STRUCTURAL GEOLOGY

scientific discipline that is concerned with rock deformation on both a large and a small scale. Its scope of study is vast, ranging from submicroscopic lattice defects in crystals to fault structures and fold systems of the Earth's crust. The methods of structural geology are nearly as diverse as those of the geologic sciences as a whole. Small-scale structural features may be studied using the same general techniques that are employed in petrology, in which sections of rock mounted on glass slides are ground very thin and are then examined with polarizing microscopes. On a larger scale, the techniques of field geology are used. These include plotting the orientation of such structural features as faults, joints, cleavage, and small folds. In most cases, the objective is to interpret the structure beneath the surface by using information available at the surface. Where mountains, continents, ocean basins, and other large-scale features are involved, the methods employed are chiefly those of geophysics and include the use of seismological, magnetic, and gravitational techniques. Furthermore, since the processes that cause rocks to deform can rarely be observed directly, it is necessary to study them by means of computer models in which they are represented mathematically.