Geoscience Reference
In-Depth Information
Let's begin this investigation by considering the shape
of deltas dominated by rivers (Figure 19.29a). This kind of
delta occurs along a coastline where wave energy and tidal
range are typically low and stream energy is relatively high.
In these situations, such as the Mississippi Delta, the delta is
elongated because the stream is able to extend into the ocean
through progradation because waves and tides are unable to
blunt this growth. Wave-dominated deltas are those that form
where the energy of incoming waves is high. These deltas tend
to be blunted and even cuspate because strong waves push sedi-
ments back toward the mainland. A good example of such a
cuspate delta can be seen where the Brazos River meets the
Gulf of Mexico along the Texas coast (Figure 19.29b). Finally,
tide-dominated deltas form along coasts that have a high tidal
range. In these settings, such as the Ganges River Delta in India
(Figure 19.29c), the velocity of incoming and outgoing tides
exceeds river velocity except during floods. As a result, the
delta takes on the form of an estuary with numerous tidal inlets.
Coral Reefs Thus far our discussion of depositional coast-
lines has focused on landforms that develop due to the accumu-
lation of sediment. Another way that coastal landforms evolve
is through the growth of coral reefs (Figure 19.30). Coral reefs
form due to the symbiotic interaction between coral polyps
and microscopic algae (called zooxanthellae ) that give the cor-
als their color. When the polyps die, they leave an exoskeleton
behind composed of calcium carbonate. These deposits grow
larger with time because, when one generation of corals dies,
Figure 19.30 Pillar coral in the Cayman Islands. Coral comes
in a wide variety of colors, shapes, and species, but they all pro-
duce calcium carbonate external skeletons.
a new one grows on top of it. Coral reefs grow best in tropical
waters between about 30° N and 25° S that are warmer than
20°C (68°F), free of suspended sediment, and well aerated.
Figure 19.31 shows the distribution of coral reefs on Earth.
Coral reefs form in three distinct types of settings. A fring-
ing reef forms on a shallow platform attached to the shore or
Coral reefs Resistant marine ridges or mounds consisting
largely of compacted coral together with algal material and
biochemically deposited calcium carbonates.
80° N
80° N
60° N
60° N
40° N
40° N
20° N
20° N
20° S
20° S
40° S
40° S
0
3,000 km
Coral Reefs
0
3,000 mi
60° S
60° S
80° S
80° S
Figure 19.31 Global distribution of coral reefs.
Coral reefs are very common in tropical areas.
 
 
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