The Formation of Columnar Joints by Cooling in Basalt at Staffa, Scotland

Columnar jointing in basaltic lava flows on the
island of Staffa, NW Scotland, was studied using a combination
of field mapping and measurement of column dimensions,
sample petrology and measurements of plagioclase crystal size
distributions (CSDs) interpreted using theoretical models of
cooling. Four different lava flow units were measured, and
column ordering was assessed using the hexagonality index
and relative standard deviations of column side length, top area
and internal angle. Upper and lower colonnades consist of
dominantly 5-, 6- and 7-sided columns, with a hexagonality index value very similar to that of Giant’s Causeway and other
basaltic columnar jointed localities. CSDs from samples at
different heights within one colonnade were used to infer the
propagation of the solidus isotherm, which was consistent with
a convective cooling mechanism within the colonnade interior.
Sample petrology and CSD measurements suggest that entablature
can form both by the interaction of propagating joint sets
and flooding of the flow surface by water, and the most widely
exposed unit on Staffa shows evidence of both mechanisms
operating on the same flow. Crystal size distribution measurements
can provide a useful tool for field interpretation of lava
flow cooling mechanisms.