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Unit 3: Oceans // Section 2: Ocean Structure and Composition

Like the atmosphere, the oceans are not uniformly mixed but are structured in layers with distinct properties (Fig. 2). Pressure increases with depth as the weight of the overlying air and water increase. Unlike the atmosphere, however, pressure changes at a linear rate rather than exponentially because water is almost impossible to compress, so its mass is equally distributed throughout a vertical water column. Atmospheric pressure at sea level is 14.7 pounds per square inch (also referred to as "one atmosphere"), and pressure increases by an additional atmosphere for every 10 meters of descent under water. This gradient is well known to scuba divers who have experienced painful "ear squeeze" from pressure differences between the air in their ears and the seawater around them.

Layers of the ocean

Figure 2. Layers of the ocean
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Source: © National Oceanic and Atmospheric Administration. National Weather Service.

The Epipelagic, or sunlight, zone (so called because most visible light in the oceans is found here) comprises the first 200 meters below the surface, and is warm and mixed by winds and wave action. Surface waters account for about 2 percent of total worldwide ocean volume. At a depth of about 200 meters, the Continental Shelf (the submerged border of the continents) begins to slope more sharply downward, marking the start of the Mesopelagic, or twilight, zone. Here water temperature falls rapidly with depth to less than 5°C at 1,000 meters. This sharp transition, which is called the thermocline, inhibits vertical mixing between denser, colder water at depths and warmer water nearer the surface. About 18 percent of the total volume of the oceans is within this zone.

Below 1,000 meters, in the Bathypelagic, or midnight, zone, water is almost uniformly cold, approximately 4°C. No sunlight penetrates to this level, and pressure at the bottom of the zone (around 4,000 meters depth) is about 5,880 pounds per square inch. Little life exists at the Abyssopelagic (abyssal) zone, which reaches to the ocean floor at a depth of about 6,000 meters. Together, these cold, deep layers contain about 80 percent of the total volume of the ocean.

The deepest points in the ocean lie in long, narrow trenches that occur at convergence zones—points where two oceanic plates collide and one is driven beneath the other. This region is called the Hadal zone. The deepest oceanic trench measured to date is the Marianas Trench near the Philippines, which reaches more than 10,000 meters below sea level. Highly specialized life forms, including fish, shrimps, sea cucumbers, and microbes, survive even at these depths.

Movement along colliding plates in convergence zones frequently generates earthquakes and tsunamis; an earthquake measuring 9.15 on the Richter scale off the coast of Sumatra triggered the Indian Ocean tsunami that killed more than 230,000 people on December 26, 2004. Volcanoes often erupt near convergence zones when hot magma escapes through rock fractures. Many of the world's largest ocean trenches, therefore, are located along the "Ring of Fire," an arc of volcanoes around the Pacific Ocean that denotes convergent plates margins (Fig. 3).

Ocean trenches and the "Ring of Fire"

Figure 3. Ocean trenches and the "Ring of Fire"
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Source: © United States Department of the Interior. United States Geological Survey.

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