The Mariana, Volcano, and Bonin Islands are a part of that great series of arcuate islands that can be traced across the Pacific from Alaska to New Zealand. They lie south-southeastward from Japan, forming the central and southern portions of a chain extending from the Izu Peninsula through Guam. All are high pelagic islands. Measured against world total they are of microscopic importance in total area, population, and resources. Location, however, gives them a major place in the spatial relations of global geography. Aviation technology may some day outdate the need for stopping points along the Pacific air routes. But, at present, over an ocean of such vast distances and one in which the total island area is small as compared with total water surface, the islands, their location, and their geographic nature are of vital importance in commercial and military aviation.
During the Second World War the various islands in the Marianas and Volcanoes were captured or bypassed in the steppingstone offensive towards Japan. Some of the larger ones were developed as staging areas to be used in the final attack against the enemy.
The Mariana, Volcano, and Bonin Islands are grouped along rising mountain ranges, or great curving anticlines, which on the convex side of their arcs towards the Pacific Basin are parallel to elongated deeps resulting from subsidence of the ocean floor. A generalized cross-section of an island group would show the surface rising from the ocean floor in a huge fold, along the top of which volcanics, coral growth, and uplift had resulted in the formation of islands.
During quiescent periods in the mountain-forming process, wave-cut terraces develop, which when later uplifted give the islands a stairstep appearance. The terraces are most strongly developed along the eastern coasts, which are subjected to more intensive wave action owing to the direction of the prevailing winds. Subsequent faulting may have somewhat modified the original island structure, resulting in surface tilt and secondary levels. A second modification has occurred on those islands on which the volcanic base was capped with coral growth; later uplifted, the limestone surface eroded into karst features. The coasts are generally cliffed and are characterized by wave-cut benches, notches, and sea caves.