Guidebook—Geology of South-central Kansas
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Micro-Lite processing plant.
Stop 2—Micro-Lite Quarry/Silver City Dome
Most of the rocks at the surface of Kansas are sedimentary in nature--that is, they are made up of sediments usually deposited at the bottom of an ocean or by a stream. In general, you have to drill hundreds or thousands of feet below the land's surface in Kansas to find igneous rocks, those once-molten rocks, such as granite. In a few places, however, you can see igneous rocks at the surface, and two of those locations are in Woodson County.
The igneous rocks here are called lamproites. About 90 million years ago, these igneous rocks rose from great depth and exploded to the surface, producing volcano-like features. Lamproites are pipes of igneous rock, which was highly charged with gas and pushed its way up through faults, fractures, and zones of crustal weakness. They are similar to kimberlites, another type of igneous rock in Kansas, found at 13 locations in Riley and Marshall counties, west of Tuttle Creek Lake. Lamproites have a different chemical composition than kimberlites, but both have produced diamonds (although not in Kansas). In fact, the world's largest diamond mine is in a lamproite at Argyle in northwestern Australia.
Generalized diagram of a lamproite pipe.
Because the landscape has endured millions of years of erosion since the lamproites here erupted, relatively little evidence of these features is visible at the land's surface. At Rose Dome, about five miles south of Yates Center, the intrusion of the lamproite created a broad dome that is apparent at the land's surface and on topographic maps. In addition to the pipe of molten rock that blew to the surface, other lamproite intruded itself into the underground rock (because lamproite and kimberlite intrude themselves into rock that is already in place, they are sometimes referred to as "intrusives"). Horizontal sheets of lamproite branch off the main pipe. These layers, or sills, extend away from the main pipe and have been encountered at depths of about 1,300 feet during core drilling, and drilling for oil and gas, in the area around the dome.
The lamproite itself is not visible at the surface of Rose Dome, but a number of granite boulders litter a pasture west of U.S. Highway 75. These granite boulders, which are generally surrounded by trees that have grown up around them, probably came along for the ride with the lamproite when it exploded to the surface. The granite was probably originally part of the crystalline basement rock that is about 2,500 feet deep here and lies beneath the overlying sedimentary rocks. These granites are about 1.2 billion years old, formed during ancient Precambrian times. Geologists use the term "xenolith" for rock fragments, such as these chunks of granite, that are "foreign rocks," or not part of the intrusive but mixed in with the lamproite, like chocolate chips in cookie dough. These granites seem so out-of-place here that early geologists thought they might have been carried in by glaciers, though we now know that glaciers did not extend this far south in Kansas.
The lamproite we will visit today is at the Silver City dome, southwest of Rose Dome. The Silver City lamproite is very similar to the Rose Dome. The actual topographic dome at Silver City is more subtle and a little more difficult to visualize than at Rose Dome; however, the lamproite itself is exposed at Silver City. The lamproite contains shiny flakes of mica; that led to early reports that the rock contained silver and a short burst of mining in the 1870's that produced a settlement called Silver City. While there is no silver here, the lamproite has been mined intermittently since then, and steadily since 1982 when Micro-Lite began quarrying one of the lamproite sills. The lamproite is quarried and hauled to the nearby town of Buffalo, where it is bagged and eventually used as a mineral supplement for cattle feed. The rock contains small amounts of the essential minerals magnesium, potassium, and iron. In 1996, Micro-Lite mined about 70,000 tons of lamproite.
While much igneous rock is very hard, the lamproite exposed at the surface here is soft and powdery, weathering to an olive brown. The rock itself is called peridotite (pronounced pah-RID-oh-tight); it is coarse grained and high in the minerals olivine and pyroxene.
Sample of lamproite from the Micro-Lite Quarry.
Surrounding the lamproite are sedimentary rocks that were deposited in Pennsylvanian times, about 300 million years ago, before the lamproite came to the surface. When the lamproite exploded to the surface, it was extremely hot, as much as 800 degrees Celsius, cooking the limestones and shales that it contacted. This process is called "contact metamorphism"--the high heat of the molten rock cooks and changes the existing sedimentary rocks (these previously in-place sedimentary rocks are sometimes referred to as "country rock" a term that, in this context, has nothing to do with music). Thus, some of these sedimentary rocks (such as the Weston Shale Member and the Tonganoxie Sandstone Member of the Stranger Formation) are now extremely hard and have a far different character than the much softer rocks that we have seen in the other stops on this field trip.
This guidebook is also available in print form as Kansas Geological Survey, Open-file Report 2001-41, from KGS Publications Sales office, 785-864-3965.
Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey. Text by Jim McCauley, Liz Brosius, Rex Buchanan, and Bob Sawin, Kansas Geological Survey.
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