Submarine Volcanoes. If an estimate of 4,000 volcanoes per million square kilometers on the floor of the Pacific Ocean is extrapolated for all the oceans than there are more than a million submarine (underwater) volcanoes. Perhaps as many as 75,000 of these volcanoes rise over half a mile (1 kilometer) above the ocean floor.
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Ocean Basin
The ocean basins account for 76 percentofthe ocean floor, and their depths range from1,500to 3,000 fathoms. They have a very slightaverageincline of no more than 1:90 miles. Forevery90 miles seaward the bottom slopes no morethan1 mile. However, superimposed on this veryflatplain are many rugged relief features, suchasseamounts, guyots, atolls, sills, and trenches.
SEAMOUNTS. —Seamounts aresubmerged,isolated, pinnacledmountains rising 3,000 feet ormoreabove the sea floor.
GUYOTS OR TABLEMOUNTS. —Guyotsare submerged, isolated, flat-toppedmountainsthat rise 3,000 feet ormore above the sea floor.
ATOLLS. —Atolls areseamounts or guyotsthat havebroken the surface, and coral depositshavebuilt up around the rim. The coral formsareef around a shallow body of water—a lagoon.
VOLCANIC ISLANDS. —Theseislands occurindividually and ingroups (island arcs). They areformedby volcanic eruptions. About 10,000volcanoesdot the ocean floor, and they areespeciallyabundant in the western Pacific basin.TheHawaiian Islands are probably the bestknownexample of volcanic islands. In the NorthAtlanticOcean, the most recent volcanic island(Surtsey)was formed south of Iceland along theMid-AtlanticRidge in 1964.
SILLS. —Sillsare elevated parts of the oceanfloorthat partially separate ocean basins. A sillrestrictsthe movement of bottom water massesandresults in their partial, and in some casesnearlytotal, isolation.
TRENCHES. —Trenches arelong, narrow,and relativelysteep-sided depressions. Theycomprisethe deepest portions of the oceans. Thetrenchesof the Pacific Ocean stretch for as longas2,500 miles (Peru-Chile Trench), are morenumerousthan in any other ocean, and have byfarthe greatest depths in the oceans. Forexample,the Mariana Trench is 35,600 feet deep;theTonga Trench, 35,430 feet deep; and theMindanaoTrench, 34,428 feet deep. Trenches arenormallyfound on the seaward side of islandarcs,while relatively shallow seas exist on thecontinentalside.
Ridges
On leaving the abyssal plains, we come to thelastof the oceanic provinces, the mid-oceanridges.The Mid-Atlantic Ridge is the mostconspicuousof all ridges. It extends from Icelandsouthwardacross the equator to about 55°S,formingan eastern and western basin in theAtlantic.The Mid-Atlantic Ridge rises from adepthof 2,500 fathoms and is continuous atdepthsof less than 1,500 fathoms over the greaterpartof its length. In several places, this ridge risesabovesea level to form islands such as the AzoresandAscension.
Learning Objective Name and describe thevarioustypes of bottom sediments.
BOTTOM COMPOSITION
Most of the ocean bottom is covered by varioustypes of bottom sediments, deposits of mineralgrains and rock fragments from the continents,mixed with dissolved shells and bones ofmarine organisms. In general, sedimentdepositsare thin or absent on the newly formedcrustof mid-ocean ridges and are thickest on theoldercrust and near continents. The four majorclassificationsof sediments are terrigenous,pelagic,glacial marine, and volcanic.
Terrigenous Sediments
Terrigenous means 'of land origin'.Terrigenoussediments are the land derived siltsandclays that are carried to sea by rivers. Windsalsocarry earth (dust) and sand out to sea anddepositthem on the surface, where they eventuallysinkto the bottom. Terrigenous deposits aremostlyfound in the region of the continentalshelf.
Pelagic Sediments
These sediments are also known as ooze. Theyformin deep water and are most commonlycomposedof shells and skeletal remains of marineplantsand animals.
Glacial Marine Sediments
The majority of these sediments (mud,rocks, sand, and boulders) were deposited when theglaciersof the ice age melted and retreated toward the poles. These sediments aredeposited today by icebergs, since in most cases, icebergs are pieces ofglaciers that break off and float to sea andmelt.These sediments are found primarily in highlatitudeswithin the continental shelf.
Volcanic Sediments
These deposits, primarilypumice and ash, arethe result ofvolcanic eruptions. They are foundinboth deep and shallow water in all the world’soceans.