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| Magma, Types of Vulcanoes, ...
Magma, Types of Vulcanoes, ...
Magma
Composition
-----------
Magma, then, is most likely neither uniform nor
homogenous. Parts may be largely crystalline, while other parts are largely
liquid. The chemical composition in different parts of a melt may also vary.
Glassy volcanic rocks originate from magma that
was probably a mixture of liquid and gas alone (see VOLCANO). Volcanologists
analyzing gases escaping from volcanic vents can infer what gases might have
escaped from magmas but not their amounts. Volcanic rocks with various
proportions of crystals and glass suggest magmas containing many solids. The
various lava types indicate a range of fluid to viscous magmas. Liquid and gas
may contribute as little as 5 percent to the volume of some magmas.
Chemically most magmas are rich in silicon
dioxide and aluminum oxide and also contain iron, magnesium, calcium, sodium,
potassium, titanium, manganese, phosphorus, and water. Carbonate magmas
(carbonatites) and other unusual compositions also exist. Molten material in
the Earth`s core may be largely nickel and iron.
William D. Romey
Overview
--------
Magma is a complex, mobile mixture of crystals,
rock fragments, liquid, and gases deep within the Earth. The source of IGNEOUS
ROCKS, it can intrude into adjacent rocks or extrude at the surface. Never
directly observed, magmas are inferred from volcanic material, such as LAVA and
natural volcanic glass, and from solid rocks that presumably crystallized from a
melt at depth. Through experimental studies with artificial melts geologists
seek to determine the physical conditions and appearance, chemical compositions,
paths of evolution, loci of origin, and causes of formation of
magmas.
Volcan
Economic Resources
------------------
Volcanic activity has yielded economic
resources such as lightweight and heavy aggregates, pumice for abrasives and
polishing compounds, and elements such as sulfur, zinc, copper, and lead in the
massive sulfide deposits associated with undersea volcanic activity. Recently,
volcanic areas have become important sources of geothermal energy, as in New
Zealand, Iceland, and California. Research into applications for volcanic
products is being pursued by various agencies, including NASA.
James A. Whitney
Eruption Prediction
-------------------
Research continues to find methods to predict
volcanic eruptions accurately. Indications that an eruption may be imminent
include swarms of small earthquakes as the magma rises up through the volcano,
increases in sulfur dioxide emissions, and physical swelling of mountain slopes.
While prediction has not been perfected, these factors were able to predict the
1991 eruptions of Mount Pinatubo in the Philippines and Mount Unzen in
Japan.
Volcanic Origins
----------------
Most of the world`s volcanoes are found along
the margins of huge plates into which the Earth`s crust is divided (see PLATE
TECTONICS). The largest volume of volcanic material is formed at SEAFLOOR
SPREADING centers between plates, such as the Mid-Atlantic Ridge. These
mid-ocean ridges are usually basaltic in composition and erupt quietly onto the
ocean floor. In contrast, the volcanoes formed over subduction zones, in regions
of plate convergence, may be among the most violent.
Along continental margins, chains of volcanoes
such as the Andes have formed. Collisions of oceanic crust produce island arcs
such as the Antilles, the Aleutians, and the Japanese Islands. In addition to
these plate margin areas, volcanoes also form over "hot spots" within a plate,
as in the cases of the Hawaiian Islands and the Yellowstone volcanic field.
Volcanic lava derives from deep-lying molten
material (see MAGMA) that tends to rise and infiltrate the Earth`s crust. The
ultimate source of the magma-producing heat is commonly thought to be the decay
of radioactive isotopes within the Earth, perhaps combined with residual heat
from the time when the planet was formed (see EARTH, HEAT FLOW
IN).
