Volcanic eruptions

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Volcanic eruptions
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Why do volcanoes erupt?
Volcanoes occur where the earths deep interior
is hot enough for rocks to melt (differentiation
in action!). Every volcano is underlain by a
pool of partially molten rock (magma) called the
magma chamber. Magma contains dissolved gases
like water and carbon dioxide When the magma
begins to rise, these gases exsolve and form
bubbles, increasing the pressure The
increasing gas pressure forces the magma up and
out of the volcano in an eruption
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Explosive vs effusive eruptions
Volcanic eruptions can be explosive or effusive,
depending on the composition of the
magma Explosive jet of hot material explodes
violently from volcano magma is fragmented
into bits of ash Example Mt. St Helens, Mt.
Vesuvius Effusive magma oozes out of vent and
forms lava flows may have fire fountains of
lava vent may be a central cone, or a long
rift Example Hawaii, Iceland
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Explosive vs effusive eruptions
What about magma composition affects eruption
type? Explosive magma has a high silica and gas
content resulting from a greater degree of
differentiation More silica makes the magma
viscous, with a consistency like that of
toothpaste Effusive basalt magma has a lower
silica and gas content because it is less
differentiated Low-viscosity magma can flow
more easily, like honey Dissolved gases can
bubble out of the magma more easily instead of
causing it to explode
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High silica magmas, being viscous, can erupt in
violent explosions in which they fragment into
clouds of ash. This can be very destructive, as
Mt. Pinatubo was in 1991.
High-silica magma can not make long lava flows
here it forms a lava dome at the Alaska volcano
Novarupta.
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Large explosive eruptions can be violent enough
to destroy their own volcanic cones, as at Crater
Lake, Oregon. Crater Lake used to be a volcano
called Mt. Mazama that was about 12,000 feet high
(the size of Mt. Adams).
Volcanologists carefully monitor Mt. Rainier in
case it ever shows signs of such a violent
eruption
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Hazards of explosive eruptions
Explosive eruptions are dangerous even if one
avoids the explosion itself. Glowing ash clouds,
called pyroclastic flows, can rush down the
mountain, burying everything in their path.
Pompeii was buried by a pyroclastic flow.
If volcanic ash mixes with water (as from a lake
or melting glacier), it can create a volcanic
mudflow or lahar. This lahar buried two towns in
Colombia in 1983, killing 23,000 people.
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Effusive eruptions
Effusive eruptions bury everything in their path,
but tend to be less violent, although they can
make spectacular fire fountains. Low-viscosity
basaltic lavas can flow for long distances, often
in channels. Sometimes they will flow in a
channel that is roofed over by cooled lava,
called a lava tube. You can visit old lava tube
caves in eastern Oregon
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Effusive eruptions produce two types of lava,
again distinguished by their viscosity Pahoehoe
or ropy lava is less viscous smooth, glassy
surface can form very long, fluid
flows Aa lava is more viscous rough,
sharp surface flows in a steep-sided mass
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Effusive eruptions shields and fissure eruptions
Some effusive eruptions form a distinct volcanic
cone, like Mauna Kea on Hawaii (peak in distance
behind pahoehoe flow). Because of the shape, the
cone is called a shield.
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Effusive eruptions shields and fissure eruptions
Other eruptions come from a rift or series of
rifts in the earths crust. They cover wide
areas with basaltic lava (mostly pahoehoe).
These are called flood basalt eruptions. Most of
eastern Washington state is covered with flood
basalts (tan area in map).
Flood basalt eruptions come from the start of
what is called a hotspot.
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Most volcanic activity on Earth takes place at
the boundaries of the tectonic plates, where they
pull apart (divergent plate boundaries) or
collide (convergent plate boundaries)
However, some volcanism occurs in the middle of
tectonic plates. This is called hotspot
volcanism. The Galapagos islands (shown here)
are considered to be hotspot volcanoes. So is
Yellowstone caldera.
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Volcanoes on the Earth
Plate tectonics vs hotspot volcanism Two
different ways of organizing heat flow
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Hotspot vs plate boundary volcanism on Earth
Plate boundary - volcanoes located along lines or
arcs Mid-ocean ridge (divergent margin)
volcanism all effusive Subduction zone
(convergent margin) volcanism mostly
explosive Hotspot - volcanoes appear in random
locations In ocean, all effusive (oceanic
plateau basalts, ocean islands) On continents,
effusive first (flood basalts), then explosive
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Volcanism on the terrestrial planets Venus
Most volcanism on Venus appears to be effusive
lava flows are long and thin. Here (left), thin
flows radiate away from the volcano Sapas Mons.
However, Venus has some interesting dome-shaped
volcanoes that appear to have formed from more
viscous lavas.
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Volcanism on the terrestrial planets Mars
Most volcanism on Mars appears to be effusive.
Shield volcanoes dot the planets surface. Most
Martian rocks that have been analyzed are broadly
basaltic in composition.
Some Martian rocks were found to be more
silica-rich in composition. Explosive volcanism
has been proposed for Mars, but never verified.
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Volcanism on the terrestrial planets Mercury and
the Moon
All volcanism on Mercury and on the Moon was
effusive and basaltic (as far as we can tell).
Basalt lava plains on Mercury (dark) and
basalt-filled basins (blue areas) on the Moon
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Topography of the terrestrial planets indicates
that Earth is the only planet with plate
tectonics Therefore, volcanism on other planets
is likely to be mainly hotspot-type (or possibly
some new type?) Little explosive volcanism
Lots of basalt, fewer more evolved magmas