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Seas and Oceans


Seas and Oceans
Photo © Vincenzo Piazza / stock.xchng.

Seas and oceans stretch from pole to pole and reach around the globe. They cover more than 70 percent of the Earth's surface and hold in excess of 300 million cubic miles of water (Luhr 2003). The world's oceans conceal a vast underwater landscape of submerged mountain ranges, continental shelves, and sprawling trenches.

The geologic features of the sea floor include mid-ocean ridge, hydrothermal vents, trenches and island chains, continental margin, abyssal plains, and submarine canyons. Mid-ocean ridges are the most extensive mountain chains on earth, spanning some 40,000 miles across the sea floor and running along divergent plate boundaries (where tectonic plate are moving away from one another as new sea floor is being churned out from the Earth's mantle).

Hydrothermal vents are fissures in the sea floor that release geothermally heated water at tempertures as high as 750°F. They are often located near mid-ocean ridges where volcanic activity is common. The water they release is rich in minerals which precipitate out of the water to form chimneys around the vent.

Trenches form on the sea floor where tectonic plates converge and one plate sinks beneath another forming deep-sea trenches. The plate that rises above the other at the convergence point is pushed upward and can form a series of volcanic islands.

Continental margins frame continents and stretch outward from dry land to abyssal plains. Continental margins consists of three regions, the continental shelf, slope, and rise.

An abyssal plain is an expanses of sea floor that begins where the continental rise ends and extends outward in flat, often featureless plain.

Submarine canyons form on continental shelves where large rivers run out to sea. The water flow causes erosion of the continental shelf and digs out deep canyons. Sediments from this erosion are dumped out over the continental slope and rise onto the abyssal plain forming a deep-sea fan (similar to an alluvial fan).

Seas and oceans are diverse and dynamic—the water they hold transmits vast amounts of energy and drives the world's climate. The water they hold sways to the rhythms of waves and tides and moves in vast currents that circle the globe.


Since the ocean habitat is so extensive, it may be broken down into several smaller subhabitats:

  • inshore waters - the shallowest areas of the oceans that line coastal areas, formed by continental shelves.
  • open sea - the vast deep waters of the oceans


The open sea is a stratified subhabitat, with light fliltering down a mere 250 meters, creating a rich habitat where algea and planktonic animals thrive. This region of the open sea is referred to as the surface layer. The lower layers, the midwater, the abyssal zone, and the seabed, are shrouded in darkness.

Animals of Seas and Oceans:

Life on earth first evolved in the oceans and developed there for most of evolutionary history. It is only recently, geologically speaking, that life has emerged from the sea and flourished on land. The animal inhabitants of seas and oceans range in size from microscopic plankton to massive whales.

Habitat Classification

  • Ecozone: Marine
  • Ecosystem: Seas and Oceans


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