111 Oceans GLY 2010 – Summer 2015 Lecture 16

22 Voyage of H.M.S. Challenger Route sailed by Challenger from

33 Bathymetry

Use of Sonar 4

55 Seismic Reflection Profile

66 North Atlantic Ocean

77 Passive Continental Margins Not located along an active plate boundary Little or no seismic activity and volcanism Found mainly in the Atlantic and most of the Indian oceans Weathering and erosion of continental material produces a wide, thick deposit of undisturbed sediments The coast of Florida is a passive margin

88 Passive Margin Features Continental Shelf Continental Slope Continental rise

999 Continental Shelf Gently sloping regions adjacent to continents – about 0.1% on average Built by transport of sediment from the continents to the ocean  Much of this material is felsic, and adds to the offshore region of the continent

10 Continental Shelf Edge of the continental shelf, at about 130 meters depth (average), is in many ways the true edge of the continent Continental shelves are much wider on passive margins (Florida) than active margins (Washington, Oregon), and may be non-existent on some margins

11 Importance of Continental Shelves 7.5% of ocean surface, but contain much of the wealth of the oceans, including petroleum, natural gas, mineral resources, and huge sand and gravel deposits Contain important fishing resources, although these are largely over-exploited and in danger of failure

12 Geology of Continental Shelves Contain many glacial deposits, from ice ages when sea-level was lower Submarine valleys are often seaward extensions of river valleys on the continent

13 Continental Slope Steeply sloping region connecting the shelf and the deep ocean

14 Features of the Continental Slope Averages about 20 kilometers in width  Average slope is 5° (50x continental shelf), but reaches 25° in some places  Marks boundary between continental crust and oceanic crust

15 Continental Rise The area between the slope and the deep ocean floor, where the slope is much less

16 Geology of Continental Rise Slope is about 1/3 degree Thick deposits of mud, delivered by turbidity currents When currents emerge from a canyon mouth, a deep-sea fan is formed

17 Active Continental Margins Occur where oceanic lithosphere is subducted beneath a continental edge Margin is narrow, with a veneer of highly deformed sediments Parallel deep-ocean trenches around the circum-Pacific margin, and along Sumatra in the Indian Ocean Volcanoes and SID earthquakes are common

18 Geology of an Active Margin Ocean-floor sediments are mixed with oceanic crust and scrapped from the descending plate Produces a chaotic mixture called an accretionary wedge Prolonged subduction leads to a large accretionary wedge, for example of Honshu Island in Japan

19 Old and Cold When the oceanic plate is old and cold, subduction angle is steep, and no accretionary wedge is formed – all sediments are subducted

20 Deep Ocean Basin Region between the continental margin and an ocean ridge About 30% of the surface area of the earth, comparable to the continents

21 Features of the Deep Ocean Basin Abyssal Plains Deep-ocean trenches Oceanic plateaus Seamounts and guyots

22 Abyssal Plains Among the flattest places on the planet Deep accumulations of sediment bury everything except high volcanic peaks Comes from a (without) & byssus (bottom)

23 Deep-Ocean Trenches

24 Ocean Trenches

25 Puerto Rico Trench 25 Click to play animatio n

26 Marianas Trench 26 Click to play animation

27 Oceanic Plateaus Resemble the flood basalt provinces found on the continents Created by mantle plume volcanism producing copious lavas which cover and smooth large areas of the ocean floor Rock consists primarily of pillow lava, which may reach or exceed 30 kilometers in thickness Examples include the Ontong Java and Rockwall Plateaus, shown on the next slide 27

28 Oceanic Plateaus 28

29 Seamounts Tens of thousands of volcanic peaks dot the ocean floors – estimates range from 22,000 to 55,000 Many rise hundreds of meters, but a few are larger The largest form islands, like the Azores, Ascension, and St. Helena As they move away from spreading centers, the plate beneath the volcano cools and contracts

30 Gulf of Alaska Seamounts Some seamounts a occur in chains, formed as plates move over hot- spots

31 Guyot Volcanic islands are worn away by weathering, landslides, stream erosion, and wave action In the surf zone, the island is worn flat, and becomes a guyot

32 Mid-Ocean Ridges (MOR) Characterized by: Rift valleys Heat flow Age of ridge

33 MOR Dimensions Exceed 70,000 kilometers in length Cover 20% of earth’s surface Typical height is 2-3 kilometers above the ocean floor Width ranges from 1000 to 4000 kilometers, although most are kilometers

34 Not Mountains MOR’s are high like mountains but are formed in an area of tension, not compression The ridges are buoyantly lifted piles of hot basaltic crust Some segments of the ridge have well developed rift valleys, named for the resemblance to the East African Rift Flanks rise very gradually, with slopes of less than one degree, toward the ridge axis

35 Oceanic Rifts Average fifty kilometers in width May be two kilometers deep

36 Slow-Spreading Ridge Topography Slow spreading leads to steep profiles and rift valleys

37 Fast-Spreading Ridge Topography Median rift valleys are usually absent Topography is much smoother

38 Supercontinent-Cycle Pangaea was the most recent supercontinent, but not the only one Reconstructions of plate positions before Pangaea are very difficult, because most older oceanic crust was destroyed by subduction By matching geologic structures, paleoclimate records, and apparent polar-wandering curves, some reconstructions are possible, as shown on the following slides

MYBP During the breakup of Rodinia

MYBP Formation of Gondwana

MYBP Collision of the northern continents

MYBP Pangaea is assembled