3. Continental Shelf
Submerged part of the continent
Slopes gently toward ocean basin
(<1°)
Composed of continental crust
Shelf ends at shelf break –
boundary between shelf and
steeper slope
4. Continental Slope
Boundary between continental
and oceanic crust
Steeply sloping compared to shelf
(5-25°)
Submarine canyons are major
features
Extends from shelf break to rise
6. Submarine Canyon
Turbidity Currents
Turbidites are layered and exhibit graded bedding (decrease in
sediment grain size from bottom to top)
Downslope movement of dense mixture of clay, silt, sand and water
Deposits are called turbidites
7. Continental Rise
At base of continental slope
Slope angle decreases
Caused by the accumulation of sediment
11. Continental Margin
Active
Convergent plate boundary
Trenches are boundaries
Tectonically active
Thin accumulation of
sediments
Narrow continental margin
12. Continental Margin
Passive vs. Active
Passive Margins
• Major rivers drain into ocean
• Sediment transported by
river builds out shelf
Active Margins
• Large rivers uncommon
• Irregular shelves
15. Ocean Basin Floor
Features
Seamounts and Guyots
Coral Reefs and Atolls
Ridges and Rises
Abyssal Plain
Trenches
16. Ocean Basin Floor
Covers about 30% of Earth’s surface
Contain abyssal plains, deep sea trenches, and seamounts
Begins at base of continental rise
Sedimentation: Passive and turbidity currents
17. Ocean Basin Floor
Abyssal Plain
Flat, deep ocean floor
Depth may be 2-3 miles or more
Thick sediment accumulation covers oceanic crust
18. Ocean Basin Floor
Deep Sea Trenches
Occur at subduction zones
where oceanic crust is
forced downward into
mantle
Associated with earthquakes
and volcanoes
Deepest is Mariana Trench
(11,020 m)
Longest is Peru-Chile trench
(5,900 km)
19. Ocean Basin Floor
Ridges and Rises
Contain central rift valleys
• 15-50 km wide
• 500-1,500 m deep
Offset by fractures
Underwater volcanic mountain chain
• Extends for 65,000 km
• 1,000 km wide
• 1,000-2,000 m high
Ridges = steep slopes
Rises = gentle slopes
20. Ocean Basin FloorOcean Basin Floor
Seamounts and GuyotsSeamounts and Guyots
Seamounts are underwater volcanoes formed along
ocean ridges or over hot spots
May be eroded flat on top and called Guyots
May emerge as an island
21. Ocean Basin Floor
Coral Reefs and Atolls
Volcanic islands (from seamount) form in warm latitudes
Fringing coral reefs form in shallow, sunlit waters
Dormant volcano subsides and flattens (becomes a guyot)
Actively-growing reef becomes a barrier reef and then an atoll
Editor's Notes
continental shelf lies at edge of continent; flat border of varying widths that slopes very gently toward ocean basin (part of continent; can be covered or uncovered by fluctuations in sea level); composed of continental crust; averages about 41 mi (10-300 km) with depths of 430 ft (130 m)
can have continental islands that are part of shelf
wide at passive margins
narrow at active margins, but width also depends on currents (rapid ones prevent sediment buildup) and sea level (narrow when sea level is low; broad when sea level is high)
continental shelf ends at the shelf break, where the slope abruptly gets steeper; usually occurs at 400 to 600 ft (100-200 m) but Antarctica, Greenland are exceptions because of their ice caps
continental slope is the exact edge of the continent
descends downward toward deep sea floor (3-5 km)
formed of sediments that reach the shelf break and slide down the slope
can also be composed of marine sediments scraped off a subducting plate at an active margin
steeper at an active margin than at a passive one, but still doesn&apos;t exceed an inclination of 25° (4° incline is average)
submarine canyons (can sometimes extend up, into, and across the continental shelf) are steep-sided and has a V-shaped cross section, with tributaries similar to those of river-cut canyons on land
carry sediments from shelf to sea floor -- is called down-slope transport of sediments; terminate in a fan-shaped wedge of sediment on deep sea floor
formed by turbidity currents which occur when turbulence mixes sediments into water and creates functional equivalent of a mud slide along the continental shelf and down the continental slope -- sediments are abrasive, so over time they cut into shelf&apos;s crust forming a canyon
continental rise is a gentle slope at the base of a steep continental slope
caused by the accumulation of sediment at a submarine canyon&apos;s base or at the base of the continental slope
descend 2-4 km to the abyssal plain (~4 km depth)
abyssal plain: flat plain extending seaward from the base of the continental slope
flatter than any plain on land; formed from sediments covering irregular topography of the seafloor
comprises 25% of earth&apos;s surface
The East Pacific Rise between the Pacific plate and the Nazca plate (where spreading rate is ~17 cm per year) causes a gentle sloping rise; in contrast, the slower spreading along the Mid-Atlantic Ridge (~5 cm per year) results in a steep slope characteristic of ridges; earthquake energy is closely related to the spreading rate and is higher at slower spreading centers
seamounts and guyots are volcanoes that arise originally at an oceanic ridge but then are carried away from the ridge by plate movement -- as they move, volcanic activity gradually decreases (the cracks in the earth&apos;s crust that serve as conduits for rising lava seal off)
however, while they are moving, there is still volcanic activity and they grow in height (seamounts)
the volcanoes can be active for 10-15 million years, but will eventually become dormant (few are active past 30 million years)
at this point the top will become flattened by wave action and will once again become submerged (guyots)
coral reefs -- origin proposed originally by Darwin before the theory of plate tectonics was formalized (called the subsistence theory of volcanic islands) -- several steps to this process:
volcanic island (from a seamount) forms in warm-water latitudes and a fringing coral reef forms in the shallow, sunlit waters adjacent to it
as the dormant volcano erodes due to the closing off of the cracks providing magma, the reef maintains its elevation near the surface because new corals build on the skeletons of old corals -- this new building produces a barrier reef where the actively growing reef is separated from the receding seamount by a well-developed lagoon
island flattens submerges (becomes a guyot) but reef continues to grow upward to produce an atoll with active reef growth confined to the outer edges (the interior of the atoll is a shallow lagoon)
Constructed primarily from skeletal remains and secretions of corals and certain algae
Confined largely to the warm, clear waters of the Pacific and Indian Oceans
Coral islands – a continuous ring of coral reef surrounding a central lagoon
Form on the flanks of a sinking volcanic island (hypothesis proposed by Charles Darwin)
Coral islands – a continuous ring of coral reef surrounding a central lagoon
Form on the flanks of a sinking volcanic island (hypothesis proposed by Charles Darwin)
