Hawaii FEP - Western Pacific Fishery Council

ock, and is recycled into the mantle. When two plates of equivalent density converge, the rockat the boundary fractures and shears like the front ends of two colliding cars, and forms a largemountain range. The Himalayan Range has this origin. There are three different types of plateconvergence: 1) ocean-continent convergence, 2) ocean-ocean convergence, and 3) continentcontinentconvergence (Fryer and Fryer 1999). A well known example of ocean-oceanconvergence is observed in the western Pacific, where the older and denser Pacific Platesubducts under the younger and less dense Philippine Plate at a very steep angle. This results inthe formation of the Marianas Trench which at nearly 11 km (~36,000 ft) is the deepest point ofthe seafloor. 7 Ocean-ocean convergent boundary movements may result in the formation ofisland arcs, where the denser (generally older) plate subducts under the less dense plate. Meltingin the upper mantle above the subducting plate generates magma that rises into the overlyinglithosphere and may lead to the formation of a chain of volcanoes known as an island arc. 8 TheIndonesian Archipelago has this geologic origin, as does the Aleutian Island chain.Transform boundaries, a third type of plate boundary, occur when lithospheric plates neitherconverge nor diverge, but shear past one another horizontally, like two ships at sea that rub sides.The result is the formation of very hazardous seismic zones of faulted rock, of which California’sSan Andreas Fault is an example (Fryer and Fryer 1999).In addition to the formation of island arcs from ocean-ocean convergence, dozens of linear islandchains across the Pacific Ocean are formed from the movement of the Pacific Plate overstationary sources of molten rock known as hot spots (Fryer and Fryer 1999). A well knownexample of hot spot island formation is the Hawaiian Ridge-Emperor Seamounts chain thatextends some 6,000 km from the "Big Island" of Hawaii (located astride the hotspot) to theAleutian Trench off Alaska where ancient islands are recycled into the mantle. 9 Although lesscommon, hot spots can also be found at mid-ocean ridges, exemplified by the Galapagos Islandsin the Pacific Ocean. 10The Pacific Ocean contains nearly 25,000 islands which can be simply classified as high islandsor low islands. High islands, like their name suggests, extend higher above sea level, and oftensupport a larger number of flora and fauna and generally have fertile soil. Low islands aregenerally atolls built by layers of calcium carbonate secreted by reef building corals andcalcareous algae on a volcanic core of a former high island that has submerged below sea level.Over geologic time, the rock of these low islands has eroded or subsided to where all that isremaining near the ocean surface is a broad reef platform surrounding a usually deep centrallagoon (Nunn 2003).3.2.3 Ocean Water CharacteristicsOver geologic time, the Pacific Ocean basin has been filled in by water produced by physical andbiological processes. A water molecule is the combination of two hydrogen atoms bonded withone oxygen atom. Water molecules have asymmetric charges, exhibiting a positive charge on the7 http://www.soest.hawaii.edu/coasts/chip/ch02/ch_2_7.asp8 http://www.soest.hawaii.edu/coasts/chip/ch02/ch_2_7.asp9 http://pubs.usgs.gov/publications/text/Hawaiian.html10 http://pubs.usgs.gov/publications/text/hotspots.html#anchor1962097937