Geology of Blanchard Caverns

by David Potts on July 3, 2011

blanchard cavernsThe Rock:  Blanchard Springs Caverns is located in sedimentary rock that was laid down during the Upper Ordovician – Lower Mississippian periods (310-460 million years ago).  At that time northern Arkansas was covered by a shallow sea that teemed with life.  During most of the period limey sediments built up from the shells of the sea animals that died.  As the layers of shells got deeper and deeper, they were compressed to form limestone rock.

Periodically, there was also deposition of layers of mud and sand.  These layers became shale and sandstone.

The Uplift:  Lower Pennsylvanian (about 310-350 million years ago). Enormous forces in the earth’s crust began a gradual uplifting of the large, oval-shaped landmass that became known as the Ozark dome.  During the uplifting, which was so slow that if we were living here we would not have known it was going on, layers of rock often cracked or fractured under the stress.  Imagine a multi-layered cake.  Now think what would happen if you started pushing the cake up in the center.

Erosion:  As the land raised, the surface streams flowed over the land in the weakest places producing deep valleys, sheer bluffs, and the rugged topography as we know it in the Ozarks.

Creation of the Cave:  (50-70 million years ago).  As the surface eroded, so did cavities underground, though by a different process.  Some of the rainwater percolated down through the cracks in the rock layers.  As the water passed through the soil, it picked up CO2 (produced during the decomposition of organic material) to add to the CO2 picked up in the air.  This formed a mild acid called carbonic acid, giving the water greater dissolving power. (A carbonated soft drink contains carbonic acid.)  As the acid water came in contact with the limestone, the limestone (CaCO3) chemically reacted with the carbonic acid.  In this way solutioning occurred along the fractures.  Very slowly, over long periods of time, crevices became channels; channels enlarged into tunnels; tunnels joined together to form rooms, water filling the whole carved out area.

Blanchard Springs Caverns is a cavity formed primarily in layers of limestone with thin layers of shale and sandstone which collapsed.  The cavern cavity zone lies between Boone Formations, which are mostly bedded chert, an insoluble siliceous rock, and St. Peter Sandstone, which is also insoluble.

Decoration of the Cave:  (The oldest formations are approximately 2-5 million years old.)  As the surface streams carved deeper into the hillsides, the water table gradually dropped. Springs, where water emerges from the hillsides, give the cave water an outlet.  Finally the water table dropped below the level of the cave, leaving  the cave air-filled and ready for decoration.  Remember this is still occurring very slowly, and over long periods of time.  Because Blanchard Springs Caverns still has water entering and flowing through it, it is considered a “living” or “active” cave.  The following processes are ongoing:

  • Deposition of Clay:  The insoluable residue of dissolved limestone is fine-grained clay called “terra rosa” (meaning red earth).  After the springs created a drain for the cave water, the underground stream began to flow and the clay eroded or moved from place to place.  It was the presence of clay near the ceiling in the Discovery Room that gave Hugh Shell and Hail Bryant the clue that there was an upper level to Blanchard Springs Caverns.
  • Breakdown:  The large piles of rock sometimes covering the cave floor probably fell as the water level in the cave dropped and no longer helped support the ceiling.  The ceiling collapse continued until the ceiling achieved a stable arch where the cavern wall helped hold up the ceiling.  (Think about playing building blocks and building stairsteps up one side and down the other.  Each preceding block supports the next one.)  It is safe to guess that over the past eons Blanchard Springs Caverns has achieved a stable ceiling.  Where the trail goes, the ceiling has been tested and is monitored to insure safety.
  • Deposition of Speleothems:  A speleothem (speliaon – cave, therma – deposit) is a “secondary mineral deposit formed in caves.”  In other words a speleothem is formed from a mineral dissolved out of the bedrock and deposited in the cave. The vast majority of speleothems in Blanchard Springs Caverns are made of calcite, a crystalline mineral of calcium carbonate (CaCO3).  The CaCO3 is dissolved from the limestone bedrock by the weak carbonic acid solution resulting from rainwater percolating down through the rock.  When the solution enters an air-filled passage the CO2 is released due to the difference in CO2 concentration between ground water and cave air.  (The CO2 content in the cave air is 25-250 times lower than the CO2 content in ground water.)  The loss of CO2 can cause calcite deposition.  The tremendous variety of speleothems is due to the number of paths the water may take when it enters the cave by dripping through the ceiling, running down the walls, splashing on the cave floor, or oozing through the walls.  Some of the variables that alter the size and shape of the speleothems are:

– Rate of dripping;

– Size and shape of the opening by which water enters the cave;

– Varying volume of water entering the cave;

– Changing route of water through the overlying rock;

– Air currents in the caverns;

– Slope of the surface over which the water is flowing;

– Variations in the concentration of CaCO3;

– Variations in the amount of carbonic acid.

