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Oceans

GLY 2010 – Summer 2015

Lecture 16

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Voyage of H.M.S. Challenger

• Route sailed by Challenger from 1872 - 1876

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Bathymetry

Use of Sonar

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Seismic Reflection Profile

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North Atlantic Ocean

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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

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Passive Margin Features

• Continental Shelf• Continental Slope• Continental rise

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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

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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

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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

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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

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Continental Slope

• Steeply sloping region connecting the shelf and the deep ocean

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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

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Continental Rise

• The area between the slope and the deep ocean floor, where the slope is much less

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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

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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

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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

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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

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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

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Features of the Deep Ocean Basin

• Abyssal Plains

• Deep-ocean trenches

• Oceanic plateaus

• Seamounts and guyots

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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)

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Deep-Ocean Trenches

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Ocean Trenches

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Puerto Rico Trench

25Click to play animation

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Marianas Trench

26Click to play animation

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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

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Oceanic Plateaus

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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

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Gulf of Alaska Seamounts

• Some seamounts a occur in chains, formed as plates move over hot-spots

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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

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Mid-Ocean Ridges (MOR)

• Characterized by:• Rift valleys• Heat flow• Age of ridge

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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

2000-3000 kilometers

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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

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Oceanic Rifts

• Average fifty kilometers in width• May be two kilometers deep

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Slow-Spreading Ridge Topography

• Slow spreading leads to steep profiles and rift valleys

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Fast-Spreading Ridge Topography

• Median rift valleys are usually absent• Topography is much smoother

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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

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600 MYBP

• During the breakup of Rodinia

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510 MYBP

• Formation of Gondwana

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430 MYBP

• Collision of the northern continents

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230 MYBP

• Pangaea is assembled