Geological Forces That Shape the Earth

Diastrophism Defined:

•Comes from the Greek word meaning a “twisting” deformation of the Earth’s crust. All processes that move, or elevate or build up portions of the Earth’s crust comes under this. •Diastrophism covers movement of solid (plastic) material, as opposed to movement of molten material which is covered by volcanism. •The most obvious evidence of diastrophic movement can be seen where sedimentary rocks have been bent, broken or tilted.

Types of Earth Movement:

a.Uplift
•Is the movement of the Earth wherein the crust rises.

•Process in which structural highs in Earth’s crust are created (e.g. mountains).
•The movement of tectonic plates and igneous plumes are two processes which may result in uplift. b.Subsidence
•A sinking or setting of part of the Earth’s crust with respect to the surrounding parts is the motion of a surface (usually, the Earth’s surface) as it shifts downward relative to a datum such as sea level. c.Thrust

•Horizontal movement of the Earth’s crust.

Theories of Diastrophism:
a.Theory of Isostasy
•States that as rock from higher region is removed by erosion and deposited on a lower region, the higher region slowly rises while the lower region

becomes heavier and sinks. b.Contraction Theory

•States that the Earth is gradually sinking. As the shrinkage occurs, the stronger and heavier blocks of the crust sink while the weaker strata are crowded and squeezed upward. c.Convection Theory

•Is a theory which would account for publishing and folding of rocks through convection currents.

This process is true when it occurs under a continental mass.

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

e.Expansion Theory
•Is a theory which states that the Earth is gradually expanding. Expansion of the Earth would change the continents’ position.

Structural Features of the Earth’s Movement:
a.Deformation of Rocks
•The effects of diastrophism are clearly seen in sedimentary rocks. Any deformation of sedimentary rocks is shown in tilts, bends, or break in the layer.
b.Folds
•A bend or flexure in a rock can be likened to waves on the ocean. Each has a crust and a trough or a down fold. •The crust of a rock is called an anticline. The trough of a rock is called syncline. c.Dip and Strike

•Dip – angle between the horizontal plane or structural surface. •Strike – direction of a line along the edge of an inclined bed where it meets the horizontal plane. It is always at right of the dip. d.Pitching Fold

•Folds whose access slants downward at the each end of an imaginary line running along the top of an anticline or along the bottom of a syncline are called the axes of the fold. •The angle between the axis and the horizontal is called Pitch. e.Joints and Fissures

•Joint – a fracture or a little separation between the rock walls. •While Fissures are cracks or actual gap between rock walls.
f.Faulting
•Refers to the surface along which a rock body has been broken and has been displaced.
•Two types of Faulting:
a.Vertical Faulting
b.Horizontal Faulting

Types of Fault
a.Thrust Fault
•It is a low angle fault in which the hanging wall is moved upward in relation to the footwall. It is characterized by the horizontal compression rather than vertical displacement. •A reverse fault in which the fault plane is inclined at an angle equal to or less than 45 degrees. •A geological fault in which the upper side appears to have been pushed upward by compression. b.Normal Fault

•Also called Gravity Fault, a dip slip fault in which the hanging wall has moved downward relative to the footwall. •A geologic fault in which the hanging wall has moved downward relative to the footwall. Normal faults occur where two blocks of rocks are pulled apart as by tension. c.Reverse Fault

•Is the material above the fault plane that moves up in relation to the material below. •A geologic fault in which the hanging wall has moved upward relative to the footwall. Reverse faults occur where two blocks of rock are forced together by compression.