SEA-BOTTOM DRILLING—II Changed Concept Of Forces That Shape Earth’s Crust

Press, Volume CIX, Issue 31935, 12 March 1969, Page 6

SEA-BOTTOM DRILLING—II Changed Concept Of Forces That Shape Earth’s Crust

(By ROBERT C. COWEN. natural science editor ol the "Christian Science Monitor”)

When oceanographers drill into the deep«sea floor, every sample they bring up is a discovery. Every inch the drill penetrates beyond the first few dozen feet of sediment represents geological record that, until this year, was closed to men.

Now, as scientists begin to read the record entries, they are finding new evidence that the sea bed undergoes farreaching motion.

Over the last decade, many geologists have abandoned the long-held view of the earth’s crust as relatively static. They no longer think the oceans’ basins very ancient or of continents as fixed in their positions. The data that are coming from the first attempt to penetrate the seabed support this view. The Deep Sea Drilling Project (D.S.D.P.), which got under way on August 11, plans to sample widely in the

Atlantic and Pacific Oceans. Scripps Institute of Oceanography manages the D.S.D.P. for a consortium of institutions including Columbia University, the University of Miami, the University of Washington, and Woods Hole Oceanographic Institution. Growth Theory Tested D.S.D.P. scientists will be looking for data on many phases of ocean history. But the second leg of the D.S.D.P. mission, which ran from New York to Dakar, between September 30 and November 25, aimed specifically at testing the modern theory of seafloor growth. Dr Melvin N. A. Peterson of

Scripps, co-chief scientist for that leg, says the evidence is about as clear-cut as geologists could hope to get. A vast system of ridges bisects the Atlantic, swings across the Indian Ocean, and on into the Pacific, thrusting under Western North America. Great rifts split the crest of the ridges. Volcanoes, earthquakes, and abnormally high flow of subterranean heat also mark their line.

Many geologists now believe that out of these ridges, the seabed is born. They envisage hot rock magma welling upward along their crests. From there it slowly spreads laterally, forming an everwider seabed by sideways growth.

Line Of Holes Tn this concept, Dr Peterson points out, you can think of the seabed as a set of giant conveyer belts. New parts of the bed move slowly outward from the ridges carrying with them whatever sediments or lava flows that are reposited on them. The farther outward the seabed moves, the more overburden it will carry as sediment accumulates through ages. Thus, the farther away from the ridges you look, the older type of sea bottom you should find. The line of drill holes at five sites which Dr Peterson and his colleagues ran from New York to Dakar across the Mid - Atlantic Ridge was designed to test this assumption. The samples recovered fit the theory neatly. As holes were drilled nearer to the ridge, the seabed became progressively younger. Expedition scientists dated the sea bed by taking the age of the sediments just on top of the basalt rock that forms the basement in that area. Presumably these are the

oldest sediments at the respective sites except, perhaps, for any older sediments that may be trapped underneath the lava flow that forms the basement rock. These oldest sediments have been far below the reach of the sampling instruments oceanographers have used in the past. The usual coring device penetrates only a few dozen feet of upper sediment. But the drill string of Global Marine’s new ship, Glomar Challenger, bites deeply into sediment and underlying. Global Marine is running the drilling operation for the D.S.D.P. It has already set a deep-sampling record in the Atlantic by penetrating 2740 feet of sea bed in 19,075 feet of water.

Fossils Date Sediment The sediment samples can be dated by studying fossils of tiny marine plants and animals. At a site 850 miles west of the mid-Atlantic ridge, Dr Peterson says the oldest sediments dated were formed 85 million years ago. However, the very oldest sediments in that sample were considerably older, for the last 200 feet reached by the drill couldn't be adequately dated from the fossils. About 500 miles from the ridge the oldest sediment was just 85 million years old. And within 100 miles of the crest of the ridge itself, the bottom-most sediment was only 18 million years old. The sea floor appears to have been growing outward from the ridge at an average rate of one centimetre a year over the past 18 million years, to judge from the drilling data. During earlier Cretaceous times, about 85 million years ago, it probably was growing several times as fast.