The Rim of the Pacific

Abstract

The Pacific is a girdle surrounding deep ocean of Pacific; if we neglect curvature of earth, may be likened to raised rim of a tray. Over vast areas of basin floor is comparatively smooth?but not quite smooth, as there is an unexplained small relief of valleys, ridges, and troughs (Menard, 1956) that has recently been discovered and is ubiquitous except where obliterated by accumulation of lava and of sediment forming archipelagic aprons (Menard). Of a much larger order, however, are scattered mounds of volcanic origin which are of huge size, though dwarfed by vast dimensions of surrounding basin. Some of volcanoes that originated on sea floor have been built up to form existing islands, but others fail to reach within several hundred fathoms of surface of sea, either having become foundations of atolls or remaining sub? merged as seamounts. Away from these (and not far from them) water is very deep, though maximum depths, up to 6 miles, are confined to narrow but elongated ocean deeps. In contrast with some monotony on floor, relief is varied in many parts of rim, consisting of mountain chains, though in other places are curving festoons of islands, not all of them high. The rim does not coincide in position with geo? graphical boundary of ocean; rather is its position definable in geophysical and geological terms, so that rim concept is as much geological as geographical. In western Pacific is far out in Pacific Ocean, though necessarily in such a place is discontinuous, consisting of islands. Behind the rim, to west, is a shelf-like belt of marginal seas that are in general shallower than basin. In terms of classic sial and sima, floor of consists of sima which is almost bare,* its only cover, as indicated by geophysical investigation supported by seismic evidence, being a thin layer which must consist for most part of abyssal sediment; and Mohorovi?ic discontinuity is at a depth rarely greater than 6 miles below sea level. Over floors of marginal seas outside rim, however, there is some sial, though is a layer very much thinner than that which forms continents (Gutenberg, 1951). A remark of Officer et al. (1957) made with regard to contrasting floors of Atlantic Ocean and Caribbean Sea is applicable to those of Pacific and western seas. Unless they were different it would be difficult to understand how an island are and deep-sea trench might develop between them.