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Distribution of land and sea
Distribution of elevations
Sea-floor provinces
Ocean sediments
Determining ocean depths, sampling the sea floor
Distribution of land and sea
Average Earth: 71% oceanic, 29% continental
Major differences between hemispheres:
Northern -- 40% of area is continental
Southern -- 83% of area is oceanic
Provinces of the sea floor -- major (cover most area)
Continental margins
Shelf, Slope, Rise
Submarine canyons
Ocean basin floor
Ave. depth = 4-6,000 m
Usually flat (mantled by sediments)
Volcanic features
Islands, coral atolls
Underwater hills, seamounts, "guyots"
Mid-ocean ridges and rises
Continuous volcanic mountain range
Provinces of the sea floor -- limited area, but still important
Trenches
6-11,000 m deep -- deepest regions of the sea floor
Occur at margins of basins, especially Pacific
East Pacific -- edge of continental margin
West Pacific -- associated with volcanic island chains
Fracture zones -- faults that offset sections of mid-ocean
ridges
Ocean Sediments
Sources:
* Continents -- weathering and erosion of exposed rock; transport
by rivers (and winds) to the sea.
* Microscopic "plants" and "animals" in surface
ocean waters ("plankton') -- precipitate tiny "shells"
of CaCO3 and SiO2
Information in sediments! -- record of present and past processes
on land and in the sea.
Determining ocean depths -- Echo Sounding
Reflection of sound impulse from sea floor
Depth = [sound velocity] X [(1/2) round-trip time]
Sampling the sea floor
Dredges and grab-samplers
Coring
Drilling and coring
For the Earth as a whole, 71 % of the surface is covered by
oceans (and 29 % by continents). But the distribution varies with
hemisphere.
o Most continental land-masses are in the Northern Hemisphere
(40% of N.H. is continental).
o The Southern Hemisphere is dominated by oceans (83% of S.H.
is oceanic)
The distribution of elevations on Earth's surface is described
by the hypsographic curve. The curve shows the percent of the
surface between two elevations (with respect to mean sea level).
The information on the curve indicates that:
* the mean elevation of continents [+840 meters (m)], and
* the mean depth of ocean basins [-3,800 m].
It is important to note that there is a sharp, narrow transition
by continents and oceans. This transition reflects fundamental
differences in the crust of those regions:
* Continental crust: thick, less dense, buoyant.
* Oceanic crust: thin, more dense, "rides lower" on
the underlying mantle.
Major provinces (in terms of areal extent) include continental margins, ocean-basin floor, and mid-ocean ridges and rises.
Continental margins -- are just as the name implies,
the transition between continental and oceanic regions.
The continental shelf is the flooded part of the continent.
The continental slope is a steep feature at the edge of
the shelf.
The contental rise is the area at the base of the slope.
A well-developed shelf, slope, and rise is typical of the wide continental margins of the Atlantic Ocean. In contrast, the margins of the Pacific Ocean are usually narrower.
Submarine canyons are common on all margins. They are deep valleys that cut the outer shelf and slope.
Ocean basin floor -- extends from continental rises
to mid-ocean ridges. The average depth is 4-6,000 m. The ocean-basin
floor is usually quite flat, especially where it is mantled by
sediment. But there are numerous topographic features of volcanic
origin, most commonly observed in the Pacific. These include
* emergent volcanic islands and coral atolls
* submergent hills, seamounts, a flat-topped seamounts called
"guyots."
Mid-ocean ridges and rises -- are a globe-encircling volcanic mountain range that rise about 2,000 m above the adjacent ocean-basin floor. They are one of the most impressive topographic features of our planet.
Other important features that are wide-spread, but do not occupy much area include marginal trenches and fracture zones.
Trenches -- are the deepest regions of the sea floor,
with depths of 6-11,000 m. They occur at the margins of ocean
basins, most commonly in the Pacific.
* In the East Pacific (along Central and South America), trenches
are at the edge of the continental margin.
* In the West Pacific (also Antarctic Scotia Sea and Carribean),
marginal trenches are associated with volcanic island chains called
"island arcs."
Fracture zones -- are faults that offset sections of mid-ocean ridges. They extend into and across the floor of ocean basins.
Sediments on the sea floor are derived both from continents and from biological activity at the sea surface.
(1) Sediment on continents is produced by the weathering (break-down) and erosion of exposed rocks. Sediment is transported by rivers to continental margins. The thickest deposits of continental sediments occur beneath the continental slope and rise. Fine-grained particles can be carried by wind or surface ocean currents to the open ocean (well away from continental margins).
(2) In surface waters of the open ocean, microscopic algae and protozoans (plankton) precipitate very small "shells" of CaCO3 and SiO2. These particles are deposited through the water much like snow falls on land. Some continental sediment and volcanic ash is deposited in the same way.
Sediment layers in the ocean are records of processes occurring
both in the oceans and on land.
* The surface layer of sediments reflect modern processes (e.g.,
the amount of sediment carried to shelf areas by rivers, the production
of biological precipitates in surface waters).
* Deeper, older layers are a record of past processes.
-- is done by some type of Echo Sounding (SONAR) device. This method is based on the reflection of a sound impulse from the sea floor. A strong impluse is generated electronically or by an explosion. Hydrophones on research vessels then "listen" and time the reflection of the impulse from the sea floor. Depth is determined from the velocity of sound in sea water (~1,450 m/sec) and the time required for the impulse to make the "round-trip" from the surface to the sea floor and back to the surface: D = v x (1/2) t.
-- can be done by a vareity of ship-board equipment.
* Dredges and "grab samplers" scoop-up sediments and
rocks at the surface.
* Core tubes are lowered overboard and then dropped into soft
sediment; the core tube is recovered, cut open, and the largely
undisturbed sediment layers are sampled.
* Drilling rigs on a few large vessels are capable of drilling
a continuous core of sediment and basalt to a total length of
more than 1 kilometer!
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