1 THE EARTH’S OCEANS

䊳 11.1 THE EARTH’S OCEANS

Oceans cover about 71 percent of the Earth’s surface. The Pacific Ocean is the largest and deepest ocean. It covers one third of Earth’s surface, more than all land combined. The Atlantic Ocean covers about half the area of the Pacific. The Indian Ocean is the smallest of the three ocean basins and lies primarily in the Southern Hemisphere. Other oceans, seas, and gulfs are portions of those three major ocean basins. For example, the Arctic Ocean is the northern extension of the Atlantic.

The sea floor is about 5 kilometers deep in most parts of the ocean basins, although it is only 2 to 3 kilo- meters deep above the mid-oceanic ridges and it can plunge to 10 kilometers deep in an oceanic trench. As a result of their area and depth, the ocean basins contain

1.4 billion cubic kilometers of water—18 times more than the volume of all land above sea level. The size of an ocean basin changes over geologic

Figure 11–1 An oceanographer extracts sediment from a time because new oceanic crust forms at spreading cen-

core retrieved from the sea floor. (Ocean Drilling Program, Texas ters, and old sea floor is consumed at subduction zones.

A&M University)

At present, the Atlantic Ocean is growing wider at the mid-Atlantic ridge, while the Pacific is shrinking at sub- duction zones around its edges—that is, the Atlantic Ocean basin is increasing in size at the expense of the Pacific.

pipe lowered on a cable from a research vessel. The Oceans profoundly affect the Earth’s climate. Some

weight drives the pipe into the soft sediment, which is marine currents carry heat from the equator toward the

forced into the pipe. The sediment core is retrieved from poles, while others carry cold water toward the equator.

the pipe after it is winched back to the surface. If the Without this heat exchange, the equator would be un-

core is removed from the pipe carefully, even the most bearably hot, and high latitudes would be colder than at

delicate sedimentary layering is preserved (Fig. 11–1). present. Because plate tectonic activity alters the distri-

Sea-floor drilling methods developed for oil explo- bution of continents and oceans, it also alters ocean cir-

ration also take core samples from oceanic crust. Large culation and thereby causes long-term climate change.

drill rigs are mounted on offshore platforms and on re- search vessels. The drill cuts cylindrical cores from both sediment and rock, which are then brought to the surface

䊳 11.2 STUDYING THE SEA FLOOR

for study. Although this type of sampling is expensive, cores can be taken from depths of several kilometers into

Seventy-five years ago, scientists had better maps of the

oceanic crust.

Moon than of the sea floor. The Moon is clearly visible

A number of countries, including France, Japan, in the night sky, and we can view its surface with a tele-

Russia, and the United States, have built small sub- scope. The sea floor, on the other hand, is deep, dark,

marines to carry oceanographers to the sea floor, where and inhospitable to humans. Modern oceanographers use

they view, photograph, and sample sea-floor rocks, sed-

a variety of techniques to study the sea floor, including iment, and life (Fig. 11–2). More recently, scientists have several types of sampling and remote sensing.

used deep-diving robots and laser imagers to sample and photograph the sea floor. A robot is cheaper and safer than a submarine, and a laser imager penetrates up to

SAMPLING eight times farther through water than a conventional

Several devices collect sediment and rock directly from

camera.

the ocean floor. A rock dredge is an open-mouthed steel net dragged along the sea floor behind a research ship.