Igneous Joints
Joints and Stresses
Jointing
Outline
Joints in igneous rocks are often associated with the tensile stresses generated
by shrinkage as the rock cools. The joints form normal (at right angles) to the
cooling surface. The margins of lava flows, sills, dykes and plutons commonly
form the cooling surfaces. In bodies of uniform thickness, perfect hexagonal,
columnar joints may form. Polygonal joints of this kind are very common.
Plutons have complex outer margins and comprise very large bodies of rock.
Cooling generates tensile joints normal to the margins but small faults
(both normal and reverse) or shear joints can also develop as a result
of forcible upward intrusion created by a bouyant granite magma in the crust.
Unloading by subsequent erosion frequently results in exfoliation joints
parallel to the outer surface of the pluton. A flow foliation can
also occur due to convections in the magma.
Joints and Stresses
Jointing
Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9