Types of Eruptions
------------------
A volcano may exhibit different styles of
eruption at different times, and eruptions may change from one type to another
as the eruption progresses. The least violent type of eruption is termed
Hawaiian and is characterized by extensive fluid lava flows from central vents
or fissures and occasionally accompanied by lava fountains. Strombolian
eruptions are characterized by moderately fluid lava flows, usually accompanied
by a violent lava-fountaining that produces an abundance of volcanic bombs and
cinders. Vulcanian eruptions are characterized by viscous magmas that form
short, thick flows around vents; very viscous or solid fragments of lava are
violently ejected from these vents. Pelean eruptions are similar to Vulcanian
eruptions but have even more viscous lava; domes form over the vents, and ash
flows commonly accompany the dome formation.The most violent eruptions, such as
that of Washington`s Mount St. Helens in 1980, are termed Plinian after Pliny
the Elder, who died in the Vesuvius eruption of AD 79 . They include the
violent ejection of large volumes of volcanic ash, followed by collapse of the
central part of the volcano (caldera collapse).
Overview
--------
A volcano is a vent in the Earth from which
molten rock (magma) and gas erupt. The molten rock that erupts from the volcano
(lava) forms a hill or mountain around the vent. The lava may flow out as a
viscous liquid, or it may explode from the vent as solid or liquid
particles.
Types of Volcanoes
------------------
The most fluid magmas are erupted quietly and
flow from the vent to form gently sloping shield volcanoes, a name derived from
their resemblance to the shields of early Germanic warriors. The lava flows
from shield volcanoes are usually only 1 to 10 m (3.3 to 33 ft) thick, but they
may extend for great distances away from the vent. The volcanoes of Hawaii and
Iceland are typical shield volcanoes.
Magmas with high gas contents and high
viscosities are usually more explosive than those flowing from shield volcanoes.
These gas-rich magmas in many instances are blown high into the air during an
eruption. The magma falls as volcanic bombs, which accumulate around the vent
and form steep-sided but relatively small cinder cones. Volcanic bombs range in
size from fine-grained ash to house-size blocks. Cinder cones most commonly
consist of volcanic fragments anywhere from ash to small-pebble size, less than
3 cm (1.2 in) in diameter.
Most of the tallest volcanoes are composite
volcanoes (stratovolcanoes). These form from a cycle of quiet eruptions of
fluid lava followed by explosive eruptions of viscous lava. The fluid lava
creates an erosion-resistant shell over the explosive debris, forming strong,
steep-sided volcanic cones.
In the past, great eruptions of extremely fluid
basaltic lava from extensive systems of fissures in the Earth have occurred.
These series of eruptions formed extensive plateaus of basaltic lava. In India
the Deccan basalts cover 260,000 sq km (100,000 sq mi), and in Oregon and
Washington the Columbia Plateau basalts cover approximately 130,000 sq km
(50,000 sq mi). No eruptions of this extent have been observed during historic
times. Even more voluminous accumulations of basaltic lava, however, are
currently being formed at the mid-ocean
ridges.<-- Das war mein Stichwortzettel zu
diesem Thema -->
active : erupt regularly
dormant :sleeping
extinct :not active
ERUPTION
erurtion character --> viscousity ,gas,
....
different eruptions : shield volcanoes(hot
spots): quiet eruption - fluid lava(1 - 10 m thick) - big
distances(Hawaii-20 volcanoes)
volcanoes with gas
rich magma(big viscousity):explosive, lava high into air,
volcanic bombs( fine
ash-->housesize blocks)
Strombolian:
moderately lava fluid, violent fountaining, a lot of volcanic
bombs
most violent eruptions termed Pilan(Mount
St. Helens, erupted in Mai 1980, the top of the2950m
high mountain was forced awai), after pilny the elder -->died in
pompeii
Äviolent
ejection of ash, collaps of central part of volcano--> caldera
collaps
magma-->mixture of crystals, rock fragments,
liquid,gases -->deep in earth
-----------------------------------------------------------------------------------
Ätna: most active volcano in sicilia,3323 m
high,90 eruption since BC 1800
<--Der Referatstext für ein
oberflächliches Referat aus Englisch. -->
Volcanoes
A volcano is a hole in the Earth from wich
molten rock (magma) and gas erupt. The molten rock that erupts from the volcano
(lava) forms a hill or mountain around the hole. The lava may flow out as a
viscous liquid, or it may explode from the hole as solid or liquid
particles.
