History and Social Science, Part 82

Question 1: Define the following terms as they relate to fault mechanics: slip, dip, strike.

Answer 1: Slip is a measurement of the net relative displacement of structural surfaces along a fault. Dip is the angle that a rock body or structural rock surface makes with the horizontal surface plane. True dip is quantified through downward measurement of the angle in the direction of greatest inclination; apparent dip is the measurement of dip in any direction. Dip is equal to zero in the direction of strike and has its maximum measurement in the direction of true dip. Strike is the direction of a line of intersection drawn between the horizontal surface plane and the surface of an uptilted rock structure.

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Question 2: Explain geologic folding.

Answer 2: A geologic fold is a region of curved or deformed stratified rocks. Folding is one process by which Earth’s crust is deformed. Rock strata are normally formed horizontally; however, geologists have identified areas where these strata arc upwards or downwards. Anticlines are upfolded areas of rock; downfolds are called synclines. In anticlines, the rocks are oldest along the axis (a horizontal line drawn through the point of the fold’s maximum curvature), and in synclines, the youngest rocks are at the axis. Monoclines, or flextures, are rock structures that slope in one direction only, and often pass into geologic faultlines. The process of folding usually occurs underneath the Earth’s surface, but surface erosion eventually exposes these formations. Folding is generally thought to be caused by the horizontal compression of the Earth’s surface, which is related to the movement of tectonic plates and fault activity.

Question 3: Define orogenesis.

Answer 3: Orogenesis refers to mountain-building processes, specifically as they relate to the movement of tectonic plates. An individual orogeny can take millions of years. Generally, mountains are created when compressional forces push surface rock upward, resulting in a landform that is higher than the land around it. There are four broad categories of mountains (which are not mutually exclusive); these categories are based on the mountain’s formative origin. Folded mountains, formed from the long-term deformation and metamorphosis of sedimentary and igneous rocks, usually occur in chains. This type of mountain often forms at convergent plate boundaries. Fault-block mountains occur at normal or reverse faults with high dips. Portions of Earth’s crust are vertically displaced along the faults. Oceanic ridges are formed at divergent boundaries beneath the ocean. When plates move apart, material from the mantle rises up and creates long mountain chains. Volcanic mountains form from the accumulation of products of volcanic eruptions, such as ash and lava. They often occur singularly, unlike other mountain types that usually exist in chains.

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