Why Do Volcanoes Erupt?


22 févr. 2014 (il y a 3 années et 4 mois)

93 vue(s)

Why Do Volcanoes Erupt?

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

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

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


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

Mt. Etna averages ~ 0.5 m

Laki (1783) Icelandic fissure

generated ~
5000 m

Andesite and dacite effusion rates are much
lower, 0.5

10 m

Variations and Complications

There are gradations between extremely
explosive eruptions and quiet effusive

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


If ground water is heated to more than

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

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)


T.J. Takahashi,


Submarine Eruptions

Generally effusive because overlying water
pressure retards boiling and rapid gas

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


volcanic eruptions do not
involve combustion on any significant

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

The fire in a volcano is simply incandescent

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



temperature of the magma.

Sudden conversion of that heat energy by
the explosive boiling of volcanic gases
, H
) 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

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

15 million m
of lava

repose time was ~ 1 month between eruptions

Such regular recurring eruptions are the

Repose periods differ greatly between

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


Stromboli (Italy)

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

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

strength of volcanic structure changes with each

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

extremely violent eruptions are usually short

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

Mauna Loa Volcano, Hawai`i

High Viscosity

Low magma temperatures

higher silica

andesite, dacite, rhyolite magma


below 600


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

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

andesite ~ 60% silica, melting T= ~1100


~ 64% silica, melting T= ~1050

rhyolite ~ 70% silica, melting T= ~ 800

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


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

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.