EARTHQUAKES & VOLCANOES

volcanoes Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the crust's interior plates, in the East African Rift and the Wells Gary Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of plate hypothesis volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so called hotspots, for example Hawaii, are postulated to arise from upwelling with magma from the core mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere or troposphere however, they also absorb heat radiated up from the Earth, thereby warming the upper atmosphere or stratosphere. Historically, so called volcanic winters have caused catastrophic famines.

An earthquake also known as a quake, tremor or temblor is the perceptible shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes can be violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter magnitude scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicentre of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity. In its most general sense, the word earthquake is used to describe any seismic event whether natural or caused by humans that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocentre. Earthquakes

facts Natural events such as volcanic eruptions and meteor impacts can cause earthquakes, but the majority of naturally-occurring earthquakes are triggered by movement of the earth's plates. The earth's surface consists of 20 constantly moving plates. The pressure increase from shifting plates can cause the crust to break. This break allows stress to be released as energy, which moves through the earth in the form of waves. Normally, it's not the shaking ground itself that claims lives during an earthquake. It's the associated destruction of man-made structures and the instigation of other natural disasters such as tsunamis, avalanches and landslides. Not only is it important to have a plan for yourself in the case of an earthquake, but your pets need a disaster plan as well. Create an earthquake plan for the animals around so if disaster strikes, you’ll be prepared. The National Earthquake Information Centre (NEIC) records an average of 20,000 earthquakes every year (about 50 a day) around the world. There are, however, millions of earthquakes estimated to occur every year that are too weak to be recorded. Each year the southern California area has about 10,000 earthquakes -- the majority of which go unnoticed. However, if there is a large earthquake the aftershock sequence will produce many more earthquakes of all magnitudes for months.