Environmental Engineering Reference
In-Depth Information
concentration is the amount (by weight) of salt in water, usually expressed as
parts per million (ppm); 1 ppm is equivalent to a shot glass of water taken from an
Olympic-sized swimming pool. Water is saline if it has a concentration of more than
1000 ppm of dissolved salts; ocean water contains about 35,000 ppm of salt (USGS,
2007). Chemical precipitation, absorption onto clay minerals, and plants and ani-
mals prevent seawater from having higher salinity concentrations; however, salinity
does vary in the oceans because surface water evaporates, rain and stream water is
added, and ice forms or thaws.
Along with salinity another important property of seawater is temperature. The
temperature of surface seawater varies with latitude, from near 0°C near the poles
to 29°C near the equator. Some isolated areas can have temperatures up to 37°C.
Temperature decreases with ocean depth.
OCEAN FLOOR
The bottoms of the ocean basins (ocean floors) are marked by mountain ranges,
plateaus, and other relief features similar to (although not as rugged as) those on the
land. As shown in Figure 7.1, the floor of the ocean can be divided into four divi-
sions: continental shelf, continental slope, continental rise, and deep-sea floor or
abyssal plain:
•
The
continental shelf
is the flooded, nearly flat true margins of the conti-
nents. Varying in width to about 40 miles and a depth of approximately 650
feet, continental shelves slope gently outward from the shores of the conti-
nents. Continental shelves occupy approximately 7.5% of the ocean floor.
•
The
continental slope
is a relatively steep slope descending from the con-
tinental shelf; it descends rather abruptly to the deeper parts of the ocean.
These slopes represent about 8.5% of the ocean floor.
•
The
continental rise
is a broad gentle slope below the continental slope con-
taining sediment that has accumulated along parts of the continental slope.
•
The
abyssal plain
is a sediment-covered deep-sea plain about 12,000 to
18,000 feet below sea level. This plain accounts for about 42% of the ocean
floor.
Continental margin
Continental shelf
Continental slope
Continental rise
Abyssal plain
FIGURE 7.1
Cross-section of ocean floor showing major elements of topography.
