Diapositiva 1

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Volcanoes
A volcano is a place on the Earths surface where
magma breaks through. A volcano is called active
if it erupts lava, rock, gas or ash, or if it
shows seismic activity. A volcano is dormant
when is not presently erupting but is considered
likely to erupt in the future. (Foxworthy and
Hill) An extinct volcano will never erupt again.
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• A volcano is a vent or 'chimney' that connects
  the magma chamber (subterranean cavity containing
  the gas-rich liquid magma) to the Earths
  surface.
• The volcano includes the surrounding cone of
  erupted material.

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In the Earth's mantle the temperature is hot
enough (800-1.600 C) to melt rock and form a
thick, flowing substance called magma. Lighter
than the solid surrounding rock, magma is buoyant
much like a cork in water being buoyant, it
rises. Most magmas contain abundant gas , as the
magma rises closer to the surface, the pressure
decreases, which cause the gases dissolved in the
magma to expand. This expansion propels the
magma through openings fractures in the Earths
surface creating a volcanic eruption Once magma
is erupted, it is called lava.
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Volcanic eruptions do not occur randomly, but are
mainly localized at plate boundaries. Some
volcanoes, can be found in the interior of plates
at areas called hot spots.
Although most of the active volcanoes we see on
land occur where plates collide, the greatest
number of the Earth's volcanoes are hidden from
view, occurring along spreading oceanic ridges.
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VOLCANIC ACTIVITY

• Two styles of volcanic eruption Explosive and
  Effusive
• Explosive where rapidly escaping gas bubbles (
  vesicles) rip apart the magma, fragmenting it.
• Effusive where the magma leaks out onto the
  surface passively as lava flows.

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Sometimes basaltic lava pours out quietly from
long fissures and floods the surrounding
countryside with lava flow upon lava flow,
forming broad plateaux. Lava plateaux can be seen
in Iceland, India and USA. (Tilling, 1985)
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Shield volcanoes are built almost entirely of
fluid lava flows forming a broad, gently sloping
cone, with a profile much like that a warrior's
shield. They are built up slowly by highly fluid
basaltic lava that spread widely over great
distances. The Hawaiian Islands are composed of
linear chains of these volcanoes, including
Kilauea and Mauna Loa, two of the world's most
active volcanoes.
Mauna Kea (left) and Mauna Loa (right),
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Some of the Earth's highest mountains are
composite volcanoes sometimes called
stratovolcanoes. They are typically steep-sided,
symmetrical cones of large dimension built of
alternating layers of lava flows, volcanic ash,
cinders, blocks, and bombs
Fuego (left) and Acatenango - Guatemala
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In a "Strombolian"-type eruption huge clots of
molten lava burst form the summit crater to form
luminous arcs through the sky.
Stromboli is a volcan characterized by frequent
activity and high viscosity lavas.
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A "Vulcanian"-type eruption is characterized by a
dense cloud of ash-laden gas that explodes from
the crater and rises high above the peak.
Steaming ash forms a whitish cloud near the upper
level of the cone.
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In a "Pelean" eruption a large quantity of gas,
dust, ash, and incandescent lava fragments are
blown out of a central crater, fall back, and
form a dense cloud that move down-slope at
velocities as great as 100 miles per hour. Such
eruptions can cause great destruction and loss of
life if it occurs in populated areas, as
demonstrated by the devastation of St. Pierre
during the 1902 eruption of Mount Pelee on
Martinique, West Indies.
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Plinian eruption on August 24, 79AD Vesuvius
blew its top, erupting tonnes of molten ash,
pumice and sulfuric gas miles into the
atmosphere. Pyroclastic flows flowed over the
city of Pompeii and surrounding areas.
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WHY DO COMPOSITE VOLCANOES TEND TO BE EXPLOSIVE
AND SHIELD VOLCANOES NON-EXPLOSIVE??

1. CHEMISTRY (COMPOSITION) OF LAVA
2. LAVA TEMPERATURE

• COMPOSITE
• Higher silica content of lavas make them more
  viscous (thick).
• Lava temperatures are generally a few hundred
  degrees cooler than those of shield volcanoes,
  making the lavas thicker in consistency.

• SHIELD
• Lower silica content of lavas make them more
  fluid (runny).
• Lava temperatures are quite hot, 2200oF,
  making it easier for these lavas to flow like
  water.

