[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]Earthquakes An earthquake (also known as a tremor or temblor) is the result of a
sudden release of energy in the Earth's crust that creates seismic waves.
Earthquakes are recorded with a seismometer, also known as a seismograph. The
moment magnitude of an earthquake is conventionally reported, or the related
and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes
being mostly imperceptible and magnitude 7 causing serious damage over large
areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and
sometimes displacing the ground. When a large earthquake epicenter is located
offshore, the seabed sometimes suffers sufficient displacement to cause a
tsunami. The shaking in earthquakes can also trigger landslides and
occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any
seismic event—whether a natural phenomenon or an event caused by humans—that
generates seismic waves. Earthquakes are caused mostly by rupture of geological
faults, but also by volcanic activity, landslides, mine blasts, and nuclear
experiments. An earthquake's point of initial rupture is called its focus or
hypocenter. The term epicenter refers to the point at ground level directly
above this.
Naturally
occurring earthquakes Fault typesTectonic earthquakes will occur anywhere within the earth
where there is sufficient stored elastic strain energy to drive fracture
propagation along a fault plane. In the case of transform or convergent type
plate boundaries, which form the largest fault surfaces on earth, they will
move past each other smoothly and aseismically only if there are no
irregularities or asperities along the boundary that increase the frictional
resistance. Most boundaries do have such asperities and this leads to a form of
stick-slip behaviour. Once the boundary has locked, continued relative motion
between the plates leads to increasing stress and therefore, stored strain
energy in the volume around the fault surface. This continues until the stress
has risen sufficiently to break through the asperity, suddenly allowing sliding
over the locked portion of the fault, releasing the stored energy. This energy
is released as a combination of radiated elastic strain seismic waves,
frictional heating of the fault surface, and cracking of the rock, thus causing
an earthquake. This process of gradual build-up of strain and stress punctuated
by occasional sudden earthquake failure is referred to as the Elastic-rebound
theory. It is estimated that only 10 percent or less of an earthquake's total
energy is radiated as seismic energy. Most of the earthquake's energy is used
to power the earthquake fracture growth or is converted into heat generated by
friction. Therefore, earthquakes lower the Earth's available elastic potential
energy and raise its temperature, though these changes are negligible compared
to the conductive and convective flow of heat out from the Earth's deep
interior.[1]
Earthquake
fault types There are three main types of fault that may cause an earthquake:
normal, reverse (thrust) and strike-slip. Normal and reverse faulting are
examples of dip-slip, where the displacement along the fault is in the
direction of dip and movement on them involves a vertical component. Normal
faults occur mainly in areas where the crust is being extended such as a
divergent boundary. Reverse faults occur in areas where the crust is being
shortened such as at a convergent boundary. Strike-slip faults are steep
structures where the two sides of the fault slip horizontally past each other ;
transform boundaries are a particular type of strike-slip fault. Many
earthquakes are caused by movement on faults that have components of both
dip-slip and strike-slip; this is known as oblique slip.
volcanoesA volcano is an opening, or
rupture, in a planet's surface or crust, which allows hot, molten rock, ash,
and gases to escape from below the surface. Volcanic activity involving the
extrusion of rock tends to form mountains or features like mountains over a
period of time.
Volcanoes are generally found
where tectonic plates are diverging or converging. A mid-oceanic ridge, for
example the Mid-Atlantic Ridge, has examples of volcanoes caused by
"divergent tectonic plates" pulling apart; the Pacific Ring of Fire
has examples of volcanoes caused by "convergent tectonic plates"
coming together. By contrast, volcanoes are usually not created where two
tectonic plates slide past one another. Volcanoes can also form where there is
stretching and thinning of the Earth's crust (called "non-hotspot
intraplate volcanism"), such as in the African Rift Valley, the Wells
Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the
European Rhine Graben with its Eifel volcanoes.
Volcanoes can be caused by
"mantle plumes". These so-called "hotspots" , for example
at Hawaii,
can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere
in the solar system, especially on rocky planets and moons.
Divergent plate boundariesAt the mid-oceanic ridges,
two tectonic plates diverge from one another. New oceanic crust is being formed
by hot molten rock slowly cooling and solidifying. The crust is very thin at
mid-oceanic ridges due to the pull of the tectonic plates. The release of
pressure due to the thinning of the crust leads to adiabatic expansion, and the
partial melting of the mantle causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans, therefore most
volcanic activity is submarine, forming new seafloor. Black smokers or deep sea
vents are an example of this kind of volcanic activity. Where the mid-oceanic
ridge is above sea-level, volcanic islands are formed, for example, Iceland.
Lava enters the Pacific at
the Big Island of Hawaii
Convergent plate boundariesSubduction zones are places
where two plates, usually an oceanic plate and a continental plate, collide. In
this case, the oceanic plate subducts, or submerges under the continental plate
forming a deep ocean trench just offshore. Water released from the subducting
plate lowers the melting temperature of the overlying mantle wedge, creating
magma. This magma tends to be very viscous due to its high silica content, so
often does not reach the surface and cools at depth. When it does reach the
surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.
HotspotsHotspots are not usually
located on the ridges of tectonic plates, but above mantle plumes, where the
convection of the Earth's mantle creates a column of hot material that rises
until it reaches the crust, which tends to be thinner than in other areas of
the Earth. The temperature of the plume causes the crust to melt and form
pipes, which can vent magma. Because the tectonic plates move whereas the
mantle plume remains in the same place, each volcano becomes dormant after a
while and a new volcano is then formed as the plate shifts over the hotspot.
The Hawaiian Islands are thought to be formed
in such a manner, as well as the Snake River Plain, with the Yellowstone
Caldera being the part of the North American plate currently above the hotspot.
Indonesia - Lombok: Mount
Rinjani - outbreak in
1994
Volcanic
featuresThe most common perception of
a volcano is of a conical mountain, spewing lava and poisonous gases from a
crater at its summit. This describes just one of many types of volcano, and the
features of volcanoes are much more complicated. The structure and behavior of
volcanoes depends on a number of factors. Some volcanoes have rugged peaks
formed by lava domes rather than a summit crater, whereas others present
landscape features such as massive plateaus. Vents that issue volcanic material
(lava, which is what magma is called once it has escaped to the surface, and
ash) and gases (mainly steam and magmatic gases) can be located anywhere on the
landform. Many of these vents give rise to smaller cones such as Puʻu ʻŌʻō on a
flank of Hawaii's Kīlauea.
Other types of volcano
include cryovolcanoes (or ice volcanoes), particularly on some moons of
Jupiter, Saturn and Neptune; and mud
volcanoes, which are formations often not associated with known magmatic
activity. Active mud volcanoes tend to involve temperatures much lower than
those of igneous volcanoes, except when a mud volcano is actually a vent of an
igneous volcano.
الأربعاء أبريل 20, 2011 9:39 pm
