. Elementary physical geography;. er, the varia-tions in temperature are usually very irregular. Thus, theentrance to the Gulf of Mexico is blocked by a submarineridge whose crest is 1,200 feet below the surface, and be-cause of this, water whose temperature is lower than that ofthe 1,200-foot level cannot enter the Gulf. But even at adepth of 12,000 feet, the temperature varies but little fromthat of the 1,200-foot level. The freezing temperature of salt water is lower by twoor three degrees than that of fresh water, the difference 196 PHYSICAL GEOGRAPHY depending mainly on the amount of mine
. Elementary physical geography;. er, the varia-tions in temperature are usually very irregular. Thus, theentrance to the Gulf of Mexico is blocked by a submarineridge whose crest is 1,200 feet below the surface, and be-cause of this, water whose temperature is lower than that ofthe 1,200-foot level cannot enter the Gulf. But even at adepth of 12,000 feet, the temperature varies but little fromthat of the 1,200-foot level. The freezing temperature of salt water is lower by twoor three degrees than that of fresh water, the difference 196 PHYSICAL GEOGRAPHY depending mainly on the amount of mineral salts in ice of the sea is therefore formed in high latitudes,where the temperature is much below the freezing-point. Bulk for bulk, ice is lighter than water. Solid sea icefloats with about one-eighth of its mass above the it contains air bubbles, however, a greater proportion isout of water. Sea Ice.—Sea ice takes various forms. The nearly leveland narrow shelf that in polar regions forms along the. ICE OF THE SEA: FLOE, PACK, AND BERG shore, and skirts almost its entire extent, is called the icefoot. Any considerable extent of undisturbed or unbrokenice forms an ice sheet or ice -field. When on-shore windsbecome so strong that the ice field is crushed and piled upagainst the shore, it forms pack ice. Detached massesfloating about constitute floes; finely broken ice floatingon the surface is called sludge. WAVES, TIDES, AND CURRENTS 197 The formation of pack ice is sometimes sudden and frequently crunching from side pressure, due to the friction of the wind, is sogreat that not only is the ice piled up in huge blocks, but the blocks,often weighing many tons, are shot up into the air ten or twenty feet. Anchor ice results from the freezing of fresh water at the bottomof an estuary into which salt water flows. The ice accumulates on thebottom until its buoyancy overcomes the force with which it adheres tothe bottom; then the whole mass
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