Page 134 - Laboratory Manual in Physical Geology

Rock Composition mineral grains are present in the rock where they
originally formed. Examples are the intergrown
Composition of a rock refers to what it is made of. mineral crystals in an igneous rock that formed
Chemical composition refers to the chemical elements from cooling of lava or magma and intergrown
that make up the rock. This determines how the rock halite crystals in rock salt that formed in an
will react with materials of different composition, such evaporating sea. The intergrown mineral crystals
as whether or not it will react with and decay (tarnish, lock together to form the rock. Also, in situ
dissolve, chemically disintegrate) in air or water. It also mineral crystals are usually arranged randomly, and
determines rock color. For example, ferromagnesian-rich they may be engulfed in glass or a mass of smaller
rocks (iron- and magnesium-rich rocks) generally have intergrown crystals. Detrital mineral grains are
a dark color and ferromagnesian-poor rocks generally not in situ —they are not intergrown, and do not
have a light color. But the chemical elements in a rock lock together to form the rock. This is because
are normally bonded together in tangible materials they were removed from the place or rock where
like minerals that, in turn, make up most rocks. So the they originally formed and were transported by
physical composition of rocks is a description of what wind, water, ice, organisms, and/or gravity to
visible materials they are made of, in whole or part. It is a new place. There, they may become or have
your job as a geologist, using your eyes and simple tools already become part of another rock. Most detrital
(like a hand magnifying lens), to describe and identify mineral grains are clasts (see below), such as
what physical materials are made of.
quartz pebbles.

Volcanic Glass ■ Clasts. Physical weathering is the cracking,
crushing, and wearing away of Earth materials.
Glass is an amorphous (containing no definite form; The cracking and crushing causes big rocks, animal
not crystalline) solid that forms by cooling molten shells, and plants to be fragmented into broken
(liquefied by heat) materials like melted rock (lava) pieces called clasts (from the Greek klastós, meaning
or quartz sand (the main ingredient that is melted to broken in pieces). Plant fragments and shells or
make window glass). Volcanic glass (obsidian) looks and bones that have been separated or broken are often
breaks just like window glass, except that it is usually singled out as bioclasts . Broken mineral crystals

dark colored. But how does it form? When a volcano are detrital mineral grains (described above), and
erupts, and lava is erupted onto Earth’s surface, it begins broken pieces of rock are called rock fragments .
to cool. If the lava is fluid enough (has low viscosity), Similarly, geologists have names for size classes of
and stays liquefied long enough, then its elements and clasts (gravel, sand, silt, clay).
molecules will bond together and form mineral crystals.
But if the lava is too viscous, and cools too quickly, then ■ Gravel, sand, silt, and clay. These terms are

mineral crystals do not form and the solid material that often used to describe what a rock or other
remains is volcanic glass. feature is made of. For example, there is sand in
a sandbox, and sandstone is made of sand. But
Grains in Rocks the terms are actually names for size classes of
clasts (called Wentworth size classes after
Most rocks are made of grains —mineral crystals or C.K. Wentworth, who devised the scale in 1922).
other hard, visible particles. To view the grains in a rock Gravel is a mass of grains that are mostly larger
hand sample, start with your own eyes and look closely. than 2 mm (like aquarium gravel, pebbles,
If you cannot see or identify the grains, then also try cobbles, and boulders). Sand is a mass of grains
using a hand lens. Most geologists use a 10x hand lens, that are mostly 1/16 to 2 mm in diameter
meaning that objects viewed through the lens appear ten (like sand in a sandbox or making up a sandy
times larger than in real life. Here is a list of the kinds beach). Silt is finer than sand, so much that
of grains that comprise most rocks. You should look for you can barely see and feel the grains. The grains
the following:
are generally too small to identify with a hand
■ Mineral grains. Mineral crystals are the most lens or your unaided eye, so geologists refer to
common kind of grains in rocks. There are them collectively as silt. Clay is even finer than
thousands of kinds of minerals, but twenty or silt. If you ever played with pottery clay, then
fewer make up the bulk of most rocks and are you know that it can dry on your hands as a
known as rock-forming minerals ( FIGURE 4.3 ). light-colored slippery powder. You can tell it
Whenever possible, try to identify and record is there, but grains are too small to feel or see
what kind(s) of mineral crystals are present in individually (even with a hand lens). Thus,
any rock that you analyze. Also try to determine geologists refer to these microscopic grains
if the mineral crystals are in situ or not. In situ collectively as clay.

Rock-Forming Processes and the Rock Cycle ■ 115