Oceans
Approximately 70% of Earth's surface is covered by water. The largest portions of water are found in the 5 oceans: the Pacific, Atlantic, Indian, Arctic, and Southern. In reality, a case could be made that we have just one ocean as all five are connected. However, we will live with the named five and move on to avoid any possible confusion. In this section, we will look at the layers or zones of the oceans, currents within the oceans, and the geographic features of the ocean floor and continental margins.
Ocean Zones
In the image to the right, you will see numerous subdivisions of a generic ocean. Horizontally, there are the intertidal and pelagic zones. Vertically, there are the photic, aphotic, abyssal, and benthic zones. All types of aquatic organisms call these areas home. Below, I will describe the characteristics of each zone and the groups of organisms inhabiting these areas. Intertidal: The area of ocean between the low tide mark and high tide mark. The organisms living here must endure being both covered and uncovered by water as well as temperature swings. Pelagic: Essentially, the ocean zone encompassing everything that is constantly covered by water. You will notice in the lower image that there are subdivisions of this zone based on depth. Photic: "Photo-" means light. This zone is the one in which sunlight penetrates the water and warms it as a result. Most ocean life lives in this zone. Aphotic - Twilight: This is the area of increasing darkness as sunlight cannot reach further than the upper few hundred meters. The temperature also drops here at what is called the "thermocline." Aphotic - Abyssal: This area of the ocean is perpetually dark and cold. Very little life exists here...but there is life. Benthic: This area occupies the ocean bottom. Plankton: Small organisms that float with the currents and tides of the ocean. Phytoplankton are photosynthetic; zooplankton are tiny animals. Nekton: Any free-swimming organism (i.e. fish, sharks, whales, dolphins, sea turtles, etc.) Benthos: Any organism that lives attached to, on, or in the ocean floor (i.e. clams, coral, crabs, tube worms, etc.) |
Ocean Currents
There are two major types of currents in the ocean: surface and deep water currents. Surface currents (left image) operate in the upper portion of the ocean (approximately the photic zone). Surface currents are moved around by the atmosphere which in turn is helped along by the surface currents. The two (atmosphere and surface currents) are closely tied together. In the northern hemisphere, surface currents move in a clockwise manner as warm water moves north from the Equator and cold water moves south toward the Equator. In the southern hemisphere, the water moves in the opposite pattern: counterclockwise as warm water moves south from the Equator and cold water moves north toward the Equator. These are called gyres.
Deep ocean currents (right image) are vertical movements of water caused by density differences below the surface currents. Cold water is denser than warm water and sinks, flowing along the bottom portion of the ocean slowly. It is pulled up in areas of upwelling where it warms and flows along the upper portions of the ocean, still below the surface currents. Upwelling is caused by warm water being pushed away from land by wind currents. As the warm water moves away from land, it pulls up the colder water from below to replace it. The colder water is richer in nutrients and cause swarms of plankton, fish, and, then, fishermen.
There are two major types of currents in the ocean: surface and deep water currents. Surface currents (left image) operate in the upper portion of the ocean (approximately the photic zone). Surface currents are moved around by the atmosphere which in turn is helped along by the surface currents. The two (atmosphere and surface currents) are closely tied together. In the northern hemisphere, surface currents move in a clockwise manner as warm water moves north from the Equator and cold water moves south toward the Equator. In the southern hemisphere, the water moves in the opposite pattern: counterclockwise as warm water moves south from the Equator and cold water moves north toward the Equator. These are called gyres.
Deep ocean currents (right image) are vertical movements of water caused by density differences below the surface currents. Cold water is denser than warm water and sinks, flowing along the bottom portion of the ocean slowly. It is pulled up in areas of upwelling where it warms and flows along the upper portions of the ocean, still below the surface currents. Upwelling is caused by warm water being pushed away from land by wind currents. As the warm water moves away from land, it pulls up the colder water from below to replace it. The colder water is richer in nutrients and cause swarms of plankton, fish, and, then, fishermen.
Geographical Features
Prior to the mid-20th century, it was widely thought that the bottom of the ocean was flat and featureless. Nobody could say otherwise as we couldn't see the bottom of the ocean. That is, until SONAR (SOund Navigation And Ranging) allowed us to see underwater. We discovered that the ocean floor is actually quite diverse with mountains, volcanoes, valleys, trenches, and plains.
Prior to the mid-20th century, it was widely thought that the bottom of the ocean was flat and featureless. Nobody could say otherwise as we couldn't see the bottom of the ocean. That is, until SONAR (SOund Navigation And Ranging) allowed us to see underwater. We discovered that the ocean floor is actually quite diverse with mountains, volcanoes, valleys, trenches, and plains.
Continental Margin - The edge of the continental crust as it meets the ocean crust.
Continental Shelf: The portion of the continent under the ocean's water; gradually sloping Continental Slope: The steeply sloped true edge of the continent made created from the difference in thickness of continental crust and oceanic crust Continental Rise: The actual beginning of the ocean crust; a gently sloped pile of sediment at the bottom of the continental slope; formed by sediment falling off of the continental shelf and slope Abyssal Plain: The flattest part of the planet; formed as sediment lightly fills in small valleys and depressions |
Seafloor Topography
Seamount: An underwater volcano that forms a mountain; if the mountain breaches the surface of the ocean, it forms an island Guyot: A seamount that develops a flat top from wave erosion and subsidence of the mountain Trench: A deep chasm formed when ocean crust subducts beneath ocean or continental crust Mid-ocean Ridge: A long range of mountains formed at divergent oceanic boundaries Submarine Canyon: A long valley formed on continental slopes from gravitational erosion Island Arc: A small chain of volcanic islands formed along ocean-ocean convergent boundaries (i.e. Japan) |