Volcanoes can be on different
places.
Most of the world`s volcanoes are found along
the margins of huge plates into which the Earth`s crust is divided . The largest
volume of volcanic material is formed at Seafloor Spreading centers between
plates, such as the Mid-Atlantic Ridge. These mid-ocean ridges are usually
shield volcanoes and erupt quietly onto the ocean floor. In contrast, the
volcanoes formed over subduction zones, in regions of plate convergence, may be
among the most violent. Along continental margins, chains of volcanoes such as
the Andes have formed. Collisions of oceanic crust produce island arcs such as
the Antilles, the Aleutians, and the Japanese Islands. In addition to these
plate margin areas, volcanoes also form over "hot spots" within a plate, as in
the cases of the Hawaiian Islands and the Yellowstone volcanic field.
Volcanoes can be active ,dormant and
extinct.Active are when they regularly erupt. Around the world, there are abouth
800. A dormant volcan is one, which is sleeping. This volcanoes can be active
again.A volcan is extinct ,when it never can erupt again.
There are a lot of types of volcanic
eruption.
A volcano may exhibit different styles of
eruption at different times, and eruptions may change from one type to another
as the eruption progresses.Vulcanian eruptions are characterized by viscous
magmas that form short, thick flows around vents; very viscous or solid
fragments of lava are violently ejected from these vents.
The most volcanoes with fluid magmas are
erupted quietly and flow from the vent to form gently sloping shield
volcanoes.These volcanoes are mostly abuoth hot spots, places, were the magma is
so hot, that it melts a hole into the crust.
The lava flows from shield volcanoes are usually only 1 to 10 m (3.3 to 33 ft)
thick, but they may extend for great distances away from the vent. The
volcanoes of Hawaii and Iceland are typical shield volcanoes.
Magmas with high gas contents and high
viscosities are usually more explosive than those flowing from shield volcanoes.
These gas-rich magmas in many instances are blown high into the air during an
eruption. The magma falls as volcanic bombs, which accumulate around the vent
and form steep-sided but relatively small cinder cones. Volcanic bombs range in
size from fine-grained ash to house-size blocks. Cinder cones most commonly
consist of volcanic fragments anywhere from ash to small-pebble size, less than
3 cm (1.2 in) in diameter.
Strombolian eruptions are characterized by
moderately fluid lava flows, usually accompanied by a violent lava-fountaining
that produces an abundance of volcanic bombs and cinders.
The most violent eruptions, such as that of
Washington`s Mount St. Helens in 1980, are termed Plinian after Pliny the Elder,
who died in the Vesuvius eruption of AD 79 . They include the violent ejection
of large volumes of volcanic ash, followed by collapse of the central part of
the volcano (caldera collapse).
But what comes out of volcanoes? It`s
magma.
Magma is a complex, mobile mixture of crystals,
rock fragments, liquid, and gases deep within the Earth.
Volcanoes can make a lot of
damage.
In AD 75 the volcano Vesuvian destroyed the
well know city Pompeii.Pompeii was founded in the 8th century BC by the oscans,
an italic tribe.The city was controled by the romans, when there was an
earthqake in AD 62.In that time, it had abouth 20000 inhabitants and was 66 ha.
big. Then at Aug. 24th 79 vesuvian erupted explosively. The volcan buried
Pompeii, Herculaneum and some other towns under his ash.This eruption killed
abouth 2000 people. The others could flew in the sea. Pompeii had been unearth
in the yaer 1748. Today the ruins of this city are a famous sightseen.Vesuvian
is still active. The last eruptions were in the yaers 1631, 1872, 1906 and
1944.When vesuv would erupt today, it would destroy Rome.
But you should not be afraid, because there are
no volcanes in Austria.
<-- Made by Marek Danis ~ 1995/96(+-1Jahr)
Note:1-2 Quellen: Compuserve, Individuell Meine E-mail: Sputnik_15_98@yahoo.com
-->
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