The ideas of continental drift and seafloor spreading were integrated into the overriding concept of plate tectonics mainly through the work of John Tuzo Wilson, of the University of Toronto, in 1965. According to the theory, the outer layers of the earth consist of about a dozen separate lithospheric plates, each about 70 to 100 meters (43 to 65 miles) thick, floating on the astenosphere. Studies with deep-ocean crustal cores in the next few years tended to confirm the theory.
Following are the names of the major plates: Pacific, Philippines, Indian-Australian, Antarctic, Nazca, Cocos, North American, Caribbean, South American, Africa, Eurasian, Iran.
Features of the Ocean Floor
- Abyssal plain:
- A sediment-covered plain about 3,500-5,500 m (11,500-18,000 ft.) below sea level.
- Continental shelf:
- A continent’s submerged and gently sloping rim, descending to about 200 m (650 ft.).
- Continental slope:
- A steep slope, descending from the continental shelf.
- Continental rise:
- A gentle slope below the continental slope.
- Guyot:
- A flat-topped seamount that had once been a volcanic island.
- Island arc:
- A curved row of volcanic islands, usually on the continental side of a trench.
- Seamount:
- A submarine volcano 1,000 m (3,3000 ft.) or more above its surroundings.
- Spreading ridge:
- A submarine mountain chain about 3,000 m (10,000 ft.) above the abyssal plain.
- Submarine canyon:
- A deep cleft in the continental slope, cut by turbid river water flowing out to sea.
- Submarine plateau:
- A high seafloor tableland.
- Trench:
- A deep, steep-sided trough in an abyssal plain.
Ridges
These are mountainous chains of basaltic rock at the active spreading centre of an ocean, stretching for 6,500 km (41,000 miles). They rise about 2 km (1.25 miles) above the seafloor. In places, they rise above the surface to form islands. A fault is a break in a rock mass, along which movement has occurred.
The major oceanic ridges are as follows: Pacific-Antarctic Ridge, Juan de Fuca Ridge, Scotia Ridge, Mid-Atlantic Ridge, Walvis Ridge, Atlantic-Indian Ridge, Carlsberg Ridge, Mid-Indian Ridge, Melanesian Rise, Galápagos Rise, East Pacific Rise, Chile Rise, Rio Grande Rise.
Trenches
A trench is an arch-shaped depression in the deep-ocean floor. It occurs where a converging ocean plate moves under the edge of the adjacent one. This is where earhquakes and tsunamis originate.
Most trenches are located along the edges of the Pacific Ocean. The major trenches are as follows: Java, New Hebrides, Kermadec-Tonga, Mariana, Philippine, Japan, Kuril, Aleutian, Middle America, Peru-Chile, South Sandwich.
The deepest one is the Mariana Trench, which is 11,022 meters (36,163 feet) deep, 2,550 km (1,600 mles) long, and 70 km (44 miles) wide.
