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Looking at the Sea Physical Features of the Ocean

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Beneath the world's oceans lie rugged mountains, active volcanoes, vast plateaus and almost bottomless trenches. The deepest ocean trenches could easily swallow up the tallest mountains on land.

Around most continents are shallow seas that cover gently sloping areas called continental shelves. These reach depths of about 650 feet (200 m). The continental shelves end at the steeper continental slopes, which lead down to the deepest parts of the ocean.

Beyond the continental slope is the abyss. The abyss contains plains, long mountains ranges called ocean ridges, isolated mountains called seamounts, and ocean trenches which are the deepest parts of the oceans. In the centers of some ocean ridges are long rift valleys, where Earthquakes and volcanic eruptions are common. Some volcanoes that rise from the ridges appear above the surface as islands.

Other mountain ranges are made up of extinct volcanoes. Some seamounts, called guyots, are extinct volcanoes with flat tops. Scientists think that these underwater mountains were once islands but their tops were worn away by waves. The diagram below shows the main features found on the ocean floor.

Facts about the Oceans

Area: about 140 million square miles (362 million sq km), ore nearly 71% of the Earth's surface.

Average Depth: 12,200 feet (3,720 m).

Deepest point: 36,198 feet (11,033 m) in the Mariana Trench in the western Pacific.

Mountains: The ocean ridges form a great mountain range, almost 40,000 miles (64,000 km) long, that weaves its way through all the major oceans. It is the largest single feature on Earth.

Highest Mountain: Mauna Kea, Hawaii, rises 33,474 feet (10,203 m) from its base on the ocean floor; only 13,680 feet (4,170 m) are above sea level.

Diagram showing the physical features of the ocean floor.

Oceanographers know these features exist because much effort has been spent on mapping the ocean bottom. In order to make maps of the ocean floor, the depth of the ocean must be known in many places. In the early days of ocean exploration, sailors made depth determinations called soundings by means of a lead line. This was simply a long piece of rope, marked off in fathoms (six-foot intervals) and having a lead weight at one end. The depth was measured by dropping the weight into the water and noting how much line went out when the lead weight reached the bottom.

Measuring Water Depth

Diagram showing sound waves beamed from a ship to the ocean floor.

Today's oceanographers use sonar instruments to generate a sound signal that is bounced or "echoed" off the sea floor and then recorded on board the ship. The speed of sound in water is 1,500 m per second, four times faster than the speed of sound in air. By carefully measuring the round-trip time of the sound waves and taking into account the variables of temperature and salinity, the depth of the water and the distance to another object can be measured accurately.

Taking soundings this way is time-consuming, especially in deep water. Today most depth measurements are made using an echo sounder. Instead of dropping a weight, a pulse of sound energy is transmitted electronically toward the bottom. The time it takes the pulse to travel to the bottom and be reflected back up to the surface is measured. From this time interval, the depth of the water can be calculated. When pulses are sent out and received in quick succession, an almost continuous recording of the ocean depth called a bottom profile may be obtained.

The sedimentary rocks that exist on the ocean bottom are much younger than any similar rocks found on the continents. The cores of mud and rock brought back by deep-sea drilling ships vary greatly in age, but no deposits from the ocean floor seem to be more than about 200 million years old. This makes oceanic crust very young compared with the continents, which contain rocks up to about 4 billion years old. How then did the oceans form?

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