EFFECT OF THERMAL REGIME ON GROWTH INCREMENT AND SPACING OF CONTRACTION JOINTS IN BASALTIC LAVA

Columnar joints in volcanic rocks are thermal contraction cracks that bound elongate rock columns. The joints grow incrementally toward the interior of a cooling extrusion by successive formation of new segments on the joint edges. Joint spacing as measured by column diameter is often smaller in the upper portion of a lava flow and larger in the lower portion. We investigated incremental growth of columnar joints and factors controlling their spacing by measuring growth increments and spacings of joints in basaltic lava and by modeling the observations using fracture mechanics principles. The field data show that growth increment and spacing are linearly correlated and increase up to a limit with distance from the cooling surface on which the joints start. The models of incremental joint growth combine fracture mechanics with thermomechanical models of layers cooling by conduction or water-steam convection in joints. A joint propagates when tensile stress concentrated at its tip builds up during cooling and exceeds the lava's resistance to fracture. The joint arrests when its tip reaches hot ductile lava with a very high resistance to fracture. Blunting of the arrested tip by ductile flow reduces joint tip stresses and thus stabilizes the joint until further cooling restores critical stress at the tip. The models predict that the joint growth increment will increase slightly from cycle to cycle up to a limit as observed in the field. Faster solidification rates with higher thermal gradients produce smaller growth increments because the level to which a joint propagates will have migrated relatively little when the arrested joint tip cools sufficiently to restart joint growth. An analysis accounting for the mechanical interaction of joints shows that a joint extended beyond its neighbors reduces the joint tip stress of its neighbors, thereby inhibiting their growth and determining a minimum joint spacing. The predicted minimum spacing increases with growth increment in agreement with field measurements. The data and model results therefore indicate that faster solidification promotes smaller joint growth increments that, in turn, produce smaller joint spacings.