Most of the silica dissolved in sea water comes from silica-rich interstitial waters of marine sediments and from rivers carrying the products of subaerial weathering. Silica-secreting microplankton, diatoms and radiolarians, extract enough opal each year to strip the oceans of dissolved silica in about 250 years. Because most of the microscopic tests dissolve rapidly in the water column and at the sea floor, however, only about 4% survive long enough to he buried, and only about 2% avoid post-depositional dissolution and enter the geologic record. Opal-rich sediments are deposited beneath biologically productive surface regions where nutrient-rich deep waters upwell to the photic zone, rather than in areas where the silica enters the sea. The opal-rich deposits ultimately undergo diagenetic transformation to chert. The occurrence of volcanic material with siliceous deposits in the geologic recurd does not reflect a direct cause-offect relation, but rather the association of volcanism with the same tectonic processes that modify oceanographic and depositional environments so as to induce high productivity of silica-secreting organisms and preservation of the tests in protected basins. At present, 85-90% of the opaline silica incorporated in marine sediments is deposited in near-shore environments where most of it is masked by terrigenous debris. Deposition of silica by inorganic precipitation appears virtually impossible since the late Mesozoic and unlikely since the Precambrian.