Larson - A t one o&clock in the morning on...

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Unformatted text preview: A t one o&clock in the morning on December 13, 1989, I was awak- ened in my bunk on board the scientic drillship JOIDES Resolution by the sounds of celebration in the adjoin- ing cabin. Since I had to relieve the watch at four anyway, I stumbled next door to join the party. The paleontolo- gists in our expedition had just report- ed to my cochief scientist, Yves Lance- lot, now at the University of Aix-Mar- seilles, that microfossils of the Jurassic period had been recovered from the hole in the oor of the western Pacic Ocean that we were drilling more than three miles below us. Two days later the drill reached the volcanic basement oceanic crust of Middle Jurassic age, about 165 million years old. A 20-year mystery was solved. At last, we had hard evidence of the world&s oldest deep-sea sediments and volcanic rocks that are still in place from eons ago. In succeeding days I reected on why the quest had taken so long. My col- leagues Clement G. Chase of the Univer- sity of Arizona, Walter C. Pitman III of Lamont-Doherty Geological (Earth) Ob- servatory, Thomas W. C. Hilde of Texas AM University and I had rst consid- ered the problem in the 1970s. The tar- get was not a small one. We had predict- ed from geophysical data that an area in the western Pacic the size of the con- tinental U.S. should be Jurassic in age, somewhere between 145 and 200 mil- lion years old. But whenever we dredged or drilled in this area, we almost invari- ably recovered rocks called basalts, formed by volcanic eruptions during the mid-Cretaceous, generally ranging in age from 80 to 120 million years and no old- er. The rst such basalt samples were dredged from the Mid-Pacic Mountains in 1950 by an early expedition of the Scripps Institution of Oceanography. Until the JOIDES discovery, however, geologists had not made much prog- ress in answering the questions con- cerning the origin of the seemingly ever present mid-Cretaceous basalts or the possible existence of underlying Juras- sic material. The 1989 discovery provided some qualitative answers. The older sediments and oceanic crust were buried during the mid-Cretaceous epoch by what we now refer to as a superplume of vol- canic material. Finally, our geophysical musings of the early 1970s could be supported with facts: the Jurassic ex- isted in the western Pacic. We had samples of it locked away on board the JOIDES Resolution. Because I am a geophysicist, I try to describe the earth and its processes quantitatively. I wanted to determine the size of the mid-Cretaceous super- plume of the western Pacic, hoping to learn something of its origins. But say- ing that and doing it are two dierent things. What do you measure, and how do you measure it? I did not even know what normal was, so how could I de- scribe the anomalous mid-Cretaceous superplume episode? The problem had to be expanded beyond the time and space framework of the mid-Cretaceous western Pacic. I decided to examinewestern Pacic....
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This note was uploaded on 02/08/2011 for the course EAS 1601 taught by Professor Lynch during the Spring '08 term at Georgia Institute of Technology.

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Larson - A t one o&clock in the morning on...

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