Geological Museum KIGAM

Folds and Gneissosity

Large-scale compressional pressures, experienced in orogenic movements, cause rock masses to be bent or broken (fractured). In the deep underground the applied pressure results in folding rather than fracturing of rocks because of their increased plasticity. With deformation the mineral grains in a sedimentary rock may recrystallize and grow so that a gneiss, a metamorphic rock, is formed. In such a metamorphic rock, banding of the constituent minerals into alternating light and dark layers results in a gneissic structure.

Syndepositional Fold & Fault A fold is a bend in rock strata or in any planar structure and is caused by prolonged strong pressure and resulting increase of plasticity of rocks deep underground. By contrast, a fault is a fracture or a zone of fracturing in rocks with displacement of opposite sides of the fracture relative to one another. Faulting usually occurs when strong pressures are applied very rapidly to a rock mass. Folds and faults may also form in soft or poorly consolidated sedimentary strata when they are subject to sudden shocks like an earthquake or they slide down an inclined surface. Such folding and faulting that formed concurrently with sedimentation is called syndepositional deformation.

Joint A joint is a fracture or parting in a rock usually formed without displacement. Joints may be classified into planar joints and columnar joints, based on the orientation of fracture surfaces. Columnar joints form the boundaries of parallel prismatic columns which are hexagonal or trigonal in cross section, and are found most frequently in basaltic lava flows and rarely in other volcanic rocks. They formed as the result of contraction of the lava during cooling. In Korea, they can be seen in Jeju Island, Yeoncheon and Pohang where basaltic rocks are exposed. The Jeongbang and Cheonjiyeon waterfalls in Jeju Island are developed on columnar jointed basalts.