Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 23

of time. On rare occasions, however, sudden events affect sedimentary processes and
urban environments. These short-duration events are typically labeled as geologic
hazards and may include tectonic processes such as earthquakes, tsunamis, volcanic
eruptions, and landslides. Some short-term geologic processes are also triggered by a
weather event such as a flood caused by extensive rains, sudden melting of snowpacks,
or a hurricane.
2.2.1 Formation of Sediments and Rocks
The fundamental building blocks of the Earth's crust are rocks. Understanding and evalu-
ating the rocks beneath urban areas requires knowledge of their composition, and how
they were deposited at a given location. On Earth, the dominant physical force that builds
and destroys rock is plate tectonics. Plate tectonics is based on the concept that the Earth's
lithosphere (crust and upper mantle) is composed of moving plates (Seyfert and Sirkin
1973; Oreskes 2003). In 1912, a German meteorologist, Alfred Wegener, noticed that some
sedimentary rocks known only to form in tropical environments were located in Arctic
and Antarctic regions. Wegener then compared these sedimentary deposits with their
approximate relative ages and distances from the equator and concluded that the conti-
nents had “drifted” to their present locations.
Wegener's hypothesis of continental drift was ridiculed because it lacked a mechanism
to forcibly move the continental land masses. Oceanographic research in the decades fol-
lowing World War II shed a great amount of detail on the structure of the earth and a
potential mechanism to move the continental crust. Today, we recognize that the continen-
tal and oceanographic crust ride passively along on lithospheric plates. Energy to drive the
movement of these tectonic plates is believed to be caused by the heat rising in the form of
convection currents generated at the core-mantle boundary. These concepts became the
theory of plate tectonics.
Figure 2.1 presents a world map showing each of the major tectonic plates. These litho-
spheric plates (crust and uppermost mantle) move on top of the upper mantle called the
asthenosphere (Figure 2.2). Their movement is responsible for most earthquakes, volca-
noes, seafloor spreading, mountain building, and ocean trench formation (Stanley 1999).
The energy source driving the movement of the plates is believed to be heat from convec-
tion currents originating deep within the interior of the Earth. Figure 2.3 shows the con-
cepts of plate tectonics.
Plate tectonics is responsible for, or plays a significant role in the formation, deforma-
tion, and weathering of most rocks on Earth. The primary classification of rocks recog-
nizes three categories (igneous, metamorphic, and sedimentary) based on their method of
formation.
Igneous rocks are formed by the solidification of molten material either beneath Earth's
surface (plutonic igneous rocks) or at the surface (volcanic igneous rocks) (AGI 1962).
Examples of plutonic (intrusive) igneous rocks include granite, diorite, gabbro, and peri-
dotite; volcanic (extrusive) igneous rocks include andesite, basalt, and rhyolite. Each type
of igneous rock is named based on its mineralogic and chemical composition.
Metamorphic rocks are formed by applying extreme temperature and pressure to
existing rocks that rearrange their structure and mineralogy to create a new rock.
Examples of metamorphic rocks include gneiss (formed from the metamorphism of
granite); quartzite (metamorphosed from quartz sandstone); and marble (metamor-
phosed limestone).