Basalt fan structure


A basalt fan structure is a basalt formation composed of columnar jointed basalt columns that have slumped into a fan shape.[1]

They are most likely created when basalt lava is intruded into a soft material, such as loose volcanic ash, which is unable to support the intrusion. As the basalt cools and forms columns, the ash collapses, causing the lava spread and form a fan shape. The formations can be vertical or any degree of horizontal.

References

  1. Faust, George T. "Joint Systems in the Watchung Basalt Flows, New Jersey" (PDF). Geological Survey Professional Paper 864-B. United States Government Printing Office. Retrieved 13 February 2014.


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