Transcript Ocean Basins and Crust

Ocean Basins and Crust
• This is first lecture of Ocean+PlateTectonics+MountainsBuilding, so it will be unusually "stamp collecting".
• When geologists talk about oceanic crust, they specifically mean the basaltic, deep-water type of crust, not
just any crust with water on top of it.
• Synopsis of oceans:
–Just offshore is a (often) broad, gently-sloping, shallow area called the continental shelf. This is part of
continent and is a sort of giant wave-cut terrace.
–Most of an ocean is deep and flat: the abyssal plain.
–In the middle of an ocean is a mid-ocean ridge, where new oceanic crust is extruded.
–The slope and rise are transitional from shelf to abyssal plain.
–In very old oceanic crust, there can be very deep trenches, which is where oceanic crust subducts. These are
near volcanic chains, and dipping earthquake zones.
v 0025 of 'Ocean Basins and Crust' by Greg Pouch at 2011-03-28 10:25:11 LastSavedBeforeThis 2010-02-08 12:08:32
Ocean Basins and Crust
3 World Topography from Tarbuck & Luntgrens
4 How do we study sea floor
Structure of an Ocean
5 Geography and Structure of an Ocean, 1 of 3
6 Geography and Structure of an Ocean, 2 of 3
7 Geography and Structure of an Ocean, 3 of 3
8 Structure of an Ocean (Slides)
9 World Topography from Tarbuck & Luntgrens
10 Geography and Structure of an Ocean (one slide)
Deposits
11 Ocean sediments
12 Coral Reefs
13 Ophiolite Sequences
14 Types of coasts
15 Passive Continental Margin
16 Active Continental Margin
World Topography
from Tarbuck & Luntgrens
How do we study sea floor
•Sampling using dredges and corers
•Drilling and recovering cores
•Submersibles
•Geophysics
Geography and Structure of an Ocean, 1 of 3
Oceans and continents are qualitatively different, but have transition zones from one to the other.
•Continental part (granodioritic, density lower than mantle)
–Continental Shelf is part of a continent that just happens to be under water.
•Sort-of a big, submarine terrace with sediments. Very gently sloping 0.1 Flat like continents.
•Extensive on passive margins (plates spreading apart), small/absent on active margins (plates colliding).
•Much shelf was exposed during last glaciation.
•Often cut by submarine canyons that debauch onto the slope.
•Transition (slope and rise are intermediate from continent to deep-ocean)
–Continental Slope is thinned continental crust.
•Pull-apart faulting (think stretched Milky Way bar, Basin and Range) with sediments and water on top.
•More steeply sloping than shelf 4-5 (but less than many hills).
•May be cut by submarine canyons.
–Continental Rise is oceanic crust with "thick" accumulation of sediments (thick for oceans).
•Gently sloping 0.5 , sort of a submarine bajada.
•Oceanic crust with “thick” deposits of continental material, mostly carried in by turbidity currents (subaqueous mudflows.) Submarine fans (a lot like alluvial fans or deltas) can coalesce, forming the
continental rise. Very flat.
•Oceanic part (basaltic, can be denser than mantle when old)
–Abyssal plain
–Mid-Ocean Ridge
–Trench
Geography and Structure of an Ocean, 2 of 3
•Continental part
–Continental Shelf
•Transitional
–Continental Slope
–Continental Rise
•Oceanic part
–Abyssal plain (old) is the deepest part of the ocean, very flat, and makes up the bulk of an ocean.
•It is oceanic crust, that was created at ridges, has gotten cold and subsided, and coated with sediments,
which smoothes out the irregularities that are found near ridges.
•Seamounts are conical volcanoes on the sea floor. (look a lot like a very fresh, uneroded volcano).
•Guyots are flat-topped mountains rising above the seafloor, but not up to the sea-surface. They were
volcanic islands that were weathered to sea-level, then subsided due to cooling of the oceanic crust
underneath them. The flat top is a combination of land surface and wave-cut terrace.
–Mid-ocean ridge (young): a linear mound on the sea floor, rising far above the abyssal plain (like km)
•Little or no sediment, very young rocks.
•Extensive volcanism (most volcanism occurs in ridges below the sea).
•Symmetric age of rock and symmetric pattern of magnetic anomalies.
