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Classification of Ocean Floor Deep Ocean Basins Deep-Ocean Basins Features of the deep ocean floor: Oceanic Ridges Hydrothermal Vents Trenches Island Arcs Abyssal Plains Abyssal Hills Seamounts Volcanic Islands Guyots © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean. •Long, underwater mountain ranges. •The largest tectonic feature on the earth. •More than 1500 km wide, 64,000 km (40,000 miles) long. •Rises 2-3 km (up to 2 mi) high above the abyssal plain. •Has a central rift valley. •Composed of basalt rocks. •Faulted with strike-slip (transform) faults. •Shallow earthquakes occur along central rift. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures or vents. Usually associated with mid-ocean ridges © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Trenches are long, narrow, deep depressions in the ocean floor with steep sides caused by the subduction of a converging ocean plate. Trenches are the deepest places on Earth. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Island Arcs, chains of volcanic islands and seamounts, are usually found parallel to the edges of ocean trenches •Found along ocean-ocean convergent boundaries As two oceanic plates converge, an island arc is formed by volcanic activity. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Abyssal Plain -Very flat area of the ocean floor at the edge of the continental rise. •Average depth is about 4-6 km (2.5-4 mi). •Covered by a layer of sediment, mostly <1 km thick. •The flattest areas on the planet. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Abyssal hills are small, extinct volcanoes or rock intrusions found between the continental margins and oceanic ridges. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins Seamounts are volcanic projections from the ocean floor that do not rise above sea level (underwater volcanoes). •Associated with hot spots and mid-ocean ridges If seamounts break the surface of the ocean, they create volcanic islands •The biggest mountain on earth is Mauna Loa (Hawaii). © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Deep-Ocean Basins •Over time, wave & wind action erodes the peak of a seamount or volcanic island to form a flat-topped tablemount or guyot. •Guyots were once volcanic peaks above sea level. They were eroded by wave action as they sank beneath the surface of the water. •As seamount or guyot is being eroded, it is also being moved away from the ridge or hotspot. •Hot Spots: Surface expression of plumes of magma rising from a stationary source of heat in the mantle. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. A volcanic island chain can form when a plate passes over a hot spot. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Hawaiian Islands-Emperor Seamount Chain • A hotspot in the middle of the Pacific plate creates a volcano. • Pacific Plate is moving northwest. • As the plate moves, a new island forms & is active. • Result: a chain of extinct volcanoes that is progressively older away from the hotspot Over time… • In the future, the HI island chain will subduct into the Aleutian Trench as new • There is a “bend” in islands form. the HI island chain: 1. The Pacific Plate used • There is a new island about 1 km below the surface to move north and called Loihi. changed directions to move northwestward OR 2. the hotspot could have changed locations. Coral Reef Development • Darwin noticed on his voyage on the HMS Beagle that coral reefs developed around sinking volcanic islands. • Corals form in shallow, warm, tropical seawater. • Temperature, salinity, and turbidity of the water have to be just right. • The coral polyp (animal) secretes limestone, which serves as the skeleton that new polyps grow on to build the coral reef. Fringing Reef • Corals develop around a landmass. • If it’s on a hotspot (active volcano), lava can destroy corals. When the volcano becomes inactive and subsides, corals develop toward barrier stage. Moorea, French Polynesia Barrier Reef • When the island subsides, or sea level rises, corals will continue to build upward and form a circular reef. • Water in between the reef and island/land is called the lagoon. • If land subsides or sea level rises faster than the coral can grow, than it will die off. Great Barrier Reef, Australia Tuanake, French Polynesia Atoll • When the island is completely submerged and conditions are right for the coral, all that will be visible on the surface is the coral reef. • If enough coral debris builds up, a narrow island can be large enough to allow habitation. A pie chart showing the distribution of Earth’s solid surface. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.