Presentation on theme: "M. Anderson, 2009. 3 major provinces Continental margins ◦ Shallow-water areas close to shore Deep-ocean basins ◦ Deep-water areas farther from."— Presentation transcript:
3 major provinces Continental margins ◦ Shallow-water areas close to shore Deep-ocean basins ◦ Deep-water areas farther from land Mid-ocean ridge ◦ Submarine mountain range
Zones separating the part of a continent above sea level from the deep‑sea floor. The true geologic margin of a continent ‑ where continental crust changes to oceanic crust ‑ is somewhere beneath the continental slope.
Passive Continental Margin ‑ Trailing end of a continental plate. Posses broad continental shelves, a continental slope and rise. Flat abyssal plains are adjacent to the rise. They lack the seismic and volcanic activity.
Active ◦ Associated with convergent or transform plate boundaries ◦ Much tectonic activity Convergent active margin Oceanic-continental convergence Where oceanic lithosphere is subducted. Seismicity, a young Mt. range and andesitic volcanism. The shelf is narrow and descends directly into a trench. Example: western South America
Continental Shelf ‑ where the sea floor slopes very gently seaward. It ends at a steep drop, the shelf ‑ slope break at @ 135m. In the Pleistocene, sea level was much lower, @ 130m. Much of the sediment on continental shelves accumulated in stream channels and floodplains. Some glaciers extended onto the exposed shelves. Affected by waves and tidal currents.
Change in gradient from shelf Average gradient 4 o Submarine canyonscut into slope by turbidity currents Submarine canyons cut into slope by turbidity currents ◦ Mixture of seawater and sediments ◦ Move under influence of gravity ◦ Erode canyons ◦ Deposit sediments at base of slope
Turbidity Currents ; sediment‑ water mixture denser than normal seawater Flow down‑slope to the deep‑sea floor. Coarsest particles are deposited first ‑ forming a graded bed. Deposits accumulate as a series of overlapping submarine fans, forming a large part of the continental rise.
Continental rise ‑ gently sloping area from the slope to a trench. Unaffected by surface processes and transport is controlled by gravity. Where most of the sediments are eventually deposited. Much of the sediment is transported by turbidity currents. Deep ocean Basin ‑ seaward of the continental margin.
Monterey Submarine Canyon Deep, steep sided submarine canyons occur on continental shelves, but are best developed on the continental slopes. Some Canyons can be traced across the shelf to associated streams on land. Some can not. Strong currents move through these canyons and are probably responsible for their erosion.
Transition between continental crust and oceanic crust Turbidite deposits ◦ Graded bedding ◦ Submarine fans Distal end of submarine fans becomes flat abyssal plains
The product of turbidity currents in km water depth Present/future targets of the oil industry once shallower resources are exhausted
Flat abyssal plains are adjacent to the rise. They lack the seismic and volcanic activity. Abyssal Plains are the flattest, most featureless areas of Earth A result of sediment deposition. Found adjacent to Continental rises. Common in the Atlantic, rare in the Pacific. Along active margins, sediments are trapped in an ocean trench so abyssal fans fail to develop.
Very flat depositional surfaces from base of continental rise Suspension settling of very fine particles Sediments cover ocean crust irregularities Well-developed in Atlantic and Indian oceans
Abyssal hills ‑ average @ 250m high. They are common on the sea floor and underlie thick sediments on the abyssal plains.
Oceanic Trenches Where lithospheric plates are consumed by subduction. Long, narrow features restricted to active continental margins. Oceanic trenches are the sites of greatest oceanic depth. Marianas Trench : 11,000 m deep. The crust here is cooler and slightly denser than elsewhere.
Trenches show a huge negative gravity anomaly; The crust is held down and is not in equilibrium. Seismic activity occurs at or near the trenches. They have Benioff zones in which earthquake foci become progressively deeper in a landward direction. Most intermediate and deep earthquakes occur in such zones. They are associated with volcanoes. (W. So. America)
Seamounts are isolated volcanic mountains scattered across the ocean floor. Most common in the Pacific Ocean, seamounts generally rise more than 1,000 meters above the sea floor, often forming islands. Emperor, Marshall and Hawaii seamounts
When the action of plate tectonics moves a seamount-formed island away from the mid-ocean ridge, the ocean crust sinks, pulling the island beneath the surface. These submerged, often flat-topped, seamounts are called guyots.
Poke through sediment cover Below sea level: ◦ Seamounts, tablemounts, or guyots at least 1 km (0.6 m) above sea floor ◦ Abyssal hills or seaknolls are less than 1 km Above sea level: ◦ Volcanic islands
Landward side of ocean trench Island arc ◦ Chain of islands, e.g., Japan, Hawaii, Aleutians Continental arc ◦ Volcanic mountain range, e.g., Andes Mountains
As the South American continent moved west, in the Cretaceous Period (140mya) the Nazca plate subducted forming a trench. By 130mya igneous activity began and a Volcanic arc was formed. By 90 mya the trench had migrated west and a new volcanic arc formed along the west coast, while mountains to, now in the interior, ceased activity.
Longest mountain chain On average, 2.5 km (1.5 miles) above surrounding sea floor Wholly volcanic Basaltic lava Divergent plate boundary Central rift valley, faults, and fissures Seamounts Pillow basalts Hydrothermal vents ◦ Deposits of metal sulfides ◦ Unusual life forms Fracture zones transform faults Fracture zones and transform faults
Oceanic ridge ◦ Prominent rift valley ◦ Steep, rugged slopes ◦ Example: Mid-Atlantic Ridge Oceanic rise ◦ Gentler, less rugged slopes ◦ Example: East Pacific Rise
When lava flows enter the ocean, or when lava outpourings actually originate within an ocean basin, the flows outer zones quickly congeal. The lava within the flow is able to move forward by breaking through the hardened surface, when this occurs over and over. The lava flow resembles large bed pillows stacked one upon another.
ALVIN: Woods Hole Submersible 1970'S @ 2,500m in the Galapagos Rift in E. Pacific Ocean Basin hydro‑thermal vents were first observed.
Hydrothermal vents ◦ Heated subsurface seawater migrates through cracks in ocean crust Warm-water vents <30 o C or 86 o F White smokers >30 o C 30 o C <350 o C or 662 o F Black smokers > 350 o C