OCEAN PROVINCES

• Most of the area is near sea level because of long-term erosion.
• Mountainous zones are relatively young features in Earth history.
• They are underlain by low-density continental crust, but in most areas the crust is covered by a thin veneer of sedimentary rocks, representing periods when they were covered by ocean (continental shelves and interior seas).

• They were once-exposed continent because erosion created them.
• They are underlain by continental--not oceanic--crust.
• They add 18% to the continental areas, so that continental crust covers about 47% of the Earth's surface.
• They are very flat (slope of 0°07'), and they end where a steep break in slope appears (beginning of continental slope).
• They are only submerged up to 135 m, except around Antarctica, where isostasy has depressed them down to 350 m.
• They contain most of the commercial fisheries.
• They are sites of active sedimentation from continental clastic material (silicates) and marine biological detritus (carbonates).
• Only they contain sub-sea oil reserves because fossil fuels form from organic matter on land, usually in coastal areas.

• They are relatively steep, ranging from 1° to 25° and averaging 4°.
• They represent the boundaries between continental and oceanic crust.
• They are cut by numerous submarine canyons--the frequency of which is related to the slope gradient--which terminate in deep-sea fans.
• Turbidity currents (high-density mixtures of water and suspended sediment that are usually initiated by earthquake tremors) flow down the slopes forming many features similar to erosional and depositional features on land.

• Deep-sea fans or cones (Indus, Ganges, Mississippi, Amazon) contain levees, meander patterns, and cutoff loops just like the lower Mississippi River.
• The Coriolis Effect causes the Western Boundary Undercurrent to flow at 1.5 km/h along the rise in the western Atlantic, and as it flows, carrying suspended sediments (benthic nepheloid layer) it produces drifts or ridges, mud waves (2-3 km period), furrows (cut by current), and ripples (10-15 cm period).

• Most lie at a depth of 4,500 to 6,000 m.
• Underlain by oceanic crust.
• They are very flat except where penetrated by seamounts (over 1 km high).
• They represent abyssal hills that have become covered with sediment (older sea floor).
• Bottom currents flow over them at 0.3 km/h and distribute fine sediments.

• Volcanic (basaltic) hills form a checkerboard topography (see Plate 12 after p. 126).
• They are younger, nearer the mid-oceanic ridges, and less covered with sediment than the abyssal plains.
• Rock becomes successively older away from the mid-oceanic ridges.
• They are cut by valleys that run perpendicular to the ridges.

• They are long, continuous, and run all the way around the globe.
• They rise far above the abyssal hills and plains and occasionally reach sea level.
• Sometimes a rift valley is found at the crest of the ridge.
• The ridge is offset by valleys running perpendicular to it.
• Large fracture zones and escarpments result from the different densities on either side of transform faults when they fuse. Most fracture zones are inactive (except Owen Fracture Zone).
• Shallow earthquakes and basaltic volcanic activity occurs at the center of the ridge and along the segments of the valleys that offset it (the rock is young to new).
• Warm-water vents (10°-20°C), white smokers (30°-330°C) containing barium sulfate, and black smokers (350°C) containing metal sulfides cause mineral concentration and support biotic communities. The entire volume of ocean water is cycled through these vents about every 3 million years, thus influencing ocean water composition.

• Are the only features that descend far deeper than the abyssal hills and plains, down to 11,220 m (over twice as deep on average).
• They are long valley-like features up to 6,000 km long.
• They can occur along continental margins (Peru-Chile Trench) or in the middle of ocean basins (Mariana Trench).
• They are the sites of shallow to deep earthquakes.
• Sites of active granitic vulcanism usually parallel them (Andes and Cascades Mountains, Japan and Sumatra-Java microcontinents, and Aleutian and Mariana Islands).
• Most occur in the Pacific Ocean, one in the Indian Ocean, and none in the Atlantic Ocean.

• About 20,000 in the Pacific, but also present in other oceans.
• Hills rising over 1 km are termed seamounts.
• Those with tops flattened by erosion are called tablemounts or guyots (most over 30 million years old). Tablemounts represent sinking of cooling, thickening oceanic crust.
• Tablemounts become tilted as they approach subduction zones.

• Fringing Reefs first develop around volcanic islands.
• Barrier Reefs then grow as the inactive island sinks.
• Atolls are all that is left when the island completely sinks.
• Reefs can only live in shallow water, but they can grow upward at 3-10 m/1000 yrs.