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Guide
to Reading 
Chapter 14 dealt with freshwater on Earth’s
surface; Chapter 15 deals with the larger realm of saltwater on Earth,
the oceans. It begins with the physical structure of ocean basins.
Although these are difficult places to access, over the past century
scientists have managed to collect and piece together information
that gives quite a comprehensive picture of both the geology and
geography of the ocean floor. They have gathered an impressive amount
of data directly, starting with research cruises of HMS Challenger
in the 1870s and continuing through sea-floor explorations of the
deep-sea submersible Alvin a century later. In this section some
of your reading is a review of plate tectonics activities and features
(oceanic crust and lithosphere, mid-ocean ridges, fracture zones,
trenches, and active and passive continental margins). There are
also many new concepts concerning ocean depths and landscapes (bathymetry,
continental shelves, slopes and rises, abyssal plains, submarine
canyons, turbidites, submarine fans, seamounts, and guyots).
The composition and characteristics of ocean water
are discussed next. You read about salinity; why is the ocean salty,
and just how salty is it?
Ocean waters exhibit numerous patterns of movement.
There are surface currents, deep currents, eddies, the Coriolis effect,
upwelling zones, downwelling zones, thermohaline circulation, water
masses, and weather-related oceanic events like storm surges. Tides,
of course, are also water motion, but they are treated thoroughly
enough in this chapter to warrant their own subcategory. Some special
advice: This is a topic that seems so elementary it’s easy
to skim too lightly over the section. Everybody knows tides go in
and out, but unless you live on the coast, you probably don’t
really know much about tidal reach, mean sea level, flood tide, ebb
tide, tidal flats, tidal bores, intertidal zones, and the tide-generating
force.
Waves are another one of those topics that you should
be careful not to skim over too lightly, for the author presents
more than the average landlubber would believe there is to know about
waves. You read about their causes and geometric shapes and nomenclature,
including the terms “wave base,” “strength and
fetch of a wind,” “ripples,” “swells,”
“amplitude and wavelength of a wave,” “breakers
and surf zones,” “swash and backwash,” “effects
on embayments and headlands,” “longshore currents,”
and “rip currents.”
Ocean study includes a look at ocean boundaries,
that is, coastal areas and shorelines. One type of shoreline is a
beach. Beaches may be composed of different types of sand, including
silicic sands or carbonate sands, and they have distinct areas, including
a beach face, foreshore zone, intertidal zone, backshore zone, and
berm. Beaches, geologically speaking, are here today, gone tomorrow,
and change is constantly occurring. In connection with this you read
about active and inactive sand layers, bioturbation of sediments,
beach drift, sand spits, baymouth bars, barrier islands, lagoons,
and sediment budgets of beaches.
A shoreline may be a rocky coast rather than a sandy
one. Here wave erosion may produce wave-cut notches, cliff retreat,
wave platforms (benches), honeycomb-weathering patterns, sea arches,
and sea stacks.
Coastal areas may be coastal wetlands that are flooded
with shallow water but experience no wave action. The three basic
types of wetlands you read about are swamps, marshes, and bogs.
Some coastal areas are flooded stream valleys called
estuaries. Here a mix of fresh- and saltwater supports complex ecosystems
inhabited by unique salt-tolerant organisms.
Fjords are dramatic coastlines that result from
the flooding of glacial valleys.
Coral reefs are specialized communities found in
shallow, warm, well-lit seawater. Their basic physical structures
(limestone mounds) are created by colonial marine animals (cnidarians),
that live in a symbiotic (mutually beneficial) relationship with
the algae called zooxanthellae. In addition to these two creatures,
the reef provides the environment for a complex community of marine
organisms. Coral reefs are classified on the basis of their shapes,
which are determined by their origins (fringing reefs, barrier reefs,
and atolls).
Over time the ocean level rises and falls, and these
changes affect coastal areas. You read about emergent and submergent
coasts, and erosional (losing area) and accretionary (gaining area)
coasts.
Coastal areas have always experienced change due
to natural events; today the human population is large enough to
add its influence. The author concludes the chapter with a discussion
of some human-induced changes and the problems they have created.
For example, people build groins, jetties, breakwaters, and seawalls
to fight coastal erosion, and when this doesn’t work well enough
to suit them, they bring in their own sand (beach nourishment). The
results of such actions are often unpredictable; quite often they
benefit one area of a beach and harm an adjacent area. Thoughtless
human activity has destroyed huge amounts of coastal wetlands and
endangered coral reefs. As in Chapter 14 the author is reminding
us that as large and powerful as the water world is, the human presence
is affecting it and not always in a desirable fashion.
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