Introduction to Marine Provinces and the Ocean Floor

1. Introduction to Marine Provinces and the Ocean Floor M. Anderson, 2009

2. 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

3. 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.

4. 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.

5. 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

6. Continental margin features • Continental shelf • Shelf break • Continental slope • Continental rise

7. 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.

8. Continental slope • Change in gradient from shelf • Average gradient 4o • Submarine canyonscut into slope by turbidity currents • Mixture of seawater and sediments • Move under influence of gravity • Erode canyons • Deposit sediments at base of slope

9. 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.

10. 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.

11. Submarine Canyons • 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. Monterey Submarine Canyon

12. Continental rise • Transition between continental crust and oceanic crust • Turbidite deposits • Graded bedding • Submarine fans • Distal end of submarine fans becomes flat abyssal plains

13. Submarine fans • The product of turbidity currents in km water depth • Present/future targets of the oil industry once shallower resources are exhausted

14. 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.

15. Abyssal plains • Very flat depositional surfaces from base of continental rise • Suspension settlingof very fine particles • Sediments cover ocean crust irregularities • Well-developed in Atlantic and Indian oceans

16. Abyssal Hills / Plains • Abyssal hills ‑ average @ 250m high. • They are common on the sea floor and underlie thick sediments on the abyssal plains.

17. Abyssal plains Fig. 3.11

18. 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.

19. 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)

20. 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

21. 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.

22. Volcanic peaks • Poke through sediment cover • Below sea level: • Seamounts, tablemounts, or guyotsat least 1 km (0.6 m) above sea floor • Abyssalhills orseaknollsare less than 1 km • Above sea level: • Volcanic islands

23. 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

24. 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.

25. 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

26. Mid-ocean ridge features • Oceanic ridge • Prominent rift valley • Steep, rugged slopes • Example: Mid-Atlantic Ridge • Oceanic rise • Gentler, less rugged slopes • Example: East Pacific Rise

27. 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.

28. Hydro‑thermal Vents • 1970'S @ 2,500m in the Galapagos Rift in E. Pacific Ocean Basin hydro‑thermal vents were first observed. ALVIN: Woods Hole Submersible

29. 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