Revised 8 / 06 (Monroe 6th ed.)
Including...
Plate Tectonics and the Seafloor
Origin of all life was in the sea
Covers 71% of the earth's surface
Only recently have we been able to study the deep oceans
Many methods - can get from book (any questions?)
I especially like the "seismic profiler" (Monroe; Fig. 12-13, pg. 371)
Last 30 years or so there's been lots of interest & exploration
Partly due to military & communications
New technology has been developed to make it possible
...and new technology demands to be used
Submersibles (Alvin, Sea Cliff, etc.)
Newly discovered life at black smokers
Economic interests are increasing
Beyond the original fishing interests
Oil and Gas
Mining possibilities
Volcanogenic massive sulfide deposits
Black smokers again
Manganese nodules
Probably a pipe dream
Excessive exploration and development costs
Already touched on this quite a bit (click here for additional information)
In many ways tonight should be a review
Spreading Centers
Ophiolites (Monroe; fig. 2-19, pg. 55)
Volcanic Arcs & Trench Complexes
Common where two oceanic plates collide
Japanese Islands, and many others in western Pacific
Similar volcanism occurs onshore at Oceanic/continental margins
Andes, Cascades
Active continental margin: the leading edge (Monroe; fig. 12-11, pg. 368)
West coast of North & South America
Leading edge of continental plate
Extensive tectonics
Erosional processes common
Subduction Zone/Trench
Passive continental margin: the trailing edge (Monroe; fig. 12-11, pg. 368)
Both sides of the Atlantic
Trailing edge of Continental plate
Minimal tectonic activity
Depositional processes common
Primarily basalt
Less than 200 m.y. old
Generated at spreading centers
Consumed at subduction zones
Descends back into mantle where it undergoes further differentiation
On the road to granite
DIGRESS TO: Granitization, and purification of the earth's crust
There are several layers
Fairly straightforward regional model for seafloor processes
Actually quite complex at the local level
Based on the study of ophiolites
Pieces of seafloor shoved up onto the continental margin
Describe in detail
The seafloor is far from flat and featureless
Highly irregular locally
Regionally, however, there is a systematic regional pattern to seafloor features
Due to plate tectonics
The seafloor is so young that there hasn't been time for the major changes associated with older continental terrains
Metamorphism, etc.
Also, its below sea level
Not subject to significant erosion
However, deposition is a continuing problem
Put it all together and it's possible to still observe the reality of recent tectonic events
(Refer to Monroe; fig. 12-7 to 12-10, pgs. 365 to 368)
There are several topographic levels to the seafloor
Globally persistent and definable
Continental Shelves
The relatively low-relief platform seaward from the shore
Usually fairly shallow water
Surrounds most of the continents
Not uniformly wide - varies quite a bit
Average width 50 miles
Range from non-existent to areas nearly 1000 miles wide
Local relief can be somewhat steep
Especially in areas subjected to glaciation
Or to stream erosion at times of lower sea level
Shelf Break - Outer edge of the shelf where it starts down to the oceanic depths
Shelf Dams
Often form the outer boundary of the shelf
Some sort of raised topographic feature
Sediments fill in behind
Several varieties have been recognized
Tectonic dam
Faulted blocks of crustal material
L.A. Basin - Palos Verdes to Catalina to 6 more offshore
Diapirs
Salt domes - common in the western Gulf of Mexico
Biological - coral reefs
Anyway, where they exist...
Can lead to thick deposits of marine sediments landward
Continental Slopes
Connect the Shelf with the deep ocean floor (Abyss)
Actually a fairly gentle gradient
Average slope 4 deg.
This is the AVERAGE gradient
Local relief can be quite substantial (or quite flat!)
Connects the two major levels of the earths surface
The major continental land masses at just above sea level (average!)
