Mechanism of entablature formation in columnar jointing

Determine the physical mechanism and thermal–hydrological conditions responsible for the formation of entablature (thin, disordered columns) within columnar jointing in cooling lava flows, and explain why and how entablature develops compared to ordered colonnades.

Background

Columnar joints in lava typically develop as a regular colonnade of prismatic columns that grow with the cooling front. Many sites also display an upper layer of narrower, more randomly oriented columns known as entablature. While regular colonnades are reasonably understood through thermoelastic fracture guided by cooling and convective heat transport, the origin of entablature remains unresolved.

The review notes that entablature has been implicated with enhanced and localized cooling, but the precise mechanism and conditions that generate entablature, rather than an ordered colonnade, are unclear and have been identified as an open problem.