Part 6 WEATHERING OF CARBONATE ROCK
Limestone commonly weathers to a bluish-grey color, as shown here, center-left of photo. McCloud River arm of Lake Shasta, CA
Carbonic acid from rainwater and water percolating through decaying forest liter weathers feldspar and calcite
Carbonate rocks are more susceptible to solutioning and enlargement of discontinuities, especially along orthogonal joints and bedding planes. • In the Midwest, rainwater can dissolve 1 mm of limestone in 25 years where there is an average precipitation of 34 in/yr (Winkler, 1965).
Terra rossa residual soils forming over limestone, typical of sub-humid conditions. Note how the reddish-brown soil infills the joints and separations between core stones. Tree roots often follow these.
Distinctly weathered surface of carbonate rock known as "rillenkarren,“ small solution pockets a few centimeters apart separated by sharp intergroove crests. These are formed by solutioning of puddled rain drops, which are slightly acidic.
GEOFORENSICS • Geoforensics involves principles of geology to solve various mysteries involving earth and ocean systems. These include applications to engineering failures as well as crimes involving our criminal justice system. • The background in this photo of Osama bin Laden appears to be a type of karst feature called rillenkarren. Not rare, but not that common either. • Osama bin Laden giving interview aired on al Jazeera television in November 2001; likely somewhere in the Tora Bora Mountains of Afghanistan.
Karst refers the topography typically developed on limestone, gypsum and other soluble rocks, primarily by dissolution. Karst regions are typified by sinkholes, cave, dry channels and extensive underground drainage networks, most of which are joint-controlled.
About 22% of the continental United States is underlain by karst. • Sinkholes are typically circular at the ground surface when their roofs collapse and funnel shaped below ground • They can develop over a wide range of diameters, as shown here
1981 Auburndale, FL depression resulted from oxidation of peats deposited in an underlying sinkhole. The depression was about 2 ft deep.
Engineering geologists surveying ground fissures caused by settlement.Sinkhole features generally express themselves in circular form.
A 1981 drought in Florida caused the collapse of this 320 ft wide by 100 ft deep sinkhole in Winter Park, Florida
Ground view of the May 1981 sinkhole in Winter Park, Florida. Note the funnel shape of the deep, central abyss.
Approximately 22% of the Continental United States is underlain by karst, including most of the Ozarks. This is a large sinkhole feature known as Slaughter Sink, near Interstate 44 on the Phelps/Pulaski County boundary.
Most limestone caverns begin through solution of calcite along steeply dipping rock joints and faults
Solution chambers can lie at considerable depth below the groundwater level for millions of years, filled with acid-charged water.
Shifts in weather patterns or natural erosion can serve to lower groundwater levels, removing buoyancy and support, causing partial collapse.
Continued solution can open up pathways to the atmosphere, allowing calcite-bearing solutions to enter the cavern, lose some of their dissolved CO2 and deposit calcite as speleothems. Residual clay begins to fill the interstices of the cavern floor.
During the wetter climes of the Pleistocene calcite-rich groundwater deposited speleothems and washed out some portion of the residual clay.
Drier Holocene climates and adjacent down cutting combined to drain many caverns, depositing more speleothems. Most caverns are filled with undetermined amounts of fall-in and clay.