And the abyssal depths at 12,000' below sea level
Seems like it should take most of the way to England
12,000 feet X sin (4 deg.) = 172,027 feet
172,027 feet / 5280 feet/mile = 32.58 miles
Surprise! DIGRESS TO: Topographic profiles and vertical exaggeration
Looks steep on most X-section due to vertical exaggeration
Refer to Monroe; fig. 12-9, pg. 367
Submarine Canyons (Monroe; Fig. 12-8, pg. 366)
Characteristic features of the continental slopes
Canyons can have quite a bit of relief - "rival the Grand Canyon"
Origin of the submarine canyons
The formation of these is difficult to explain
Hard to imagine surface erosional processes under the sea
Relatively poorly studied - as is much of the ocean floor
Lots of theories proposed in the past
Early Favorites :
Probably started by sub-aerial erosion processes
During glacial events when sea level was lower
This only accounts for the upper portions of the canyons
Sea level was never low enough to expose the entire length of the longer and deeper canyons
Structural origin (for at least some of the canyons)
Mendocino Fracture Zone & Canyon - off the California coast
Strong bottom currents may help in the erosion of the canyons
Most now agree that Turbidity Currents are primarily responsible
"Density currents of debris-laden water" - DEFINE
Quite viscous
Like a Mud- or Earth-flow
Can move fast and far
Up to 100 kph for distances of up to 700 km
Can be set off by seismic or other disturbances
Example: Grand Banks - off Newfoundland 1929 (Monroe; fig. 12-9, pg. 367)
Earthquake set off a large turbidity current
Severed Trans-Atlantic phone lines
Many cables over 13 hours
Speed of the current 66 ft/sec. (75 kph)
Anyway, all this debris piles up at the mouth of the canyons
Submarine fans
Like alluvial fans in an arid landscape (Monroe; Fig. 18-23, pg. 588)
Continental rise
Coalesced fans (like a bajada in an arid landscape) (Monroe; Fig. 18-24, pg. 588)
Forms the boundary between the slope and the abyssal plain
The basic oceanic depths
Quite a bit of relief (DIGRESS TO: low vs. high relief)
Abyssal Plains
Generally fairly low-relief
Cover large portions of the ocean floor
Most of them probably have at least a thin veneer of sediments (or oozes) covering them
Increasing depth of sediments with distance from spreading center
Abyssal Hills
Topographic mounds on the abyssal plain
Remain well below sea level
Mere "pimples" on the sea floor
Oceanic Ridge systems (Monroe; fig. 2-14, pg. 46) (Monroe; fig. 12-6, pg. 364) (Monroe; fig. 12-12, pg. 370)
Spreading centers for the earth's tectonic plates
Formation of mafic basaltic crust
Can rise above sea level
Iceland, the Azores, Ascension Island
Up to 7500' above sea level (Pico Island in the Azores)
Median valley (Monroe; fig. 12-14, pg. 371)
The actual rift at the crest of the ridge/rise system
High heat flow
High level of seismic activity
Thin crust so generally a lot of small quakes
Extensional environment
Trenches
Increased depths below the main level of the abyssal plains
Generally long and narrow features - like the ridges
Monroe; Fig. 12-12, pg. 370 and relief map
Associated with subduction zones and volcanic arcs
Represent zones of oceanic plate subduction
Review subduction processes
These are the lowest elevations on earth!
Mariana Trench @ -35,785'
Tonga Trench @ -35,326'
Lower heat flow (relative to spreading centers)
High level of seismic activity
Thicker crust so generally less frequent but potentially greater quakes
Compressional environment
Seamounts
Volcanic mounds on the ocean floor which don't extend above sea level
"Thousands of them"
10,000 alone in the Pacific Basin
Basaltic composition - no surprise here
Commonly occur in clusters or linear arrangements
Related to faulting at spreading centers?
Guyots - originally described by Hess (what a guy)
More puzzling than seamounts
Nearly level submarine plateaus
3000' to 5000' below sea level!
Volcanic origin, but
Some are covered with rounded boulders
Evidence of fossil corals
also fossils of Globigerina
Single-celled surface dwelling organism
Origin is VERY unclear
Almost certainly the result of wave action
Clearly represent erosional surfaces at great depth
Would require remarkable sea level fluctuations, or
Extreme regional subsidence of the sea floor
This is the more likely explanation
Plate tectonics may supply an answer
They were originally islands associated with a spreading center
Like Iceland, Azores
Near enough to the surface to be eroded by wave action
Plate motions transport these ridge features off the topographic high associated with the ridge
And down into the mid-ocean depths
Submarine volcanoes - can be very large
Like the Hawaiian/Emperor chain
Composed of more fluid basalt than their terrestrial counterparts
Called "aseismic" islands
Means "without seismic activity"
Not actually true
Lots of activity, just not as strong as at plate boundaries
Origin of these "mantle plumes" or "hot spots" is unclear
Impacts sites?
