Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 25

Divergent
plate boundary

Ocean basin Continental plate

Spreading center

Convergent
plate boundary

Trench

Oceanic Plate

Convection

FIGURE 2.3
(See color insert.) Concepts of plate tectonics and continental drift.

Sedimentary rocks are formed from the mechanical weathering or erosion of preexist-
ing rock or from dissolved material that precipitates from solution. Therefore, there are
two types of sedimentary rocks (Pettijohn 1975):

• Clastic : Composed of grains of fragments of rocks and minerals that have been
eroded, transported, deposited, and eventually become cemented together to form
a rock.
• Chemical or biochemical : Composed of minerals that have been precipitated from
water by inorganic processes or biological processes. For example, coal is formed
from plant debris.

Common examples of clastic sedimentary rocks include conglomerate, sandstone, silt-
stone, and shale.
Some of the more common chemical and biochemical sedimentary rocks include lime-
stone, dolostone, coal, and the evaporites, halite and gypsum. When coal undergoes
significant changes through exposure to high temperatures and pressures, it forms a pro-
gressively higher-grade of coal, eventually becoming anthracite, considered to be a meta-
morphic rock (Pettijohn 1975).
A rock is simply composed of minerals (Flint and Skinner 1974). A mineral is a naturally
occurring solid formed through geological processes, which has a characteristic chemical
composition, a highly ordered atomic structure, and specific physical properties (Hurlbut
and Klein 1985). Minerals range in composition from pure elements to very complex sili-
cates, oxides, sulfates, and others with thousands of known forms. Rocks do not require
a specific chemical composition but have ranges of compositions of typically two or more
different minerals. For instance, igneous rocks differ in composition based on the chemical
makeup of their parent magma and their rate of cooling. The composition of sedimentary
rocks differs based on the parent material and the methods of transport and deposition.