•There is often a medial valley (a graben [down dropped fault block] in the middle). Extensive faulting due
to movement of the plates and collapse of magma chambers results in rugged topography.
•Offset by transform faults, where the location of the ridge jumps.
•This is where oceanic crust is created. Elevation is mainly due to hot crust, and consequent expanding.
–Trench (very old)
Geography and Structure of an Ocean, 3 of 3
•Continental part
–Continental Shelf
•Transitional
–Continental Slope
–Continental Rise
•Oceanic part
–Abyssal plain (old)
•Seamounts
•Guyots
–Mid-ocean ridge (young)
–Trench might be actual topographic trench, or might be a basement "trench" with sediments filling the hole.
•Can be a very deep (like 10 or 15 km), often parallel to a coast or an island arc
•Away from the ocean, there is usually an island arc or volcanic chain, and earthquakes in sloping zone.
•Often, there is an accretionary wedge where sediments and oceanic crust that were scraped/broken off the
descending plate have accumulated. (What happens when seamount, guyot, coral reef, basalt plateau, or
mid-ocean ridge go into subduction zone?)
•Typically, oceanic crust oceanward is very old, so cold and dense, probably denser than mantle.
•These mark subduction (convergent) plate boundaries.
.6
Structure of an Ocean
World Topography
from Tarbuck & Luntgrens
Geography and Structure of an Ocean
Oceans and continents are qualitatively different, but have transition zones from one to the other.
•Continental part
–Continental Shelf Very gently sloping 0.1 part of a continent that is under water. Flat like continents
–Extensive on passive margins (plates spreading apart), small/absent on active margins (plates colliding).
–Much shelf was exposed during last glaciation.
–Often cut by submarine canyons, that debauch onto the slope.
–*Continental Slope More steeply dipping than shelf 4-5, this is usually thinned continental crust. (Pull-apart faulting, like basin and range,
but with water on top of it.)
–Marks the transition from continent to ocean. May be cut by submarine canyons.
–**Continental Rise Gently sloping 0.5 accumulation of sediments, sort of a submarine bajada.
–Oceanic crust with “thick” deposits of continental material, mostly carried in by turbidity currents (sub-aqueous mudflows.) Submarine fans
(a lot like alluvial fans or deltas) can coalesce, forming the continental rise. Very flat.
•Oceanic part
–**Continental Slope
–*Continental Rise
–Abyssal plain
–This is the bulk of an ocean. It is oceanic crust, that was created at ridges, has gotten cold and subsided, and coated with sediments, which
smoothes out the irregularities that are found near ridges.
•Seamounts are conical volcanoes on the sea floor. Guyots are flat-topped mountains rising above the seafloor, but not up to the seasurface. They were volcanic islands that were weathered to sea-level, then subsided due to cooling of the oceanic crust underneath them.
The flat top is a combination of land surface and wave-cut terrace.
–Mid-ocean ridge: a linear mound on the sea floor rising far above the abyssal plain, little or no sediment, very young rocks. Extensive
volcanism (most volcanism occurs in ridges below the sea).
–Symmetric age of rock and symmetric pattern of magnetic anomalies.
–There is often a medial valley (a graben [down dropped fault block] in the middle). Extensive faulting due to movement of the plates and
collapse of magma chambers results in rugged topography. Offset by transform faults, where the location of the ridge jumps.
–This is where oceanic crust is created. Elevation is mainly due to hot crust, and consequent expanding.
–Trench This is a deep trench, often parallel to the coast or an island arc (convergent plate boundaries).
–These occur where an oceanic plate is being subducted. Away from the ocean, there is usually an island arc or volcanic chain. Often, there
is an accretionary wedge where sediments and oceanic crust that were scraped/broken off the descending plate have accumulated.
Ocean sediments
• Most sediments on land accumulated as shallow, marine sediments.
• Shallow (Shelf and slope)
–Terrigenous Much oceanic sediment is derived from land. Near-shore, there is more wave-action and
only sands accumulate. Further offshore, silt and clay are settling out.
–Biogenic Shells and other remains of organisms. Some plankton secrete calcite or silica shells,
sometimes this includes apatite from bones and shells, maybe organic matter in reducing environments.
There are also reef-building organisms like corals.
