Why Do Volcanoes Erupt?

busyicicleMechanics

Feb 22, 2014 (3 years and 5 months ago)

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Why Do Volcanoes Erupt?


Magma deep inside the Earth is generally
less dense than the solid rock surrounding
it.


Tends to rise toward the Earth’s surface
from the buoyant force of gravity.


As magma nears the Earth’s surface it’s
dissolved gases boil out of the molten lava.


Force of this gas expansion propels the
molten lava, or pyroclastics from the vent.


Explosive Eruptions


If gas content is high and the magma thick
and viscous, sudden release of confining
pressure causes the gases to explosively boil
out from the magma.


A sudden burst will tear the magma into hot
fragments that jet upwards, or blast outward
from the vent.


The direction of travel for the rock
fragments, hot ash, and expanding gases
depends in part on the initial shape and
direction of the vent.


Depends even more on the density of the
mixture.


If the mixture is denser than air:


moves down
-
slope as a high speed avalanche


nuée ardente, ash flow, pyroclastic flow


temperature can be > 700

C


speeds > 100 kph


If the mixture is lighter than air:


can rise upward in a dark roiling cloud


many turbulent cells


Effusive Eruptions


Gas content of the magma is low


Magma of relatively low viscosity


Gases boil out less violently


example: lava fountains associated with
Hawaiian and Icelandic volcanic eruptions


If the gases have escaped before the lava
reaches the surface the lava will simply well
out of the vent as a stream of molten rock


Effusion Rates


Highly variable, historical eruptions of
basaltic lava range from 0.5


5000 m
3
/sec


Mt. Etna averages ~ 0.5 m
3
/sec


Laki (1783) Icelandic fissure


generated ~
5000 m
3
/sec


Andesite and dacite effusion rates are much
lower, 0.5


10 m
3
/sec

Variations and Complications


There are gradations between extremely
explosive eruptions and quiet effusive
eruptions

Ground Water


Numerous possibilities and dangers exist if
ground water is present


At a depth of 200
-
300m below the Earth’s
surface pressure increases the boiling point
of water to more than 200

C.



If ground water is heated to more than
200

C and pressure is suddenly released



hot water will flash to steam in a massive
hydrothermal eruption


example: Mt. St. Helens


Any volcanic eruption that suddenly mixes
ground water and molten rock in ~ equal
quantities will produce extremely violent
eruptions.


Ground water explosions can occur at
volcanoes with viscous, gas
-
rich magma
intensifying the eruption.


It is also possible at normally effusive
volcanoes in Iceland or Hawaii.

Hydrovolcanic eruption at

Ukinrek, Alaska (1977)


Kilauea,

T.J. Takahashi,

USGS.


Submarine Eruptions


Generally effusive because overlying water
pressure retards boiling and rapid gas
expansion.


In shallow water they tend to be more
explosive because of the rapid generation of
steam from the mixing of magma and sea
water at low pressures.



Pillow basalts on the south Pacific seafloor

--

Courtesy of NOAA


Volcanic Fire


Misnomer
-

volcanic eruptions do not
involve combustion on any significant
scale.


Venting gases, or the ash cloud, are
sometimes referred to as smoke, that is not
correct.


The fire in a volcano is simply incandescent
rock.


The energy and power in a volcanic
eruption is contained in the the enormous
store of calories in the 900

-

1200


temperature of the magma.


Sudden conversion of that heat energy by
the explosive boiling of volcanic gases
(CO
2
, H
2
O, SO
2
) and adjacent ground water
or surface water generates the power.


Eruption will stop when the rapid release of
energy from the expanding gases exceeds
the rate of energy replacement from new
magma.

Repose Times


The slow accumulation of magma and the
ensuing release of volcanic energy can be
very cyclic.


Example: Kilauea


from January 1983
-

July 1986 there were 47
episodes of high lava fountaining from the
same basic vent


each episode lasted several hours, produced
10
-

15 million m
3
of lava


repose time was ~ 1 month between eruptions


Such regular recurring eruptions are the
exception.




Repose periods differ greatly between
volcanoes
-

and an individual volcano can
have different repose periods as well.


Examples:


Stromboli (Italy)


15
-
20 minutes


El Chichon (Mexico)


1000 years


Asama (Japan)


has erupted thousands of
times since its first recorded eruption in A.D.
685, since 1900 shortest period of repose was
less than a day, the longest, 5 years.

Why the Variation in Repose
Time?


Enormous diversity of volcanic types, rock
types, surface environments, stages of
volcanic growth …


Individual repose time may vary because


rate of magma movement from depth is
variable


strength of volcanic structure changes with each
eruption


Because an eruption ceases when energy is
released faster than it is replaced:


extremely violent eruptions are usually short
-
lived


steady low
-
volume flow eruptions, or a series
of small explosive eruptions can continue for
many years (Stromboli)

Explosive vs. Effusive Eruptions


Or why are some volcanoes explosive and
others are not?


Involves at least four important factors


the viscosity of the magma


the amount of dissolved gases in the magma


the suddenness with which pressure is released
as the magma nears the surface


amount of heated ground water





Viscosity of magma ranges from a liquid the
consistency of warm honey to a near solid


Low Viscosity



high temperatures


low silica content


generally occurs in basaltic magmas


fissure eruptions (Icelandic, Hawaiian
volcanism)

Mauna Loa Volcano, Hawai`i

High Viscosity


Low magma temperatures


higher silica


andesite, dacite, rhyolite magma


stratovolcanoes


below 600
-

700

C volcanic rock is
essentially solid

Colima Volcano, Mexico





The ease with which gases boil out of a
magma is largely controlled by the viscosity
of a magma


Low Viscosity


Gases escape rapidly


lava fountains


boiling lava pools

High Viscosity


Pressure of the gases forming bubbles in the
molten rock increases until it shatters the
rock



basalt, ~ 50% silica, melting T = ~ 1400
°
C


andesite ~ 60% silica, melting T= ~1100
°
C


dacite

~ 64% silica, melting T= ~1050
°
C


rhyolite ~ 70% silica, melting T= ~ 800
°
C


The source of these additional gases in
magmas associated with subduction zones
may be related to:


incorporation of seafloor sediments with high
water and CO
2

content


incorporation of large amounts of sea water


The rate at which pressure is released as a
magma rises:


slow ascent


allows gases time to come out of
solution with the magma


fast ascent


gases trapped in the magma until
the final moment
-

burst like an exploding
pressure cooker


Volcanic eruptions are extremely variable,
but they have two fundamental common
principles:


1. Gravity forces the lighter magma from within
the Earth to the surface.

2. The gentle to explosive boiling of gases
determines the type of eruption.