3 results found.
... Science Frontiers ONLINE No. 21: May-Jun 1982 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects What's ok in the mediterranean is verboten in the atlantic and the pacific Geologists and geophysicists have now satisfied themselves that a few million years ago the Mediterranean dried up nearly completely. The Deep Sea Drilling Project discovered in 1970 and 1975 that layers of evaporites existed beneath the Mediterranean's floor. In addition, over 80 years ago, the bed of the Rhone River was found to consist of river sands and gravels superimposed upon hundreds of feet of oceanic sediments. Beneath these deposits -- some 3000 feet down -- was a gorge cut in granitic rock. Other rivers emptying into the Mediterranean had cut similar gorges into solid rock long ago. No one could provide an acceptable explanation for the deep-cut gorges until the evaporites proved that the water level had been low enough for the rivers to cut the gorges subaerially. In other words, the Mediterranean's level fell several thousand feet, allowing the rivers to erode gorges much as the Colorado does today in the Grand Canyon. (Smith, E.G . Walton; "When the Mediterranean Went Dry," Sea Frontiers, 28:66, 1982.) Comment. The Med's buried gorges are obviously close cousins of the many submarine canyons found around the world's continental shelves. Most geologists strongly resist any explanation of the submarine canyons involving subaerial erosion, because no one believes the oceans ever dropped thousands of feet. True, the ...
Terms matched: 1 - Score: 55 - 15 May 2017 - URL: /sf021/sf021p11.htm
... Science Frontiers ONLINE No. 46: Jul-Aug 1986 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Salt structures on venus?The following quotation is the abstract of a paper appearing in the Bulletin of the American Association of Petroleum Geologists. "The discovery of a surprisingly high deuterium/hydrogen ratio on Venus immediately led to the speculation that Venus may have once had a volume of surface water comparable to that of the terrestrial oceans. We propose that the evaporation of this putative ocean may have yielded residual salt deposits that formed various terrain features depicted in Venera 15 and 16 radar images. "By analogy with models for the total evaporation of terrestrial oceans, evaporite deposits on Venus should be ar least ten to hundreds of meters thick. From photogeologic evidence and insitu chemical analyses, it appears that the salt plains were later buried by lava flows. On Earth, salt diapirism leads to the formation of salt domes, anticlines, and elongated salt intrusions -- features which have dimensions of roughly 1 to 100 km. Due to the rapid erosion of salt by water, surface evaporite landforms are only common in dry regions such as the Zagros Mountains of Iran, where salt plugs and glaciers exist. Venus is far drier than Iran; extruded salt should be preserved, although the high surface temperature (470 C) would probably stimulate rapid salt flow. Venus possesses a variety of circular landforms, ten to hundreds of kilometers wide, which could be either megasalt domes or salt intrusions colonizing impact craters. Additionally, arcuate bands seen in the ...
Terms matched: 1 - Score: 24 - 15 May 2017 - URL: /sf046/sf046p04.htm
... Science Frontiers ONLINE No. 87: May-Jun 1993 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Whence the earth's pulse?Geological history records a restless planet subject to a succession of chemical and physical upheavals. Have these great paroxysms been random in time? M.R . Rampino and K. Caldeira do not think so: Number of geological events during geologic time "Published data sets of major geologic events of the past 250 Myr (extinction events, sea-level lows, continental flood-basalt eruptions, mountain-building events, abrupt changes in sea-floor spreading, ocean-anoxic and blackshale events and the largest evaporite deposits) have been synthesized (with estimated errors). These events show evidence for a statistically significant periodic component with an underlying periodicity, formally equal to 26.6 Myr, and a recent maximum, close to the present time. The cycle may not be strictly periodic, but a periodicity of 30 Myr is robust to probable errors in dating of the geologic events." The obvious question is: What could cause a 30-million-year periodicity? Internally, the earth's innards might be periodic, possibly in terms of plume eruption, mineral phase changes, core convection, etc. Externally, comets and asteroids are cyclic. Rampino and Caldeira point out that the solar system crosses the heavily populated plane of the Galaxy every 30 million years. (Rampino, Michael R., and Caldeira, Ken; "Major Episodes of Geologic Change: Correlations, Time ...
Terms matched: 1 - Score: 15 - 15 May 2017 - URL: /sf087/sf087g11.htm
