Diastrophism and Earthquake

• To identify the geologic forces that shape the earth

• Recognize the type of movement and its relation to the structural features and movement of the earth.

• Define faults and their relation to earthquakes and prediction.

To a casual observer, the crust of the earth is permanent and fixed, but a great deal of evidence, both direct and indirect, shows that the earth is in constant motion. This is because of the forces acting on its surface and within it. Evidence of crustal movement comes in many forms and for all to see. The dynamic internal forces, collectively known as diastrophism, generally tend to elevate the earth’s surface. They are in constant battle against external forces that tend to wear away the land surface

Theories of Diastrophism

• Theory of Isostasy (from a Greek word meaning “equal standing”) states that as rock from higher regions is removed by erosion and deposited on a lower region, the higher region slowly rises while the lower region becomes heavier and slowly sinks.

• Contraction Theory states that the earth is gradually shrinking. As the shrinkage occurs, the stronger and heavier blocks of the crust sink while the weaker strata are crowded and squeezed upward.

• Convection Theory is a theory which would account for the pushing and folding of rocks through convection currents. This process is true when it occurs under a continental mass.

• Continental Drift Theory is a theory which accounts for diastrophic movement and for the folding and faulting along the edges of the continents.

• Expansion Theory is a theory which states that the earth is gradually expanding. Expansion of the earth would change the continents’ positions.

Types of Earth Movement

Uplift - is the movement of the earth wherein the crust rises.

Subsidence - a sinking or setting of a part of the earth’s crust with respect to the surrounding parts.

Thrust - horizontal movement of the earth’s crust.

Structural Features of Earth Movement

Deformation of rocks - The effects of diastrophism is more clearly seen in rocks, especially in sedimentary rocks. Any deformation of sedimentary rocks is shown in tilts, bends, or break in the layer.

Folds - A bend or flexure in a rock can be likened to waves on the ocean. Each has a crust or upfold and a trough or downfold. The crust of a rock is called an anticline and the trough of the rock is called sycline.

Dip and strike - Dip is the angle between the horizontal plane and a structural surface (like a bedding plane, a joint, or other planar). Strike is the direction of a line along the edge of an inclined bed where it meets the horizontal plane. The strike is always at the right of the dip.

Pitching fold - Folds whose axes slant downward at each end of an imaginary line running along the top of an anticline or along the bottom of a sycline are called the axes of the fold. The angle between the axis and the horizontal is called pitch.

Joints and fissures -A joint is a fracture or a little separation between the rock walls while fissures are cracks or actual gaps between the rock walls.

Faulting - It refers to the surface along which a rock body has been broken and has been displaced. There are two types of faulting: vertical faulting occurs when an entire block of rock is raised evenly so that the layers remain in their original horizontal position; horizontal faulting occurs when an entire block of rock is lowered evenly so that the layers are in their original vertical position.