- 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
- 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
- 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
- Deep-sea fans or cones (Indus, Ganges, Mississippi, Amazon) contain
levees, meander patterns, and cutoff loops just like the lower Mississippi
- 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
Abyssal Hills (hills under 1 km high)
- 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
- 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
- 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.
Deep-Sea Trenches (Subduction Zones)
- They are long, continuous, and run all the way around the globe.
- They rise far above the abyssal hills and plains and occasionally reach
- 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
- 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.
Seamounts and Tablemounts
Coral Reef Development
- 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
Timothy H. Heaton:
Phone (605) 677-6122, FAX (605) 677-6121