Geologists Find Unusual Origins For Plateau On Pacific Sea Floor

December 18, 1996

Cincinnati -- The origins of life. The origins of the universe. In the scientific quest to understand beginnings, University of Cincinnati graduate student Raelyn Welch would be happy just to understand the origin of a single geologic feature at the bottom of the western Pacific Ocean.

It's called the Ontong Java Plateau, and the rocks at the base of the plateau defy explanation by any current geologic theory. "They're intriguing animals. That's for sure," said Welch, who will report her findings during a special symposium Wednesday, Dec. 18, at the American Geophysical Union meeting in San Francisco.

With UC geology professor Attila Kilinc, Welch studied the plateau's basalts, rocks which formed after magma cooled, crystallized and rose to the surface to form new oceanic crust. The Ontong Java Plateau is one of several oceanic plateaus which have mystified scientists, because enormous amounts of new crust are formed in the middle of tectonic plates in a very short period of time. It's estimated that the Ontong Java Plateau formed from eruptions which only extended over three million years. In comparison, the Hawaiian Islands were formed by eruptions over tens of millions of years.

The basalt layers can be as much as 40 kilometers thick which is five times thicker than normal ocean crust. "These are massive areas," said Welch. "You could spread it out over the whole United States, it would still be five kilometers thick."

"The sheer volume cannot be explained by normal plate tectonic models. Geologists want to find out how these formed." The Ontong Java Plateau has an estimated volume 245 times larger than the mantle source below Hawaii.

To determine the origins of the basalt, Welch did extensive geochemical and physical analyses on parts of two cores pulled up by the Ocean Drilling Program in 1991. Ten samples came from one ocean core, representing different depths within that core. Eight samples came from another drill site about 100 kilometers away from the other core.

Welch found that each sample had distinct geochemical characteristics. The characteristics were slightly different and didn't match normal oceanic basalts formed at spreading ridges. "These clearly have different source regions. They quite possibly have different parental magmas."

In addition, Welch studied thin slices of the rock to look at the mineraology of the samples. She found a significant percentage of minerals known as plagioclase and pyroxenes. There was very little of a mineral known as olivine. That suggests that the rocks formed in a high-pressure environment, possibly up to pressures of eight kilobars.

"Most researchers are looking at these samples in terms of one atmosphere of pressure," said Welch. "The absence of olivine indicates this is a higher pressure system than suggested by a lot of the workers in the field."

To back up her laboratory work, Welch also ran a computer program which can simulate the conditions under which basalts form. When she plugged in characteristics of several high magnesium samples, the computer program also concluded the rocks formed under pressures higher than one atmosphere conditions. The estimated pressure is between 4-8 kilobars.

In recent years, geologists have recognized that plumes of magma can rise from lower in the Earth's mantle. That could explain the high pressures; however, Welch's samples don't match known plume basalts either. They lie in a no-man's land somewhere in the middle between low-pressure basalts and the high-pressure plumes.

Welch says the origins of the Ontong Java Plateau basalts won't fully be understood until scientists return there and drill a deeper core into the rock. And although the Ocean Drilling Program is considering such a trip, it's unknown when the next expedition might take place.

University of Cincinnati

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