Transcript PLATE TECTONICS and OCEANS

Oceans
GLY 2010 – Summer 2012
Lecture 16
1
Voyage of H.M.S. Challenger
• Route sailed by Challenger from 1872 - 1876
2
Bathymetry
3
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
9
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 th 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 subducted19
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
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Marianas Trench
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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 hotspots
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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
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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
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