Recognizing Rocks: Igneous, Sedimentary, and Metamorphic Types - Geology - page 88

Dike: Dikes are tabular, discordant features created by magma filling narrow cracks
or fractures in crustal rocks.
Sill: Sills are tabular concordant features created when magma fills space horizont-
ally (usually).
Laccolith: Laccoliths are created by a horizontal flow of magma between sediment-
ary rocks that are close to the surface and cause an upward bulge or dome shape
to the earth's surface.
Batholith: A below-ground igneous feature that is more than 100 square kilometers
(62 square miles) in size at the surface of the earth is called a batholith. These
massive, discordant plutons commonly make up the core of large mountain sys-
tems, such as the Sierra Nevada mountains in California. An exposed pluton that is
smaller than a batholith is called a stock.
Merging Many Single Grains of Sand: Sed-
imentary Rocks
Another type of rock, sedimentary rock, is much more common than igneous rock. Sedi-
mentary rocks are composed of pieces of other rocks. When any rock is exposed to the
elements — sunlight, wind, and water — it is eventually broken down into smaller
particles. These particles are moved around the surface of the earth by gravity, water,
ice, and wind (see Part IV for details). They eventually settle somewhere, are glued to-
gether, and become a sedimentary rock.
Digging in the dirt: Soils
All this talk about sediments may have you thinking about dirt. Dirt as you know it is somewhat different
than sediments. A more technical name for dirt is soil. Soils develop from the interaction of sediment
with air, water, organic materials (plants and animals), and bedrock. Soils are the sediments that sup-
port plant growth by providing mineral nutrients, as well as bringing water and air to the plant roots.
As illustrated here, soils are categorized into zones based on their content. This sequence of zones
is called a soilproile. A soil profile develops as air and organic materials move down into the sed-
iments, and minerals weathered and eroded from the underlying bedrock move up through the sedi-
ments. Water also moves from the surface into the sediments, and it flows through the sediments as
ground water.