–Turbidites are submarine mudflows, and give rise to graded beds (upward fining sequence, where coarse
material settles out first). Caused by storms or submarine mudslides.
–Contour currents run parallel to shores, and re-work fine-grained sediment.
• Deep (Slope, rise and abyssal plain)
–Terrigenous: includes windblown and turbidity current transported clastic sediment. Often distinctively
colored due to the great amount of time spent in an oxidizing environment.
–Biogenic: Includes calcareous shells, siliceous shells, and apatite remains. May include organic matter.
–Abiogenic precipitates: rare, but some minerals can precipitate directly from seawater.
–Astrogenic: If little else is deposited, micrometeorites can make up a significant fraction of sediment .
Coral Reefs
• Coral (symbiotic coral + alga) are filter feeders, (at present) only live in fairly warm, clear, agitated waters.
• Coral reefs are due to accumulation of the calcareous remains of corals and algae, along with minor
contributions from other organisms, like sponges and shellfish.
• Reefs come in three main types. First two are found in the geologic record, as well as live. Atolls are not found in the geologic record.
–Fringe reefs occur along the margins of land. Only conditions needed by coral are necessary for fringe
reefs to form. If sea-level is constant, they propagate seaward, leading to barrier reefs.
–Barrier reefs occur offshore from land at the surf zone. As a fringe reef grows, the water above it
becomes less agitated, a condition necessary for coral. Coral keeps growing in the surf zone, so the live
part of a reef can be separated from the land. Fragmented bits of reef fill in the lagoon (quiet water area
behind the reef). (Why would this be different for atolls?)
–Atolls occur in deep water, and are usually circular rims of reef around a central lagoon. They form as
fringe or barrier reefs around an island. The central lagoon is due to karst dissolution of the central part
of an originally flat island when sea level was lower during the last glaciation. (We know this because a) we don't
find ancient atolls and b) when we drilled a lot into Bikini Atoll, the lower part looked like a perfectly normal fringe reef, and the upper bit
looked like it had been karsted, and partially filled in)
Ophiolite Sequences
• An ophiolite is a rock sequence, often found near an ocean, consisting of deep-ocean sediments,
pillow basalts, sheeted dike complex, gabbro plutons, and often ultramafic rocks that are restites.
• Ophiolites have a seismic velocity structure that matches oceanic crust fairly well, and may be
slices of oceanic crust that have been obducted onto land during collisions. Ages of ophiolites
suggest that they are usually formed around the time a mid-ocean ridge should have subducted,
and may be non-typical oceanic crust.
Types of coasts
Most coasts are either very exciting (Chile) or very dull
(New Jersey)
• Active margins (Chile) have volcanoes and earthquakes.
There is almost always a trench, and usually a volcanic
chain or island arc. If there is a continental shelf, it is
narrow. Immature, reducing sediments, with volcanics.
Structures: volcanoes, lots of faults and folds (chewed
up). Called flysch or eugeosyncline
• Passive margins (New Jersey) lack volcanoes,
earthquakes, nearby young mountain chains (at least
volcanic or folded), and trench. Passive margins have
broad continental shelves and have slowly subsided and
accumulated thick sequences of mature sediments (no
unstable minerals, very well sorted, rounding is usually
good), limestones, no volcanics. Bottom layers often
have thick salt, less mature sediments, some volcanics.
Structurally: grabens parallel to shore line. Called
miogeosyncline.
Passive Continental Margin
Active Continental Margin
Oceanic Crust
• Unlike continents, oceans have surprisingly little structure. Oceanic basement is fairly young (all less than
180Ma) basaltic/gabbroic material. Ophiolites are thought/hoped to represent oceanic crust
• On a passive margin, the near-shore continental shelf is a broad area of continental crust and goes with the
continent. This transitions into the continental slope (thinned continental) and continental rise (oceanic with
continental junk) out to the abyssal plain (older oceanic crust) to the mid-ocean ridges (younger oceanic
crust).
• On an active margin, there is little or no continental shelf and often a trench with very old oceanic material.
Active margins often have volcanoes and earthquakes, like Chile.
• Oceanic sedimentation ranges from alluvial-fan-like features near shore to space-dust only deposition in
very deep, quiet water.
From http://www.chbr.noaa.gov/default.aspx?category=cep&pageName=projects