Thursday, 26 July 2012
CASE 414 - The oceans
Oceans cover about 70% of the Earth's surface. The oceans contain roughly 97% of the Earth's water supply.
The oceans of Earth are unique in our Solar System. No other planet in our Solar System has liquid water (although recent finds on Mars indicate that Mars may have had some liquid water in the recent past). Life on Earth originated in the seas, and the oceans continue to be home to an incredibly diverse web of life.
The oceans of Earth serve many functions, especially affecting the weather and temperature. They moderate the Earth's temperature by absorbing incoming solar radiation (stored as heat energy). The always-moving ocean currents distribute this heat energy around the globe. This heats the land and air during winter and cools it during summer.
The Earth's oceans are all connected to one another. Until the year 2000, there were four recognized oceans: the Pacific, Atlantic, Indian, and Arctic. In the Spring of 2000, the International Hydrographic Organization delimited a new ocean, the Southern Ocean (it surrounds Antarctica and extends to 60 degrees latitude).
There are also many seas (smaller branches of an ocean); seas are often partly enclosed by land. The largest seas are the South China Sea, the Caribbean Sea, and the Mediterranean Sea.
Formation of oceans
Geologically, Earth's ocean is the area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the mantle. The ocean floor spreads from mid-ocean ridges where two plates adjoin. Where two plates move towards each other, one plate subducts under another plate (oceanic or continental), leading to an oceanic trench. From this perspective, the earth has three oceans: the World Ocean, the Caspian Sea, and Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia.
The Mediterranean Sea is at times a discrete ocean, because tectonic plate movements have repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is a natural canal that was cut through continental rock around 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.
The ocean has a significant effect on the biosphere. Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall, and ocean temperatures determine climate and wind patterns that affect life on land. Life within the ocean evolved 3 billion years prior to life on land. Both the depth and the distance from shore strongly influence the biodiversity of the plants and animals present in each region.
Lifeforms native to the ocean include: Fish; Radiata, such as jellyfish (Cnidaria); Cetacea, such as whales, dolphins, and porpoises; Cephalopods, such as octopus and squid; Crustaceans, such as lobsters, clams, shrimp, and krill; Marine worms; Plankton; and Echinoderms, such as brittle stars, starfish, sea cucumbers, and sand dollars. Economic value The oceans are essential to transportation: most of the world's goods move by ship between the world's seaports. Oceans are also the major supply source for the fishing industry. Some of the more major ones are shrimp, fish, crabs and lobster.