Karst, Sinkholes, and Human Activity | Earth Science Week
Sinkholes are most common in what geologists call, “karst terrain.” These are regions where the type of rock below the land surface can naturally be dissolved . Sinkholes are the most characteristic feature of karst topography. Carbon dioxide (CO2), when combined with the water in air and soil, forms carbonic acid, . Karst topography forms in areas where the underlying bedrock is composed of dioxide (which creates carbonic acid that reacts with the rock, dissolving it).
Karst formations are cavernous and therefore have high rates of permeability, resulting in reduced opportunity for contaminants to be filtered. Groundwater in karst areas is just as easily polluted as surface streams. Sinkholes have often been used as farmstead or community trash dumps. Overloaded or malfunctioning septic tanks in karst landscapes may dump raw sewage directly into underground channels. The karst topography also poses difficulties for human inhabitants.
Sinkholes can develop gradually as surface openings enlarge, but progressive erosion is frequently unseen until the roof of an underground cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery. In the United States, sudden collapse of such a cavern-sinkhole swallowed part of the collection of the National Corvette Museum in Bowling Green, Kentucky in Typically this will involve a cover of sandstone overlying limestone strata undergoing solution.
According to Will Gregg, during the event, citizens expected to find dead trout in the dry lake bed, since Rainy Lake is normally a productive trout lake. Much to their surprise, no dead fish were found, yet the following year, when the lake reappeared, the trout also came back.
Obviously, the fish had been swept into the subterranean passageways with the water, only to reappear with the water at a later point. In the same account, a story is told of another early attempt to float lumber down the Rainy River.
What is the difference between a sinkhole and land subsidence?
Immediately prior to the log run, the lake drained through the sinkholes. The lumberers consequently built a rail line on the dry lake bed. After the logs were shipped, the Rainy Lake sinkholes again became plugged, and the lake reappeared. The images below are from a personal collection, and have been reprinted here. The depict conditions on the dry lakebed of Rainy Lake during the event. Note, in the images below, how the waters have cut a gorge into the soft mud of the lakebed, as they rushed out of the lake.
More images, this time with an annotation: The image above is of the dried clays on the bed of Rainy Lake. The caption handwritten on the back of this picture said: Robinson my other grandfather. Note the cracks in the clay bottom.
My father has told me that some of those cracks you could put your arm full length in, and that some places you could hear a roar like an under ground current.
In the late 's and 's, however, citizens began buying lots on Rainy Lake in several subdivisions. It was an ideal location for a summer cottage until latewhen the sinkholes in the bed of Rainy Lake again became unplugged and the lake began draining, after a period of time when the lake was higher than normal.
Inthe level of Rainy Lake had dropped over 45 feet, and the shoreline had receded over meters in places. During the incident, water in Rainy Lake was draining at an estimated rate of 10 gallons per second.
Karst & Sinkholes
A significant number of cottages and summer homes were built around Rainy Lake between the times of the and incidents. The residents were understandably concerned, and after a year dormancy, interest in Rainy Lake was again revived. In an attempt to help bring the lake level up, the Michigan DNR opened a stop-log wildlife flooding dam in and allowed water to flow approximately six miles down the Rainy River into Rainy Lake.
As expected, the resulting increase in lake level was of small magnitude, and only temporary. Rainy Lake has now stabilized, and it is back at its normal level.
Karst and Sinkholes
Late inthe major sinkhole through which the lake drained once again became naturally plugged, allowing the lake to fill. Post-glacial sinkholes Postglacial sinkholes are likewise a unique feature of the karst landscape. These sinks are typically nearly perfectly round, about to feet in diameter, with a depth up to feet.
- What is the difference between a sinkhole and land subsidence?
- Karst and sinkholes
Postglacial sinks look very similar to kettle holes with which they are often confused by the untrained eye sinks are about 5 degrees steeper and are VERY round. Basic structural and tectonic processes in the Michigan Basin were reviewed, and almost lineaments in northern Alpena and southern Presque Isle Counties which are discernible on aerial photographs, were mapped. Two predominant average azimuth orientations of photolinears were calculated.
Karst, Sinkholes, and Human Activity
Many of the linear features discernible in northeastern lower Michigan developed at least in part by ground water solution processes. The primary development of fractures probably took place concurrently with the uplift of the Michigan Basin at the close of the Paleozoic Era.
Many of the northwest-southeast and northeast-southwest trending sets of lineaments developed along fracture intersections. Although evidence exists for an upper Devonian karst system, much of the post-Paleozoic karst appears to be oriented along the northwest-southeast linear trend, which is roughly parallel to the regional strike of the bedrock in the area. Much of the subsurface ground water movement in this area probably follows linear zones of weakness which developed along fracture intersections.
Much has yet to be learned about karst phenomena in northeastern lower Michigan, and what controls the seemingly periodic cycle of lake lowering due to karst, such as has happened at Rainy Lake. Modern technology will not prevent karst lakes from draining in the future, but much insight is to be gained so we can better predict cyclical trends in karst phenomena and be better prepared to deal with them as they develop.
The work of the Michigan Karst Conservancy is carried out by volunteers, who believe in the value to the public of protecting examples of karst features in Michigan for education and scientific uses. Donations to the MKC are tax deductible in accord with federal law. Form a column of sugar by filling the tube with sugar. Then surround it with sand by pouring sand in the rest of the cup. Slowly pull the paper tube up out of the cup and pour a thin layer of sand over the sugar.
Cut the bottom off your soda bottle so it is about the same height as the foam cup. Fill the dish about one-third full of water. Place the foam cup in the dish of water. The process may be sped up by lifting the cup out of the dish, allowing sugary water to drain out the bottom of the cup.
Why does a sinkhole form only over the sugar deposit? What type of rock might the sugar represent? What does the water in the dish stand for? Now look at the map of Florida, where most freshwater is sourced from karst aquifers. Notice the rectangle highlighting a concentration of blue, which represents population density, along the western coast of the state.