Can you think of others?

  • Resolutioning:  Since the surface is ever changing, animals and plants are living and dying and the water may change its course as it passes through the rock, the ratio of CaCO3 carried in the water to the amount of acidity is variable.  When the incoming solution is under saturated with calcite, resolutioning will take place.  In other words, the calcite that has been deposited to form speleothems will be taken back into the solution and will be washed away.  Speleothems that are  dry and look dull and chalky (like they are covered by dried toothpaste) are good examples of resolutioning in the Coral Room.
  • Dormancy:  There are some dry speleothems in Blanchard Springs Caverns which attest to the changing course of the water.  These formations are considered dormant, not just dead, because water may return one day.  Our viewing of the cave is just a tiny glimpse of its life.  It’s like seeing one frame of a long playing movie – one frozen instant.  The cave has constantly been changing in the past and it will continue to do so in the future.

Coloring of Speleothems:  Pure calcite is a milky, translucent white.  However, pure white speleothems are rare.  Most speleothems are stained by impurities carried into the cave with water and deposited between the crystals of calcite.  Iron impurities typically stain speleothems yellow, brown, or orange.  Black, blue-gray, and pastel blue colors are probably due to manganese compounds.  A sample of dark stains on the cave ceiling over the loop in the Coral Room was found to be manganese dioxide.  However, a sample of blue-gray flowstone tested by a spectrophotometer contained only calcium, sodium and iron.  Cave micro-organisms and organic materials also probably contribute to

the colors found in a cave, especially black, brown and red.  The stain of color is concentrated on the outer layers of the speleothem.  As the speleothem grows, the crystal bonding structure of the calcite forces the color to these outer layers.

In the very distant future:  What remains for Blanchard Springs Caverns is for the surface erosion to wear the cavern ceiling thinner and thinner. Eventually, hundreds of thousands of years in the future, the cavern ceiling will collapse as a huge sinkhole.  The sediment will wash in to fill the sinkhole and may obliterate all signs of the cavern system.  Meanwhile more passages may be actively forming and enlarging underground and new caves may be given entrances.

References:

Ford, T.O. & C.H.D. Allingford, The Science of Speleology, Academic Press, 1976.

Hill, Carol A., Cave Minerals, National Speleological Society, Huntsville, AL, 1976.

Moore, George W. & G. Nicholas Sullivan, Speleology, The Study of Caves,

Zephyrs Press, Inc., Teaneck, NJ, 1978.

“The Amazing World Below” movie.

“Welcome to Blanchard Springs Caverns” brochure.

Vertical file at Blanchard:

Cohoon, Richard R. “Blanchard Springs Caverns, Tour A – A Geological Report Concerning the Features of the Cave Significant enough for Public Interpretation'” 1968.

Tony V. Field, “Geological History of Blanchard Springs Caverns,” (Geology – BSC).

GEOLOGY

Among numerous inquiries received by the U.S. Geological Survey are those concerning the age of the earth, division of geologic time, and how the earth’s rocks, meteorites, and moon specimens are dated.  The following is based on information contained in a booklet, “Geologic Time,” one of a series of

non-technical publications prepared by the USGS to answer inquiries about a variety of the earth science subjects.

Questions and answers about geologic time:

How old is the earth? The earth is at least 4.6 billion years old, according to recent estimates.  A great part of the evidence for this age is contained in the earth’s rocks.

What scales are used to tell geologic time? Two scales are used to date the various earth-shaping episodes – a relative time scale, based on the sequence of layering of the rocks and the slow but progressive development of life as displayed by fossils preserved in the rocks; and an atomic time scale, based on the natural radioactivity of chemical elements in the rocks.

