<P> Within the closed loop of the andesite line are most of the deep troughs, submerged volcanic mountains, and oceanic volcanic islands that characterize the Pacific basin . Here basaltic lavas gently flow out of rifts to build huge dome - shaped volcanic mountains whose eroded summits form island arcs, chains, and clusters . Outside the andesite line, volcanism is of the explosive type, and the Pacific Ring of Fire is the world's foremost belt of explosive volcanism . The Ring of Fire is named after the several hundred active volcanoes that sit above the various subduction zones . </P> <P> The Pacific Ocean is the only ocean which is almost totally bounded by subduction zones . Only the Antarctic and Australian coasts have no nearby subduction zones . </P> <P> The Pacific Ocean was born 750 million years ago at the breakup of Rodinia, although it is generally called the Panthalassic Ocean until the breakup of Pangea, about 200 million years ago . The oldest Pacific Ocean floor is only around 180 Ma old, with older crust subducted by now . </P> <P> The Pacific Ocean contains several long seamount chains, formed by hotspot volcanism . These include the Hawaiian--Emperor seamount chain and the Louisville Ridge . </P>

Compared with equatorial open ocean waters open ocean water in the northern pacific