A basalt fan structure is a basalt formation composed of columnar jointed basalt columns that have slumped into a fan shape.1
Such formations 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
References
- 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 17 December 2019.