Thursday, March 7, 2013

Minnesota Geology Pictures - Sinkholes

Sinkholes are in the news today, largely because of the sinkhole that swallowed and killed a Florida man recently.  Sink holes are also a relatively common occurrence in Minnesota.  In fact, the city of Fountain claims to be 'The Sinkhole Capital of the World".

Fountain, MN city sign.
Sinkholes mainly occur in three distinct ways in the state of Minnesota.

1.  Torrential rainfall washes out sections of roads.  An example of this is last summer's 10+ inches of rain on Duluth area causing numerous sinkholes across the city.  For a newspaper article of the event (including pictures of the sinkholes) click here.

2.  Water mains break, typically due to the freeze/thaw cycles during fall, winter or spring months.  A recent example from St. Paul can be found here.

3.  Due to dissolving of the carbonate bedrock found primarily in Southeastern Minnesota.  As passing groundwater dissolves the limestone/dolostone bedrock near the surface, it creates cavities that eventually cannot support the weight of the ground above and collapses.

In fact, you do not need to look very hard in Southeast Minnesota for evidence of sinkholes in Google Earth.  As sinkholes form in farmers fields, farmers continue to farm the land avoiding the holes.  Over time, trees and vegetation begin to grow from the bottom of these sinkholes, making the sinkholes easily seen.
Google Earth image of farm field in SE MN.
Sinkholes (and caves, disappearing streamsare a feature commonly found in Karst landscapes.  A more detailed post written on the Karst landscapes of Minnesota can be found here.