Regional elevation depends on age of the particular parcel of 

 crust. The original igneous crust is mantled by sediments into zones 

 reflecting age of the basin, proximity to continents and sediment 

 sources, and areas of biologic productivity. The three primary 

 sedimentary processes for smoothing of the original igneous rock are: 

 (1) continuous pelagic "snowfall"; (2) gravitational slumps and slides, 

 including turbidity flows from both terrigenous sources and, on a 

 smaller scale, the elevated undersea peaks; and (3) redistribution of 

 sediment by bottom currents. 



Figure 2-7 shows the geographic distribution of deep sea 

 sediments by source as explained below. Sediments may be classified 

 in increasing size order as clay, silt, sand, and gravel. Sediments 

 are also classified by source. 



(1) Terrigenous — Organic or inorganic land-derived materials; 

 material derived from inorganic rock is lithogenic. 



(2) Biogenic — Organic or inorganic material from marine 

 organisms. 



ash) 



(3) Volcanic — Submarine or airborne volcanic material (lava, 



(4) Hydrogenic — Materials crystallized from seawater, such 

 as manganese nodules. 



(5) Cosmic — Materials of extraterrestrial origin such as 

 cosmic dust and tektites. 



Sediments are often classified by the biotic form that produces 

 their major biogenic component. These sediments are called oozes. 

 Microscopic free-floating plants and animals of the sea leave hard 

 remains or tests of either a calcareous or siliceous nature. Calcareous 

 deposits, though widespread, are restricted to shallower regions of 

 the pelagic zone than siliceous deposits, since calcareous remains 

 are more soluble than siliceous remains. Brown or red clays predominate 

 in deeper parts of the ocean basins under surface waters of low 

 biological production. These clays are derived from the very smallest 

 particles of wind-carried atmospheric dust from land. 



Coring of the sea bottom and collection of material by use of 

 dredges and grab samplers are techniques for obtaining samples from the 

 upper layers and surface of the sediment. These methods are slow, 

 laborious, and expensive. Deep sea photography is also used for studying 

 the characteristics of the sea bottom. However, seismic profiling in 

 the deep ocean, introduced for routine shipboard use in 1956, has 

 become one of the most widely used tools of marine geologists and 

 geophysicists. Seismic profiling is a method of studying sediment 

 thickness and layering, using the penetrability of low-frequency acoustic 

 energy into the bottom (fig. 2-8). If the acoustic energy density is 



