Oceanography and Water Study Guide for McGraw-Hill's ASVAB
The oceans cover 71 percent of the planet. Around the edges of the continents is the continental shelf. Further out is the continental slope, then the rise, and then the ocean floor. The shape of the continental shelf often determines the magnitude of the tides along the coast.
An ocean tide refers to the cycle of rising and falling seawater caused by the gravitational pull of the moon and the sun. The moon, being closer to Earth, has the strongest effect on tides. Tides during full and new moons, when Earth, the moon, and the sun are in a nearly straight line (spring tides), are the highest. When the sun is more or less perpendicular to Earth and the moon, less dramatic tides are created, called neap tides. Neap tides take place during the first and last quarter of the moon.
The continental slope has the steepest angle. The ocean floor contains a thin layer of basaltic rock. The ocean floor also has many interesting features, such as volcanoes, fissures, trenches, and ridges.
Surface water moves around the oceans with currents that are generated by winds. The friction between the ocean and the wind causes the water to move in the direction of the wind. Some currents are temporary and others are permanent, such as the Gulf Stream on the east coast of the United States (bringing warm water northward) and the California Current along the west coast of the United States (bringing colder water southward).
Water also moves around and up and down through the oceans as a result of temperature, density, salinity, and chemical differences between masses of water. Water that is denser (colder and/or saltier) sinks to the bottom, to be replaced by lighter water. This creates internal circulation or convection and mixing of the oceans.
The Hydrologic Cycle
The diagram above shows the cycle of water as it moves between Earth's surface and the atmosphere and back. Water is stored in reservoirs, such as the ocean, lakes or rivers, glaciers, and groundwater. From these reservoirs, the water evaporates into the atmosphere. When conditions permit, the water in the atmosphere condenses and returns to Earth's surface in the form of precipitation like rain or snow. Water can also flow downward over the land in the form of runoff from rivers or groundwater, eventually ending up back in the ocean, where it too evaporates, condenses, and precipitates back to Earth. The process of sublimation can occur when water molecules from snow or ice pass directly into the atmosphere without becoming a liquid. Some 97 percent of Earth's water is held in the oceans.
Water also reaches the atmosphere through a process called transpiration—water loss from the stomata of plants (see the relevant section in biology). It is difficult to tell the difference between evaporation and transpiration. As a result, the word evapotranspiration is used to describe the entire process.
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