Survey Researchers Find Rare Volcanic Features

Kimberlites . . . provide clues about the deep subsurface: snapshots of an otherwise inaccessible part of the earth

CONTENTS

Kimberlites in Kansas–page 1

From the Director–page 2

Reservoir Maps Available–page 3

Teachers Honored–page 3

A Place To Visit–page 4

With rare exceptions, the rocks we see at the surface in Kansas are sedimentary in origin—that is, they’re made up of particles deposited by wind or water. Among the exceptions are the kimberlites of Riley and Marshall counties, igneous rocks formed from the cooling of molten magma that exploded to the surface during the Cretaceous Period, about 90 million years ago.

Kimberlite is a soft, dull-gray rock with thin white veins that originates deep in the earth, in the upper mantle—the layer that lies about 25 to 250 miles beneath the earth’s surface. As kimberlitic magma erupts, it expands, breaking up rock in its path and exploding through the crust (perhaps at speeds of 1,300 feet per second). These eruptions form deep, carrot-shaped pipes that often occur in small clusters. The explosion produces small craters at the surface and leaves behind a mixture of igneous and sedimentary rock that often contains garnets, a dark-red, semi-precious stone.

In Kansas ten kimberlite pipes, all located in Riley County, have long been known to geologists. During the summer and fall of 1999, researchers at the Kansas Geological Survey located and drilled into three new kimberlites, two in Riley and one in Marshall County. All of the newly discovered kimberlite pipes are covered with soil, 15 to 20 feet below the ground surface. All are on private property.

Kimberlites are scientifically important because they provide clues about the deep subsurface: snapshots of an otherwise inaccessible part of the earth. They also generate economic interest because they are the source of most of the world’s diamonds. Diamonds form in the upper mantle, generally at depths between 90 and 125 miles. Kimberlites serve as elevators that bring diamonds to the surface. However, of the 4,000 kimberlites known worldwide, 90% do not contain diamonds. So far, diamonds have not been found in Kansas kimberlites.

To find the kimberlites, Survey researchers Tom Weis and Kevin Dobbs conducted detailed surveys of the earth’s magnetic levels. Because kimberlites are composed of igneous rocks, which contain minerals that are different from the surrounding sedimentary rocks, they produce a distinctive magnetic signature.

Weis and Dobbs conducted their ground surveys at locations where previous aerial surveys had indicated possible kimberlites. These aerial surveys were conducted by Cominco American, Inc., a mining company headquartered in Spokane, Washington, that explored for diamonds in the early 1980’s. As a result of their detailed ground surveys, Weis and Dobbs identified three locations that were likely to have kimberlites below the surface. Starting in late September 1999, the Survey drilled at these locations and confirmed the presence of three new kimberlites.

At each location, the Survey drilled 300 feet into the kimberlites and obtained continuous core samples of this rock that originated deep within the earth. Researchers then performed tests to measure the amount of magnetite, a mineral inherent in magma, present in the cores from each site. Two of the cores had fairly consistent readings, suggesting a similarity in rock characteristics throughout the 300 feet of sample, according to Survey geologist Pieter Berendsen. But the Marshall County core showed a surprising variation in the amount of magnetite present. “These huge variations relate to differences in rock characteristics,” said Berendsen, “and could indicate that the kimberlite at this site erupted in a series of different pulses.”

Research on the kimberlites is part of a larger Survey study of the geology of the Manhattan quadrangle, a roughly 7,200-square-mile area in north-central Kansas. That study involves mapping the area’s bedrock geology, analyzing the deep subsurface, and looking at geologic hazards such as landslides.

Online February 10, 2003

Comments to: lbrosius@kgs.ku.edu

Kansas Geological Survey

URL:http://www.kgs.ku.edu/Publications/GeoRecord/2000/vol6.1/Page1.html