Monthly Archives: January 2015

Geopic of the week: Speleothems

 

Geopic Blanchard

Above are speleothems- they’re also called dripstones, stalagmites and stalactites, or other names depending on their shape.  They can be seen when touring Blanchard Springs Caverns located about 10 miles north of Mountain View, Arkansas.  The cave is full of breathtaking features like these which form by precipitation from ground water that drips from the cave ceiling.  The groundwater, which has traveled through rock and soil before reaching the cave, is saturated with minerals it dissolved along it’s path.  The principal mineral that forms the dripstones is calcite.  As the groundwater enters the open air of the cave a pressure drop causes the calcite to come out of solution and precipitate a tiny coating on the outside of the growing speleothem.  Over time some of them can reach from the roof to the floor of the cave and elaborate, and marvelous shapes develop. 

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Geopic of the week: injection feature

 

Sandstone injection feature hollywood quarry

Pictured above is what geologists term an injection feature or sand dike.  It formed when sand was violently forced upward into overlying clay before the sediment was cemented to form rock.  In environments where sediment is accumulating very quickly, water can get trapped and buried in a sand body;  as more sediment is deposited on top of the sand, the pressure causes the sand body to compress.  When water erupts upward to relieve the pressure, it carries sand with it which fills the fissure created by the escaping water. 

Geologists look for clues like injection features when trying to unravel the mystery of what conditions were like when a rock was deposited.  This particular rock is part of the Jackfork Formation which is exposed at the surface around Little Rock Arkansas and surrounding areas; it was deposited when the area was at the bottom of a deep ocean basin more than 300 million years ago.  Ink pen is for scale.