Mechanics of columnar joint formation in igneous rocks
Abstract
Columnar joints are interconnected tension fractures that divide rocks into long prismatic columns. Geologists have long speculated about the formation of columnar joints in igneous rocks in order to explain the existence of surficial bands normal to column axes, the development of nearly hexagonal joint patterns, and variations in column size. Previous investigations of columnar joints generally have not utilized existing knowledge concerning fracture-surface morphology or fracture mechanics. The present research addresses the kinematics of columnar joint formation through analysis of joint-surface morphology and joint intersections; it addresses joint-growth mechanics and variable column size by using thermomechanical concepts and fracture mechanics. Each band forms when an individual crack starts at a point on the edge of the preceding crack. Cracks propagate mostly normal to column axes along the leading edges of columnar joints; systematic addition of new cracks to the edges of older ones produces overall joint growth parallel to column axes. Application of overall joint-growth criteria based on these findings indicates that downward growing joints of many lava flows grew much longer than upward growing joints, which implies very fast solidification rates in the upper portions of the flows. A thermal model consisting of water-steam convection in the upper joint set and conduction in the lower joint-set region explains this phenomenon. Polygonal joint patterns evolve from nearly tetragonal ones at flow surfaces to nearly hexagonal ones in the interiors by the gradual change of T intersections to pseudo Y intersections. Cracks form sequentially at column triple junctions, and they overshoot and cut corners of triple junctions to produce systematic changes of the joint patterns. A fracture-mechanics model based on joint-tip blunting and joint interaction indicates that joint-growth increments are relatively large when cooling rate is small, and that large growth increments produce large joint spacings, in agreement with field observations. The results of this research are relevant to formation of joints in newly formed oceanic crust and layered sedimentary rocks, to thermal history and correlation of lava flows, to engineering investigations of columnar jointed rock at two candidate sites of a nuclear waste repository, and to schemes for enhancing geothermal energy extraction by inducing thermal fractures in hot rock.
Degree
Ph.D.
Advisors
Aydin, Purdue University.
Subject Area
Geology
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