Hawaii FEP - Western Pacific Fishery Council

denser than the continental portion composed of granite which is enriched with silica. 3 Tectonicprocesses and plate movements define the contours of the Pacific Ocean. Generally, the abyssalplain or seafloor of the central Pacific basin is relatively uniform, with a mean depth of about4270 m (14,000 ft). 4 Within the Pacific basin, however, are underwater plate boundaries thatdefine long mountainous chains, submerged volcanoes, islands and archipelagos, and variousother bathymetric features that influence the movement of water and the occurrence anddistribution of marine organisms.Figure 3: Schematic Diagram of the Earth's Lithospheric PlatesSource: Dr. C.H. Fletcher III, UH Dept. of Geology and Geophysics, personal communicationDivergent plate boundaries —locations where lithospheric plates separate from each other—form“spreading centers” where new seafloor is constructed atop high mid-ocean ridges. These ridgesstretch for thousands of kilometers 5 and are characterized by active submarine volcanism andearthquakes. At these ridges, magma is generated at the top of the mantle immediatelyunderlying an opening, or rift, in the lithosphere. As magma pushes up under the spreadinglithosphere it inflates the ridges until a fissure is created and lava erupts onto the sea floor (Fryerand Fryer 1999). The erupted lava, and its subsequent cooling, forms new seafloor on the edgesof the separating plates. This process is responsible for the phenomenon known as “seafloorspreading”, where new ocean floor is constantly forming and sliding away from either side of theridge. 6Convergent plate boundaries are locations where two plates move together and one plate, usuallycomposed of denser basalt, subducts or slides beneath the other which is composed of less dense3 http://www.soest.hawaii.edu/coasts/chip/ch02/ch_2_7.asp4 http://www.physicalgeography.net/fundamentals/8o.html (accessed January 2007)5 http://www.washington.edu/burkemuseum/geo_history_wa/The Restless Earth v.2.0.htm (accessed July 2006)6 http://www.washington.edu/burkemuseum/geo_history_wa/The Restless Earth v.2.0.htm (accessed July 2006)36