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PRODUCTS OF EXPLOSIVE ERUPTIONS

• Pyroclastic material
• Rock fragments created by explosive eruptions
• magma explodes from volcano and solidifies in the
  air
• existing rock is shattered by powerful eruptions

Lapilli
Volcanic bombs
Volcanic blocks
Volcanic ash
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• Three products from an explosive eruption
• Ash fall volcanic ash that has fallen through
  the air from an eruption cloud. A deposit so
  formed is usually well sorted and layered.
• Pyroclastic flow a hot, fast-moving and
  high-density mixture of fine and coarse particles
  and gas formed during explosive eruptions or from
  the collapse of a lava dome
• Base surge turbulent, low-density cloud of rock
  debris and water and (or) steam that moves over
  the ground surface at high speed.

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Deposits associated to ash fall are generally
called tuffs, volcanic rock made up of rock and
mineral fragments in a volcanic ash matrix. Tuffs
commonly are composed of much shattered volcanic
rock glass chilled magma blown into the air and
then deposited regularly on the ground like a
snow fall. If the deposits are made by coarse
grained materials they are called volcanic
breccia.
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Pyroclastic flow consists in the lateral flowage
of a turbulent mixture of hot gases and unsorted
pyroclastic material. The deposits are not
layered and tend to fill ground hollows, they are
generally called ignimbrites
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Direct measurements of pyroclastic flows are
extremely dangerous!!!
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Explosive volcanic activity can also produces mud
flows called lahar

• Hot lava can melt snow and ice on the top of the
  cone, so that melt water picks up ashes and rocks
  forming fast flowing, high energy torrents.

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Base surges are generated by phreatomagmatic
eruption a type of volcanic explosion that
occurs when water comes in contact with hot rocks
or ash near a volcanic vent, causing steam
explosions .
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Effusive (lava) activity
If magma is thin and runny, gases can escape
easily from it. When this type of magma erupts,
it flows out of the volcano. Lava flows rarely
kill people because they move slowly enough for
people to get out of their way. Lava flows,
however, can cause considerable destruction to
buildings in their path.
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Because of basalt's low silica content, it has a
low viscosity .Therefore, basaltic lava can flow
quickly and easily move gt20 kilometers from a
vent.
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When lavas contain large amounts of gases,
bubbles tend to escape while lava is flowing
giving rise to highly irregular surfaces. Aa (a
Hawaiian term), is lava that has a rough, spiny
and generally clinkery surface. In thick aa
flows, the rubbly surface of loose clinkers and
blocks hides a massive, relatively dense
interior. (Tilling, Heliker, and Wright, 1987)
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If magma has a low content of gas, lava flow
quikly solidifies at his top, while inside the
hot fluid keeps on running downhill. Pahoehoe is
a Hawaiian term for basaltic lava that has a
smooth, or ropy surface. A pahoehoe flow
typically advances as a series of small lobes
that continually break out from a cooled crust.
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Fluid lava erupted under water may form a special
structure called pillow lava. Such structures
form when molten lava breaks through the thin
walls of underwater tubes, squeezes out like
toothpaste, and quickly solidifies. Typically
ranging from less than a foot to several feet in
diameter, each pillow has a glassy outer skin
formed by the rapid cooling of the lava by water.
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FEATURES ASSOCIATED WITH COMPOSITE AND
SHIELD-TYPE VOLCANOES
Sometimes basaltic lava flows give rise to some
spectacular geological features such as the
Giant Causeway in Northern Ireland
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A lava dome is a steep-sided mass of viscous lava
extruded from a volcanic vent, often circular in
plan view and spiny, rounded, or flat on top. Its
surface is often rough and blocky as a result of
fragmentation of the cooler, outer crust during
growth of the dome. (Foxworthy and Hill, 1982)
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Cinder cones, also known as scoria cones,
represent the smallest type of volcano, rarely
exceeding 300 m in height. They are entirely
composed of pyroclastic materials (volcanic ash).
They are quite common and associated with both
composite or shield volcanoes.
Volcano Sudoeste and the smaller cinder cones at
its base are part of the San Quintin volcanic
field located in Mexicos Baja Peninsula.
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When volcanoes explode or eject a significant
volume of material onto the surface the cone may
collapse into the voided reservoir, forming a
steep, bowl-shaped depression called a Caldera.
These can be quite large, 1-100 km in diameter!
Crater Lake, Oregon (10km) Aniachak Caldera,
Alaska
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Diatreme is a general term for a volcanic vent or
pipe drilled through enclosing rocks (usually
flat-lying sedimentary rocks) by the explosive
energy of gas-charged magmas. The diamond-bearing
kimberlite pipes of South Africa are diatremes.
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References

• http// egsc.usgs.gov /isb//pubs/teachers-packets/
  volcanoes
• http//www.nature.nps.gov/views/kcs/volcanism
• http//geology.com/volcanoes/volcanic-hazards