Ch. 2— Resource Assessments and Expectations * 41 



past. Diamonds have been recovered from near- 

 shore areas in Namibia, Republic of South Africa, 

 and Brazil. 



The recovery of sand and gravel from offshore 

 far exceeds the extent of mining of other marine 

 minerals.* In the United States, offshore sand and 



'J.M. Broadus, "Seabed Materials," Science, vol. 235, Feb. 20, 

 1987, pp. 853-860. Also see M. Baram, D. Rice, and W. Lee, Ma- 

 rine Mining of the Continental Shelf (Cambridge, MA: Ballinger, 

 1978), p. 301. 



gravel recovery is primarily limited to State waters, 

 mostly in New Jersey, New York, Florida, Mis- 

 sissippi, and California. Japan and the European 

 countries have depended more on marine sand and 

 gravel than has the United States because of limited 

 land resources. Special uses can be made of ma- 

 rine sand and gravel deposits in the Alaskan and 

 Canadian Beaufort Sea — the offshore oil and gas 

 industry uses such material for gravel islands and 

 gravel pads for drilling. 



GENERAL GEOLOGIC FRAMEWORK 



The potential for the formation of economic 

 mineral deposits within the Exclusive Economic 

 Zone (EEZ) of the United States is determined by 

 the geologic history, geomorphology, and environ- 

 ment of its continental margins and insular areas. 

 Continental margins are a relatively small portion 

 of the Earth's total surface area, yet they are of great 

 geological importance and of tremendous linear ex- 

 tent. In broad relief, the Earth's surface consists 

 of two great topographic surfaces: one essentially 

 at sea level — the continental masses of the world, 

 including the submerged shelf areas — and the other 

 at nearly 16,000 feet below sea level — representing 

 the deep ocean basins. The boundaries between 

 these two surfaces are the continental margins. 

 Continental margins can be divided into separate 

 provinces: the continental shelf, continental slope, 

 and continental rise (see figure 2-1). 



Continental margins represent active zones 

 where geologic conditions change. These changes 

 are driven by tectonic activity within the Earth's 

 crust and by chemical and physical activity on the 

 surface of the Earth. Tectonic processes such as vol- 

 canism and faulting dynamically alter the seafloor, 

 geochemical processes occur as seawater interacts 

 with the rocks and sediments on the seafloor, and 

 sedimentary processes control the material depos- 

 ited on or eroded from the seafloor. All of these 

 processes contribute to the formation of offshore 

 mineral deposits. 



Advanced marine research technologies devel- 

 oped since World War II and the refinement and 

 acceptance of the plate-tectonics theory have cre- 

 ated a greater understanding of the dynamics of 

 continental margins and mineral formation. Ac- 



cording to the plate-tectonics model, the Earth's 

 outer shell is made up of gigantic plates of continen- 

 tal lithosphere (crust and upper mantle) and/or 

 oceanic lithosphere. These plates are in slow but 

 constant motion relative to each other. Plates col- 

 lide, override, slide past each other along transform 

 faults, or puU apart along rift zones where new ma- 

 terial from the Earth's mantle upwells and is added 

 to the crust above. 



Seafloor spreading centers are divergent plate 

 boundaries where new oceanic crust is forming. As 

 plates move apart, the leading edge moves against 

 another plate forming either a convergent plate 

 boundary or slipping along it in a transform plate 

 boundary. Depending on whether the leading edge 

 is oceanic or continental lithosphere, this process 

 may result in the building of a mountain range 

 (e.g., the Cascade Mountains) or an oceanic island 

 arc (e.g., the Aleutian Islands) or, if the plates are 

 slipping past one another, a transform fault zone 

 (e.g., the San Andreas fault zone). 



Four types of continental margins border the 

 United States: active collision, trailing edge, ex- 

 tensional transform, and continental sea. Where 

 collisions occur between oceanic plates and plates 

 containing continental land masses, the thinner 

 oceanic plate will be overridden by the thicker, less 

 dense continental plate. The zone along which one 

 plate overrides another is called a subduction zone 

 and frequently is manifested by an oceanic trench. 



Coastal volcanic mountain ranges, volcanic is- 

 land arcs, and frequent earthquake activity are re- 

 lated to subduction zones. This type of active con- 

 tinental margin borders most of the Pacific Ocean 

 and the U.S. EEZ adjacent to the Aleutian chain 



