SEAFLOOR ROCK DESCRIPTION 



Seafloor rock may be basalt, often coated with manganese; manganese 

 precipitated as discrete nodules or coalesced into a continuous pavement: 

 phosphate deposits as nodules or slabs; coral, both living and relict; 

 and, on the continental shelves and slopes and on trench sides, the 

 whole spectrum of known rock types. 



Basalt 



Occurrence . The ocean ridges and rises and submarine volcanoes 

 are all composed of basalt. Many submarine volcanoes have built them- 

 selves up above the sea surface to form basaltic islands. Many others 

 have become extinct while near the sea surface and later, while sinking 

 relative to sea level, were capped by coral varying in thickness up to 

 thousands of feet. Some idea of scale may be obtained from the overall 

 dimensions for some seafloor features: the Mid-Atlantic Ridge has a 

 height of 6000 feet above the deep basins on either side^ with side 

 slopes ranging up to 30 degrees, even at great depths; 3 volcanoes of 

 the Pacific Basin reach 30,000 feet with side slopes up to 25 degrees. ^ 

 From the rate of discovery and various other considerations it appears 

 that there are approximately 10,000 volcanoes in the Pacific Basin with 

 a relief of more than 1 km;" the Atlantic contains considerably fewer. '^ 



Deep submarine basalt occurs as flows: the surface of these 

 flows may be (1) a layer of "glass" fragments named hyaloclastite and/ 

 or (2) a mass of ellipsoidal bodies called pillow lava, or (3) a bare 

 coherent rock surface possibly representing a sheet flow as thin as 5 

 centimeters (Ih inches) .-'--'- Hyaloclastites are sand-textured materials 

 ranging from one millimeter to a few centimeters across , apparently 

 formed through chilling and granulating of the front and surface of a 

 lava flow advancing beneath water. This material chemically weathers 

 to form a yellowish-brown earthy or waxy palagonite. Basaltic pillows 

 are usually less than two meters (6.5 feet) in diameter, and most 

 commonly less than one meter (three feet) . Pillows may be tightly 

 molded to each other, but commonly they are separated by detrital 

 material, either ordinary sedimentary material or hyaloclastite. Whole 

 pillows are commonly intermingled with fragments of pillows, typically 

 resembling a wedge-shaped slice of watermelon. Some seafloor photos 

 show large piles composed solely of such pillow fragments , -'■^ much like 

 terrestrial talus piles. Pillow lavas quite commonly grade either 

 laterally or vertically into non-pillowed flows. ' 



Lava erupted subaerially, or explosively erupted into shallow 

 waters, may often ultimately reach depths of interest to the seafloor 

 foundation engineer. Subaerial lava often flows into the sea; typical 

 explosive products fall into the sea about erupting vents; and sub- 

 aerially solidified lava often subsides relative to sea level reaching 

 considerable water depths, as at Mauna Kea where formations are 

 believed to have subsided 1000 meters relative to sealevel. ■'■^ 



