The Mystery of Terra Rossa

the mystery of MICHAEL MAY IS A WKU GEOLOGIST WHO IS soil its red color. Exactly how “bulldozed” the red soil as far south as LOOKING FOR THE DIRT this unusual soil forms has been southern Indiana and other terminal ON HIS RESEARCH TOPIC a mystery.” areas. Michael May — TERRA ROSSA SOILS. Geologists have proposed that In the past several years, May has by better understanding the origin been collaborating with Dr. Enrique “Terra rossa, the red, iron- and of this red material there may be Merino and other researchers at clay-rich soil common in karst areas, a more solid basis for finding and Indiana University on another theory. is found above limestone bedrock,” extracting enriched ores of iron, “We’re throwing out the idea that it’s May said. “What parents in the aluminum, and similar economi- just a simple insoluble residue,” May Bowling Green area haven’t encoun- cally important elements, he said. said. “It doesn’t make sense geochem- tered their children’s sports, school, For years, researchers have tried to ically to be able to take a rock that is or play attire stained with terra rossa? determine the origin of terra rossa about ninety-nine percent calcium, It literally touches all of our lives. soil. One theory was that the red carbon, and oxygen, and magically Get some on your soccer clothes, soil resulted from the dissolution of get all that iron by simple dissolution.” or gardening clothes, and it will not limestone and is simply an insoluble “Then the second discounted idea wash out easily. That’s the iron in the residue or leftover from that process. was that glacial ice could just push in terra rossa, the element that gives our Another theory was that glaciers had such soil. We found the soil in central, 6 The Western Scholar | Spring 2006 B Y T O M M Y N E W T O N south central, and western Kentucky, the control volume per unit time. no volumetric change in the limestone and the glaciers never touched those Assuming a steady state flow of mass, — only a replacement of the rock’s parts of Kentucky. So the question mass in equals mass out, and there is original minerals by minerals. still remains: What kind of evidence “Using that as a kind of thesis, should we be looking for to help us what we came up with was that we understand where all this red dirt could collect evidence in the field that originated?” would be suggestive of a mineralog- Their idea is fairly simple and ical or chemical replacement process based on the “conservation of mass within the limestone,” May said. “The principle.” The principle is applied problem always has been that people to fixed volumes, known as control making the observations weren’t volumes (or surfaces), as in a known trained in microscopic analysis of volume of limestone. For a given rocks. They didn’t look for it so they control volume, the principle states didn’t see it.” that mass entering per unit time In recent years, May and others Conservation of mass principle equals the mass leaving per unit Limestone being replaced by iron-rich have been collecting samples. time plus the increase of mass in clays forming “terra rossa” Western Kentucky University 7 The samples, such as those from Samples taken farther away from the rock removed at the WKU site or the limestone-terra rossa interface from a grocery store construction site and closer to Earth’s surface show in Bloomington, Indiana, revealed more impact from weathering and that the calcite (limestone’s main other natural phenomena such as mineral) is being replaced by iron- earthworms and microbes. “The ques- rich minerals. tion that is really out there is where “We’re looking for bona fide did all this iron come from,” May said. textures that were characteristic of “Perhaps the best idea, but it can’t a certain limestone. For instance if be proven at this point, is that it’s Microscopic evidence showing original it had seashells in it, we might find literally dust in the wind. We have a limestone being ‘eaten away’ or replaced by ghost outlines of the seashells in the lot of iron-rich, airborne particulates iron-rich minerals leaving ghost images of terra rossa right at the limestone-terra that get moved all across the earth,” ancient marine fossils. rossa interface,” May said. he said. The best samples come from construc- Photographs from the microscopic WKU students have traveled to tion sites, such as the huge limestone analysis of the limestone show the Bahamas in recent years and have rock resting near the Environmental burrows of organisms that once documented some red layers in other- Science and Technology Building. That lived on the sea floor. May said that wise white limestone there. “Where rock was removed from the site of texture also shows up in the nearby did all this red sediment come from?” WKU’s new Complex for Engineering and Biological Sciences. “That rock “The question that is really out there is shows the interface where chemical and physical processes are changing where did all this iron come from?” our white limestone into something red,” May said. terra rossa soil. “If you have the same May said. The red, yellow, and brown May began working on the replace- shapes of fossil grains in the terra dust has blown from Africa across the ment theory during his Ph.D. work at rossa that you have in the immedi- Atlantic Ocean and is affecting areas Indiana University. “It’s an ongoing ately adjacent limestone, you should in the Caribbean causing demise of interest of mine to look at sedimentary be able to see that it is not simply some coral reefs. rocks such as the limestones around an insoluble residue. It’s actually an And if you travel up the Natcher Bowling Green with an eye toward active phenomenon where the lime- Parkway, you’ll see reddish or brown the changes they undergo over time,” stone goes from calcite to an iron-rich sedimentary rocks such as shale and May said. “And when I say time I mean series of minerals that are totally sandstone. “Guess what they have?” geologic time — several hundred thou- different, but it retains its original May said. “Lots of iron in them. sand to a few million years.” depositional texture,” he said. So as those were stripped off and eroded over hundreds of thousands or millions of years, they contributed to the raw materials of iron or iron oxide that could have easily been washed or blown into the karst plain area. And simply put, nothing is lost or gained. It is just recycled. Again just because you can’t prove how the iron got there, doesn’t mean it didn’t get there. But I think it’s a pretty good explanation.” The replacement phenomenon is ongoing. “Nature has been tricking people about terra rossa for a number of years just by what we call simple weathering or soil forming processes. Prevailing winds transporting Potential iron sources from younger Once this is converted mineralogically iron-rich clays from southwest Mississippian and Pennsylvanian bedrock (sandstone and shale). 8 The Western Scholar | Spring 2006 “ We have a lot of iron-rich, airborne particulates that get moved all across the earth” or chemically, it also gets churned “We’re not looking way out there at some more by natural soil forming big mountains or other large features. processes — it freezes and thaws,” It’s the opposite. It’s a matter of he said. focusing on the microscopic and “The phenomena of weathering putting it in the context of the selec- and replacement are little understood tion of samples in the field. People by a lot of people who do landscape simply haven’t had the opportunities analysis including geomorphologists to look at it in this way.” Generalized geologic cross section because they don’t get microscopic For WKU students, the research showing karst plain with terra rossa in views,” May said. has given them opportunities to Mississippian rocks on right. Dripping “All the rocks in the area appear visit places like the Bahamas, to springs escarpment and younger Mississippian and Pennsylvanian rocks to be eventually turning into this red conduct hands-on applied research, on left potentially supplying karst plain soil.” The replacement theory also and to present their findings at local, with iron-rich material. has economic value because compa- regional, and national conferences. nies that deal in enriched ores, such “At this point, my work is to engage as bauxite for aluminum and laterite the students,” May said. “Through for iron, want to know the concentra- study abroad programs, we want tions of elements in the ore. They to continue to provide students the also want to know the processes opportunity to examine these rocks responsible for variations in concen- and ancient soils. We want to let trations. Some globally important students look at these things in the metallic ores owe their genesis to field and also under the microscope mineralogical replacement processes, when we get back.” May said. May hopes the students will May’s work is a rare combination continue to share in his enthusiasm Generalized geologic cross section that bridges soil science, petrography, for the terra rossa research. “We showing the regional bedrock dip sedimentary geology, and geochem- know so little about the soil we walk resulting in exposure of rock layers of different ages. Missippian rocks are host istry. “This research is interesting on.

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