![]() ![]() In the history of the study of faults, Robert Mallet, an Irish engineer, was the first to believe that simple mechanics of Earth ’s crust cause earthquakes. About 100-200 earthquakes per year cause some damage, and one or two per year are catastrophic. Only one third of these can be felt by humans, most of which are very small and do not cause any damage. Together, the pulses caused by the heat engine inside Earth result in over a million earthquakes per year that can be detected by instruments. The slower motions of the mantle pulse through the thin crust, causing earthquakes, volcanic activity, and the movement of tectonic plates. The mantle responds to this, as well as to centrifugal force caused by the rotation of Earth on its axis and to the force of gravity. Because it is fluid, the core moves constantly. Deeper still is Earth ’s core, which is molten rock. The mantle, a zone underlying the crust, is very dense rock that is almost liquid. ![]() The engine driving the movement of the plates originates deep in the earth. The process of plates grinding together causes near-surface earthquakes, and the collision and subduction of plates causes the most intense earthquakes much deeper in the crust. Spreading plates most commonly occur in the oceans in the phenomenon known as sea-floor spreading when plates spread within landmasses, they create huge valleys called rifts. Each plate is relatively rigid, and, where the plates meet, they can spread apart, grind against each other, or ride one over the other in a process called subduction. Scientists believe the crust is composed of about 12 of these plates. Instead, it is made up of huge blocks of rock that fit together to form the entire surface of the planet, including the continents or landmasses and the ocean floors. ![]() To understand faults, it is helpful to understand plate tectonics. A fault zone may be hundreds of feet (meters) wide and each has a unique character some include countless faults and others have very few. The Northridge earthquake in the Los Angeles area in January 1994 occurred along a thrust fault that had not previously been known but is within the San Andreas zone. It may be more accurate to call these systems “fault zones ” or “fault belts ” that contain known and unknown faults. Major fault systems are typically found where plates meet for example, the San Andreas fault in California is really a fault system that includes many smaller faults that branch off of the main trace of the San Andreas as well as faults that parallel the main fault. Faults may be inches (centimeters) to hundreds of miles (kilometers) in length, and movements or displacements have the same range in length. Much larger movements or displacements occur along faults where plates of Earth ’s crust abut each other. Bedrock faults are those in which bodies of rock meet small, local movements may occur onīedrock faults. Mountain-building by small movements along faultsĮarthquake generation by large sudden movements along faultsĪ fault is a geologic term describing a fracture at which two bodies of rock have been displaced relative to each other. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |