MARINE BENCH 



theory. In nature only dipoles, not isolated 

 magnetic poles, exist. ( 5 ) 



magnetic secular change — The slow change over 

 many years of either intensity or direction of 

 the earth's magnetic field due to causes lying 

 deep within the earth. 



magnetic storm — See magnetic disturbance. 



magnetic variation — 1. Changes in the magnetic 

 field with time or space. 

 2. Magnetic declination. 



magnetometer — An instrument for measuring 

 the intensity and/or direction of the earth's 

 magnetic field. 



magnetostriction transducer — A transducer 

 that depends for its operation on the interaction 

 between the magnetization and the deformation 

 of a material having magnetostrictive proper- 

 ties. 



magnitude — 1. In mathematics, a number as- 

 signed to a quantity, by which the quantity may 

 be compared with other quantities of the same 

 class. 



2. A quantity which represents the total 

 energy released by an earthquake, as contrasted 

 to "intensity," which describes its effects at a 

 particular place. In 1935 C. F. Richter de- 

 vised the magnitude scale in current use. The 

 Richter magnitude scale ranges numerically 

 from near to about 8.5. The smallest shocks 

 felt have a magnitude of 1.5, which represents 

 an energy release of about 10" ergs. A magni- 

 tude 5 shock represents the release of 10-^ ergs of 

 energy, which is equivalent to the eneray re- 

 leased by 20,000 tons of TNT. 



3. The intensity of a short-period magnetic 

 fluctuation, usually expressed in milligausses or 

 gammas. 



major constituents — Those chemical elements 

 present in sea water which together make up over 

 99.9 percent of the kiiown dissolved solid con- 

 stituents of sea water. These include the follow- 

 ing ions: chloride, sulfate, bicarbonate, bro- 

 mide, fluoride, boric acid, sodium, magnesium, 

 calcium, potassium, and strontium. 



makatea — The raised fin of a coral reef. 



manatee — Any of the three species of sea cow 

 which constitute the genus Tricliechus. All 

 three are confined to sliallow tropical marine 

 waters, estuaries, and rivers on both sides of the 

 Atlantic Ocean. The tail is broad and rounded, 

 not whalelike as in the dugong. 



manateegrass — See seagrass. 



manganese nodules — See nodules. 



mangrove — One of several genera of tropical 

 trees or shrubs which produce many prop roots 

 and grow along low-lying coasts into shallow- 

 water. 



manta ray — (or devilfish). One of a family 

 (Mobulidae) of large rays. This is the largest 

 of the rays, with widths over 20 feet and weights 

 over 3,000 pounds having been recorded. See 

 ray. 



mantle — (also called (iHthenoHfliere) . Tlie rela- 

 tively plastic region between the crust and core 

 of the earth. {See figure for earth structure.) 



mantle rock — See regolith. 



many-year ice — See artic pack (sense 2) . 



map — A representation on a plane surface, at an 

 established scale, of the physical features (nat- 

 ural, artificial, or both) of a part or the whole of 

 the earth's surface, by means of signs and sym- 

 bols, and with the means of orientation indi- 

 cated. (37) 



marginal basin — This term is not recommended 

 by the ACUF for a basin at the foot of a con- 

 tinental slope bounded by an outer ridge. See 

 basin. 



marginal conglomerate — Coarse pebble deposits 

 along the shore wliich form the landward mar- 

 gins or facies of other types of sediments into 

 which they gi'ade. (2) 



marginal crushing — The destruction of the outer 

 edges of ice cakes because of collision of the 

 cakes. (65) 



marginal deep — This term is not recommended 

 by the ACUF for a narrow linear zone of deep 

 water adjacent to an island arc. See trench, 

 trough. 



marginal platform — This term is not recom- 

 mended by the ACUF for a shelf adjacent to a 

 continent and similar topographically to a con- 

 tinental shelf but located at a greater depth 

 that may reach 1,260 fathoms. See terrace, 

 plateau. 



marginal seas — (or adjacent seas) . Semienclosed 

 seas adjacent to, widely open to, and connected 

 with the oceans at the water surface but bounded 

 at depth by submarine ridges; for example, 

 Yellow Sea. When shallow (less than about 150 

 fathoms) they are called shelf seas ; for exam- 

 ple, Hudson Bay. 



marginal trench — This term is not recommended 

 by the ACUF for a trench which parallels the 

 continental trend between the base of the con- 

 tinental slope and abyssal plain. See trench, 

 trough. 



marginal zone — The outer (seaward) ridge and 

 generally the highest portion of a coral reef. 



marigram — A graphic record of the rise and fall 

 of the tide in the form of a curve which shows 

 the time of any stage of the tide represented by 

 abscissas and the lieight in feet by ordinates. 

 (50) (/S'ee figure for tide curve.) 



marigraph — Sec tide gage. 



marine abrasion — The erosion of a bedrock sur- 

 face by wave movement of sand and gravel. 



marine alluvium — A contradiction in terms which 

 is best avoided. I'se marine sediment. 



marine arch — See sea arch. 



marine bench — A small wave eroded terrace 

 along the shore which is level or gently inclined 

 seaward. 



101 



