Page 213 - NS-2 Textbook

208 NAUTICAL SCIENCES

Basin Mid-Atlantic Ridge Basin

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Depth (fathoms)
A profile of the Mid-Atlantic Ridge.
about 2,000 miles from the west coast of South America, oceanic islands-the tops of fanner volcanic mountains.
it runs northward to the perlinsula of Baja California. The When erosion has ,vorn away much of a volcanic peak in
whole 40,OOO-mile-Iong mountain chain is sometimes the ocean, a strand of coral islands is left around the old
given a single name, the Mid-Ocean Ridge, although it is volcanic rim. 111is formation is known as an atoll. The
somewhat off center in the Pacific. Many lUldenvater centrallagoall of the atoll is what remains of the old vol-
earthquakes occur in a rift running down the ridge's cen- canic crater.
terline. Large portions of the major plate margins of In some cases, these coral islands continue to subside
Earth's surface lie along the centerline of the Mid-Ocean and finally disappear beneath the sea surface, leaving
Ridge. what is known as a seamollnt. Many strings of seamounts
Oceall Islands, Seamollllts, alld GllyotS. All true oceallic dot the floor of the central Pacific, the ancient remains of
islallds are volcanic in origin. They cliffer from island fonner islands. They are fOlmd in all oceans but are most
fragments that have broken away from continental comnlon in the Pacific Ocean.
masses, such as New Zealand, New Guinea, and Green- Scattered lmdenvater mowltains with peaks that
land. Almost all of the small islands of the Pacific are never reached the suTface retain the name seamounts,
cones formed by accumulated layers of volcanic me,te,'iel
/v'olc"nic island
seamount

The development of oceanic islands from submarine volcanoes. At the center, the drawing shows a volcanic island with its active volcano. At
left is a seamount, an underwater volcanic peak that has become extinct or inactive before reaching the surface. At the far right, the decay-
ing volcanic island has eroded to become an atoll with the crater now a lagoon surrounded by coral-covered island fragments of the crater's
rim. To the right of the seamount is a guyot, once an island or submerged peak that had its top eroded away by sea action.