The Sea Floor and Seawater
Beneath the ever-moving water lies the seemingly motionless ocean floor. If the basins of the world ocean could somehow be drained, the ocean bottom would look like a rugged, varied landscape, sculpted by volcanoes, earthquakes, erosion, and plate tectonics. Plate tectonics is a geological process in which the rigid pieces--called tectonic plates--that make up Earth's crust gradually shift in position over eons. Thus, even the sea floor moves, if ever so slowly.
As the plates move, they alter the topography (surface features) of the ocean floor. In the deep ocean, at sites where plates move apart from each other, magma (liquefied rock) oozes from cracklike rifts on the sea floor. The magma hardens into volcanic rocks called basalts, which form new oceanic crust on the sea floor. The build-up of new crust produces large, rocky ridges on both sides of the rift. The ridges form an almost continuous underwater mountain range that extends for approximately 60,000 kilometers (37,000 miles) across the Pacific, Indian, and Atlantic basins. The Mid-Atlantic Ridge is a segment of this feature.
Although ridges, trenches, and seamounts mark much of the deep ocean's landscape, extensive parts of the ocean floor are flat, featureless, and covered in a thick layer of sediment. These areas are called the abyssal plains. The sediment builds up from minerals and other material eroded from the continents and from the shells and other remains of tiny sea organisms.
Another sea-floor region lies along the continental margins, the submerged extensions of the continents. A continental margin consists of the continental shelf, the upper edge of the continent; the continental slope, the side of the continent; and the continental rise, a thick layer of sediment at the bottom of the slope. Deep underwater canyons mark the continental slope in a number of places.
The ocean's topography and currents make the sea a unique place, but the characteristics of seawater itself are also important in distinguishing the ocean from lakes and other bodies of fresh water. Seawater differs from fresh water because it contains salts. The most common salt in seawater is sodium chloride (table salt).
Salts alter the basic physical and chemical properties of water. Besides making saltwater heavier than fresh water of the same temperature, salts also lower the freezing point of water. This allows seawater to remain in a liquid state when its temperature is as low as -2 degrees C (28 degrees F).
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