Page 630 - 9780077418427.pdf

/Volume/201/MHDQ233/tat78194_disk1of1/0073378194/tat78194_pagefiles
tiL12214_ch24_597-622.indd Page 607 9/23/10 11:09 AM user-f465
tiL12214_ch24_597-622.indd Page 607 9/23/10 11:09 AM user-f465 /Volume/201/MHDQ233/tat78194_disk1of1/0073378194/tat78194_pagefile

FIGURE 24.11 A modern-day tuna fishing boat. Note the time-tested “crow’s nest” lookout on the mast and the more modern helicopter
fastened on top. Both are used to look for schools of tuna, but the helicopter also does other jobs.

of the chemical processes at work in the ocean, and the interac- “rim of fire.” It is called this because of the volcanoes associated
tions of the ocean and the atmosphere, and to chart the topog- with the converging plates. The “rim” also has the other associ-
raphy of the ocean floor. ated features of converging plate boundaries such as oceanic
trenches, island arcs, and earthquakes. The Atlantic Ocean is
2
second in size, with a surface area of 107 million km (about
OCEANS AND SEAS 41 million mi ) and the shallowest average depth of only 3.3 km
2
The vast body of salt water that covers more than 70 percent of (about 2.1 mi). The Atlantic Ocean is bounded by nearly parallel
Earth’s surface is usually called the ocean or the sea. Although continental margins with a diverging plate boundary between. It
there is really only one big ocean on Earth, specific regions have lacks the trench and island arc features of the Pacific, but it does
been given names for convenience in describing locations. For have islands, such as Iceland, that are a part of the Mid-Atlantic
this purpose, three principal regions are recognized: (1) the Ridge at the plate boundary. The shallow seas of the Atlantic,
Atlantic Ocean, (2) the Indian Ocean, and (3) the Pacific Ocean. such as the Mediterranean, Caribbean, and Gulf of Mexico, con-
As shown in Figure 24.12, these are not separate, independent tribute to the shallow average depth of the Atlantic. The Indian
2
bodies of salt water but are actually different parts of Earth’s sin- Ocean has the smallest surface area, with 74 million km (about
2
gle, continuous ocean. In general, the ocean is a single, continu- 29 million mi ) and an average depth of 3.8 km (about 2.4 mi).
ous body of salt water on the surface of Earth. Specific regions As mentioned earlier, a sea is usually a part of an ocean that
(Atlantic, Indian, and Pacific) are often subdivided further into is identified because some characteristic sets it apart. For exam-
North Atlantic Ocean, South Atlantic Ocean, and so on. ple, the Mediterranean, Gulf of Mexico, and Caribbean seas are
A sea is usually a smaller part of the ocean, a region with bounded by land, and they are located in a warm, dry climate.
characteristics that distinguish it from the larger ocean of which Evaporation of seawater is greater than usual at these locations,
it is a part. Often the term sea also is used in the name of certain which results in the seawater being saltier. Being bounded by land
inland bodies of salty water. and having saltier seawater characterizes these locations as being
The Pacific Ocean is the largest of the three principal ocean different from the rest of the Atlantic. The Sargasso Sea, on the
2
regions. It has the largest surface area, covering 180 million km other hand, is a part of the Atlantic that is not bounded by land and
2
(about 70 million mi ), and the greatest average depth of 3.9 km has a normal concentration of sea salts. This sea is characterized
(about 2.4 mi). The Pacific is circled by active converging plate by having an abundance of floating brown seaweeds that accumu-
boundaries, so it is sometimes described as being circled by a late in this region because of the global wind and ocean current

24-11 CHAPTER 24 Earth’s Waters 607