Ring-shaped islands composed of the skeletons of corals
Original central island is below sea level
Three major stages of development (Monroe; "Reefs," pg. 380)
As defined by Darwin during his voyage on the Beagle
Fringing reefs
Initial development of coral reef near shore
Barrier Reefs
Near complete barrier of coral around an island
Atolls
The final reef-enclosed lagoon
The island is essentially gone due to subsidence (and erosion)
The 3 types are related to each other in a gradational sequence
Fringe will evolve, with time, through a barrier reef into an atoll
Islands commonly begin on or near ridge axis
Combination of upward coral development and subsidence of the island
The rate of coral growth must keep pace with the rate of sinking of the island
Physical characteristics of atolls
Can get quite large
Kwajelein - 15 miles X 75 miles
Most are far smaller
Generally low relief
The island mass has essentially been eroded to below sea level
Fringing reef made of solid coral
Doesn't extend above sea level
Very exposed to ocean storms
Waves can easily override them
Especially storm waves
Encloses the lagoon and Reef Island
Drilling to test Darwin's Atoll theory
Hypothesis: "If the islands are sinking, then the coral reefs should be extremely thick"
A-Bomb testing required some deep holes
Marshall Islands
Deepest hole was 2516'
In coral all the way
Eniwetok - nearby atoll
Deepest holes were 4152' and 4456'
Penetrated 3936' coral and then seafloor basalt
Dated as Eocene (60 m.y.)
Rate of subsidence not constant
Diminished with time
Average 50 to 170 ft per million years
This testing certainly supports Darwin's theories
At least for the two islands which were drilled
It also introduced several problems, at least as far as the indigenous population was concerned
Extensive atmospheric and underground testing
Possible sites for long-term storage of nuclear wastes
There's another real problem with the assumed origin of the Pacific Atolls
Fluctuating sea level due to glaciation
Complicates the upward growth of the coral reefs
By and large, though, the Darwin model is generally accepted
Different types of sediments cover most of the ocean floor
Near shore
Terrigenous sediments
Sand and silt predominate on the beaches and Continental Shelf
Facies changes with distance from shore
Dependent on depositional energy
DESCRIBE AND EXPAND AS NEEDED
Deep ocean sediments
Initial work by the H.M.S. Challenger (1872-1876)
Earliest (?) work on seafloor (abyssal plains)
Defined the broad picture of seafloor sedimentation (Monroe; fig. 12-20, pg. 377)
Ooze
Descriptive term which characterizes the majority of deep ocean sediments
Represents accumulations of debris which settles to the bottom
Called Pelagic sediments
Usually microscopic marine organisms
Lack of terrestrial sediments causes them to be concentrated in the deep ocean
Pelagic sediments also occur near shore but are masked by the overwhelming volume of terrestrial debris
Ooze composition varies systematically across the ocean floor
Calcareous oozes
Tropical and temperate seas <15,000' deep
FORAMINIFERA - common source
Single-celled calcium based creature
Like many single-celled organisms, they divide into two individual creatures
The vacated shells sink to the bottom
Forms Calcareous oozes
In cold and/or deep water, the calcium can re-dissolve
Calcium Compensation Depth (CCD)
Siliceous oozes
Single-celled silica based organisms
Radiolaria - animals
Diatoms - plants
Form in deep water where calcium can't remain stable
Or in localized areas where excess silica increases their production
Siliceous ooze tends to concentrate in colder/deeper waters
But also have a temperature/pressure where they become unstable and re-dissolve (SCD)
Red/Brown clays - occur in the deepest oceanic basins
Most widespread of all sedimentary deposits on the earth
Almost totally inorganic
Actually terrigenous in origin
Very fine grain
Accumulate at a very slow rate
The only thing which can survive the extreme pressure of the deepest basins
Sediment thickness and rate of deposition varies throughout the ocean
Thickness generally increases away from the spreading ridges
Rate of deposition varies with:
Proximity to the continental land masses
Erosion rate on land
Local proliferation of marine organisms
And the solubility of calcium and silica
Water depth
Calcium to silica to clays
Ocean currents
Some coarse-grained sediments do occur
Usually near the base of the continental slope
Probably the result of turbidity currents
Submarine boulder fields
Probably carried offshore by icebergs (glacial erratics)
These are called Glacial Marine deposits
The deep ocean basins provide an excellent, unbroken record of the recent geologic past
Lots of recent studies of sediment cores taken throughout the ocean depths
The type of cold- or warm-water organisms indicate the relative temperatures
Can be used to date glacial and interglacial events
Indications of at least 6 major glacial periods in the last 400,000 years
Oil & gas
Continental shelf
Manganese nodules (Monroe; fig. 12-19, pg. 376)
Feature of the abyssal depths
Golf-ball to bowling ball sized nodules
Grow very slowly
"Ten layers of atoms per year"
Approx. 3 mm per million years
Contain Mn, Fe, Cu, Ni
Estimated reserves: 1 trillion tons
Not currently economic
Tough mining and political problems
Volcanogenic sulfide deposits
Associated with hydrothermal vents at spreading centers (Monroe; fig. 12-15, pg. 372)
Turner-Albright Massive Sulfide Deposit, Josephine County, Oregon
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