Basalt fan structure
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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.
- 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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