Earth, the internal structure of Earthquakes, and volcanoes are manifestations of processes at work deep within Earth. These processes in turn are evidence of Earth’s internal structure, models of which have undergone considerable evolution in the 20th century. On a large scale, the internal features of Earth may be categorized as follows:
1. Core. This is the innermost layer of Earth, a dense, approximately spherical mass of very hot rock that accounts for roughly one-third of the planet’s mass and one-sixth of its volume. The core is divided into a solid inner core and a liquid outer layer.
2. Mantle. The mantle, the thick layer of rock surrounding the core, contains about two-thirds of Earth’s mass and more than four-fifths of its volume. An outer layer of the mantle is called the asthenosphere and is involved closely with driving plate tectonics. There is also a division within the upper and lower mantle based on mineral structure.
3. Crust. The crust, or surface layer, of Earth, is very thin compared to the mantle and core (five to 80 miles [8 to 129 km] thick) and accounts for only a tiny fraction of Earth’s total mass and volume. The crust floats on the asthenosphere by a process called isostasy, in which the relatively lightweight rocks of the crust are supported by the denser rocks below. The crust is divided into several individuals, rigid plates that interact with one another through various processes, generating earthquakes and volcanic activity. The crust contains many faults that produce numerous earthquakes.
Earthflow - an earthflow is a type of mass movement. It is a viscous flow of saturated, fine-grained materials that move downslope at speeds ranging from barely perceptible up to about 10 miles per hour (about 0.1 m/sec). Materials susceptible to earthflow include clay, fine sand and silt, and fine-grained pyroclastic material (primarily ash).
The velocity and distance of the earthflow are controlled by water content, with faster and longer movements through higher water content. Some earth flows may continue to move for years. Earth flows normally begin when the water content increases either through rain, melting (normal heating or volcanic eruption), or liquefaction during an earthquake. Shaking from an earthquake may also initiate flow in saturated soil. The flows tend to bulge in the middle as they move, which in turn channels more fluid to the middle while the edges dry out. Earth flows will stop moving when the water content drops.
Earthquake hazard. An earthquake hazard is considered to be any of the damaging effects and processes of an earthquake that may affect the normal activities of people. Examples of earthquake hazards include surface faulting, ground shaking, landslides, liquefaction, slumping, fissures, avalanches, tsunamis, and seiches, among others.
Earthquake light - This phenomenon consists of a peculiar glow that sometimes is reportedly seen in the sky during earthquakes. What causes earthquake light is uncertain, but it has been suggested that methane escaping from underground during earthquakes is ignited somehow and burns near the surface, giving off light.
Earthquake risk - Earthquake risk is the probable damage to buildings, roads, services, and other infrastructure and the number of people that are expected to be killed or injured during an earthquake of a size in a location. Earthquake risk is a probabilistic model for a specific seismic event. It varies considerably with each area depending upon population and emergency readiness.
Earthquake swarm - Many earthquakes preceding a volcanic eruption. As magma moves upward in the crust, it pushes rock out of the way. It forces open cracks in the rock. Each break results in a small earthquake. These earthquakes are typical of magnitudes of 4 or less, but some larger earthquakes are possible. There can be hundreds to thousands of earthquakes in such swarms. volcanologists use this kind of seismic activity to predict eruptions. The correlation is not foolproof. Many earthquake swarms do not foretell a coming eruption but rather just the movement of magma.