent and solidifying while
airborne.
Granite is a light-colored igneous rock with grains large enough to be visible with the
unaided eye. It forms from the slow crystallization of magma below Earth’s surface.
Granite is composed mainly of quartz and feldspar with minor amounts of micas,
amphiboles and other minerals. This mineral composition usually gives granite a red,
pink, gray or white color with dark mineral grains visible throughout the rock.
Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains
quartz and feldspar minerals. Rhyolite tends to streak very easily.
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and
pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but
can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a
composition similar to gabbro. The difference between basalt and gabbro is that basalt is
a fine-grained rock while gabbro is a coarse-grained rock.
Sedimentary Rock (6-10)
Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2
millimeter diameter) weathering debris. Environments where large amounts of sand can
accumulate include beaches, deserts, flood plains and deltas.
Shale is a fine-grained sedimentary rock that forms from the compaction of silt and claysize mineral particles that we commonly call "mud". This composition places shale in a
category of sedimentary rocks known as "mudstones". Shale is distinguished from other
mudstones because it is fissile and laminated. "Laminated" means that the rock is made
up of many thin layers. "Fissile" means that the rock readily splits into thin pieces along
the laminations
Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in
the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine
waters. It is usually an organic sedimentary rock that forms from the accumulation of
shell, coral, algal and fecal debris. It can also be a chemical sedimentary rock formed by
the precipitation of calcium carbonate from lake or ocean water.
Breccia is a term most often used for clastic sedimentary rocks that are composed of
large angular fragments (over two millimeters in diameter). The spaces between the large
angular fragments can be filled with a matrix of smaller particles or a mineral cement that
binds the rock together.
Conglomerate is a clastic sedimentary rock that contains large (greater than two
millimeters in diameter) rounded clasts. The space between the clasts is generally filled
with smaller particles and/or a chemical cement that binds the rock together.
Metamorphic Rocks (11-15)
Schist is metamorphic rock with well developed foliation. It often contains significant
amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate
metamorphic grade between phyllite and gneiss.
Gneiss is foliated metamorphic rock that has a banded (lines) appearance and is made up
of granular mineral grains. It typically contains abundant quartz or feldspar minerals.
Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale
or mudstone by low-grade regional metamorphism. It is popular for a wide variety of
uses such as roofing, flooring and flagging because of its durability and attractive
appearance.
Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of
sandstone. It is composed primarily of quartz.
Marble is a non-foliated metamorphic rock that is produced from the metamorphism of
limestone. It is composed primarily of calcium carbonate. Usually a very light color.