Introduction to Marine Provinces and the Ocean Floor

M. Anderson, 2009

Ocean provinces

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

Continental Margins

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 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.

Continental margins

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 margin features

Continental shelf

Shelf break Continental

slope Continental

rise

Continental Shelf

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.

Continental slope Change in gradient from shelf Average gradient 4o

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

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

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.

Submarine Canyons

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.

Continental rise

Transition between continental crust and oceanic crust

Turbidite deposits◦Graded bedding◦Submarine fans

Distal end of submarine fans becomes flat abyssal plains

Submarine fans The product of turbidity currents in km water depth Present/future targets of the oil industry onceshallower resources are exhausted

Abyssal Plains

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.

Abyssal plains

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 / Plains Abyssal hills ‑

average @ 250m high. They are common on

the sea floor and underlie thick sediments on the abyssal plains.

Abyssal plains

Fig. 3.11

Oceanic Trenches

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.

Ocean Trenches 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

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

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.

Volcanic peaks 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

Volcanic arcs

Landward side of ocean trench Island arc

◦Chain of islands, e.g., Japan, Hawaii, Aleutians

Continental arc◦Volcanic mountain range,

e.g., Andes Mountains

Andean Arc 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.

Mid-ocean ridge 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 and transform faults

Mid-ocean ridge featuresOceanic ridge

◦Prominent rift valley◦Steep, rugged slopes◦Example: Mid-Atlantic Ridge

Oceanic rise◦Gentler, less rugged slopes◦Example: East Pacific Rise

Pillow lava / Pillow basalt

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.

Hydro‑thermal Vents

ALVIN: Woods Hole Submersible

1970'S @ 2,500m in the Galapagos Rift in E. Pacific Ocean Basin hydro‑thermal vents were first observed.

Volcanic features of mid-ocean ridge

Hydrothermal vents◦Heated subsurface seawater migrates through cracks in ocean crust Warm-water vents <30oC or 86oF White smokers >30oC <350oC or 662oF Black smokers > 350oC