What are some early geologic speculations? In the 5th century, B.C., the historian Herodotus made one of the earliest recorded geological observations. He found fossil shells far inland in what are now parts of Egypt and Libya and correctly inferred that the Mediterranean Sea had once extended much farther south.  In the 3rd century B.C., Erathosthenes depicted a spherical earth and even calculated its diameter and circumference.  However, less than 500 years ago, sailors aboard the Santa Maria begged Columbus to turn back lest they sail off the earth’s “edge.”  Most people appear to have traditionally believed the earth to be quite young – that its age might be measured in terms of thousands of years, but certainly not in millions or billions.

When were fossils linked to geologic time? Around 1800, William “Strata” Smith, an English civil engineer and surveyor, had a hobby of collecting and cataloging fossil shells from areas in southern England where “limestones and shales are layered like slices of bread and butter.”  His hobby led to the discovery that certain layers contained fossils unlike those in other layers. Using these key or index fossils as markers, Smith could identify a particular slow but progressive development of life; therefore, scientists use them to identify rocks of the same age throughout the world.

What are the major divisions of geologic time? Such recurring events as mountain building and sea encroachment, of which rocks themselves are records, mark units of relative geologic time, even though actual dates of the events are unknown.  By comparison, the history of mankind is similarly organized into relative units of time.  We speak of human events as occurring either B.C. or A.D. – broad divisions of time.  Shorter spans are measured by the dynasties of ancient Egypt or by the reigns of kings and queens in Europe.  Similarly, geologic time divides the earth’s history into eras – broad spans based on the general character of life that existed during these times, and periods – shorter spans based partly on evidence of major disturbances of the earth’s crust.  Following are the geologic eras, ranging successively from the present to the oldest:

  • Cenozoic (from the present to about 70 million years ago);
  • Mesozoic (from about 70 million to 225 million years ago);
  • Paleozoic (from 225-500 million years ago); and
  • Precambrian (600 million years ago and older) marking the time between the birth of the planet and the appearance of complex forms of life.  More than 80% of the earth’s estimated 4.6 billion years fall within this Precambrian Era.

What is Radioactive Decay? Atoms of the same element with differing atomic weights are called isotopes.  Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter).  The rate of decay is conveniently

expressed in terms of an isotope’s half-life, or the time it takes for one-half the nuclei in a sample to decay.  The isotopes of certain elements decay very slowly.  Those of potassium-40 have a half-life of 1.3 billion years.  The potassium-40 method is one of the most useful dating methods available to the geologist because it can be used on rocks as young as a few thousand years as well as the oldest rocks known.

What is the “Carbon-14” Dating Method? An important atomic “clock” used for dating purposes is based on the radioactive decay of the isotope carbon-14, which has a half-life of 5,730 years.  Carbon-14 is being produced continuously in the earth’s upper atmosphere as a result of nitrogen-14 isotopes being struck by neutrons that have their origin in cosmic rays.  This newly formed radiocarbon becomes mixed with the nonradioactive carbon in the carbon dioxide of the air, eventually finding its way into all living plants and animals. After the death of an organism, the amount of radiocarbon gradually decreases through the radioactive decay as it reverts to nitrogen-14.  By measuring the amount of radioactivity remaining in organic materials, the amount of carbon-14 in the materials can be calculated, and the time of death can be determined. Because of the relatively short half-life of carbon-14, the radiocarbon clock can be used for dating events that have taken place only within the past 50,000 years.

What are some of the oldest dated rocks on earth? Rocks in southwestern Minnesota have been found to be 3.8 billion years old — the oldest rocks thus far found on earth.  The ancient rocks — a granite-gneiss — occur along the valley of the Minnesota River and are particularly well exposed near the town

of Granite Falls, Minn.  The age of the rocks was calculated to be about 3.8 billion years old — plus or minus 100 million years — from using the rubidium-strontium and uranium lead dating methods.  Rocks of comparable age have been found in western Greenland and other rocks — 3 to 3.5 billion years old — are known to occur in southern Africa and in the Soviet Union.  Rocks older than 3 billion years probably have survived due to the continuing erosion of the earth’s surface, and the “reconstruction” of rocks deep within the earth.

How old are meteorites? An approximate age for the earth has been determined from studies of meteorites — matter from space.  Stony meteorites contain sufficient uranium to produce appreciable quantities of radiogenic lead, which can be used to measure their ages.  Calculations yield an age of about 4.6 billion years for these meteorites.  Since the earth and meteorites likely have a similar origin, it seems reasonable to assume that the age of the earth is about the same age as the meteorites.

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