Volcanoes
What determines whether a volcano extrudes
magma violently or gently?
The primary factors include the magma’s
composition, its temperature, and the
amount of dissolved gases it contains. To
varying degrees these factors affect the
magma’s viscosity. The more viscous (thicker)
the material, the greater its resistance to flow.
For example, syrup is more viscous than water.
The effect of temperature on viscosity is easily
visualized. Just as heating syrup makes it more
fluid (less viscous), the mobility of lava is
strongly influenced by temperature changes.
As a lava flow cools and begins to congeal, its
mobility decreases and eventually the flow
halts.
But more significant to volcanic behavior is the
chemical composition of magmas. A major
difference among various igneous rocks is
their silica (SiO2) content. The same is true of
the magmas from which rocks form. Magmas
that produce basaltic rocks contain about 50
percent silica, whereas magmas that produce
granitic rocks contain over 70 percent silica.
Note that a magma’s viscosity is directly related to
its silica content. In general the more silica in
magma, the greater is its viscosity. The flow of
magma is impeded because silicate structures
link together into long chains, even before
crystallization begins. Consequently, because of
high silica content, granitic lavas are very viscous
and tend to form comparatively short, thick
flows. By contrast, basaltic lava, which contain
less silica, tend to be more fluid.
In Hawaiian eruptions, the magmas are hot and
basaltic, so they are extruded with ease. By
contrast, highly viscous granitic magmas are more
difficult to force through a vent. On occcasion,
the vent may become plugged with viscous
magma, which results in a buildup of gases and a
great pressure increase, so a potentially explosive
eruption may result. However, a viscous magma
is not explosive by itself. It is the gas content that
puts the bang into a violent eruption.
Pahoehoe lava is characterized by a smooth, billowy, or ropy
surface. A ropy surface develops when a thin skin of cooler
lava at the surface of the flow is pushed into folds by the
faster moving, fluid lava just below the surface.
Pahoehoe lava flow.
There are three types of lava and lava flows:
pillow, pahoehoe (pronounced pah-hoy-hoy),
and aa (pronounced ah-ah) . Pillow lavas are
volumetrically the most abundant type
because they are erupted at mid-ocean ridges
and because they make up the submarine
portion of seamounts and large intraplate
volcanoes, like the Hawaii-Emperor seamount
chain. Pahoehoe is the second most abundant
type of lava flow.
Aa is characterized by a rough, jagged, spinose, and
generally clinkery surface. Aa flows advance much like
the tread of a bulldozer. This photo is looking across an
aa channel. Note the character of the aa that makes the
wall of the channel.
Dissolved gases in magma provide the force that
extrudes molten rock from the vent. These gases
are mostly water vapor and carbon dioxide. As
magma moves into a near-surface environment,
such as within a volcano, the confining pressure
in the uppermost portion of the magma body is
greatly reduced. This reduced confining pressure
allows dissolved gases to be released suddenly,
just as opening a soda bottle allows dissolved
carbon dioxide gas to bubble out of the soda.
At high temperatures and low, near-surface pressures,
these gases will expand to occupy hundreds of times
their original volume. Very fluid basaltic magmas allow
the expanding gases to bubble upward and escape
from the vent with relative ease. As they escape, the
gases will often propel incandescent lava hundreds of
meters into the air, producing lava fountains. Although
spectacular, such fountains are mostly harmless and
not generally associated with major explosive events
that cause great loss of life and property. Rather,
eruptions of fluid basaltic lavas, such as those that
occur in Hawaii, are relatively quiet.
At the other extreme, highly viscous magmas
impede the upward migration of expanding
gases. The gases collect in bubbles and
pockets that increase in size and pressure until
they explosively eject the semimolten rock
from the volcano. The result is a Mount St.
Helens or Mt. Pinatubo.
Mount St. Helens before
and after.
To summarize, the viscosity of magma, plus the
quantity of dissolved gases and the ease with
which they can escape, determines the nature
of a volcanic eruption. We can now
understand the gentle volcanic eruptions of
hot, fluid lavas in Hawaii and the explosive,
violent, dangerous eruptions of viscous lavas
in some volcanoes.
The gases in highly viscous magmas become
superheated, and upon release they expand a
thousandfold as they blow pulverized rock,
lava, and glass fragments from the vent. The
particles produced by these processes are
called pyroclastics (meaning “fire fragments”).
These ejected lava fragments range in size
from very fine dust and sand-sized volcanic
ash, to large pieces.
The fine ash particles are produced when the
extruded lava contains so many gas bubbles that
it resembles the froth flowing from a newly
opened bottle of champagne. As the hot gases
expand explosively, the lava is disseminated into
very fine glassy fragments. When the hot ash
falls, the glassy shards often fuse to form welded
tuff. Sometimes the frothlike lava is ejected in
larger pieces called pumice. This material has so
many voids (air spaces) that it is often light
enough to float in water.
Pumice
Pyroclastics the size of walnuts, called lapilli (“little
stones”) and pea-sized particles called cinders are
also very common. Cinders contain numerous
voids and form when ejected lava blocks are
pulverized by escaping gases. Particles larger than
lapilli are called blocks when they are made of
hardened lava and bombs when they are ejected
as incandescent lava. Since volcanic bombs are
semimolten upon ejection, they often take on a
streamlined shape.
Volcanic bomb
Successive eruptions from a central vent result
in a mountainous accumulation of material
known as a volcano. Located at the summit of
many volcanoes is a steep-walled depression
called a crater. The crater is connected to a
magma chamber via a pipelike conduit, or
vent. Some volcanoes have unusually large
summit depressions that exceed one
kilometer in diameter and are known as
calderas.
When fluid lava leaves a conduit, it is often
stored in the crater or caldera until it
overflows. On the other hand, viscous lava
forms a plug in the pipe. It rises slowly or is
blown out, often enlarging the crater.
However, lava does not always issue from a
central crater. Sometimes it is easier for the
magma or escaping gases to push through
fissures on the volcano’s flanks.
The eruptive history of each volcano is unique.
Consequently, all volcanoes are somewhat
different in form and size. Nevertheless,
volcanologists recognize three general
eruptive patterns and characteristic forms:
shield volcanoes, cinder cones, and composite
cones.
Mauna Kea, Hawaiʻi, a shield volcano on the Big Island of Hawaii.
When fluid lava is extruded, the volcano takes
the shape of a broad, slightly domed structure
called a shield volcano. They are so called
because they roughly resemble the shape of a
warrior’s shield. Shield volcanoes are built
primarily of basaltic lava flows and contain
only a small percentage of pyroclastic
material.
Cinder cones are built from ejected lava
fragments. Loose pyroclastic material has a
high angle of repose (between 30 and 40
degrees), the steepest angle at which the
material remains stable. Thus volcanoes of
this type have very steep slopes. Cinder cones
are rather small, usually less than 300 meters
high, and often form as parasitic cones on or
near larger volcanoes. In addition, they
frequently occur in groups.
Cinder cone volcano
Composite Cones
Earth’s most picturesque volcanoes are
composite cones. Most active composite
cones are in a narrow zone that encircles the
Pacific Ocean, appropriately named the Ring
of Fire.
A composite cone or stratovolcano is a large,
nearly symmetrical structure composed of
interbedded lava flows and pyroclastic
deposits, emitted mainly from a central vent.
Just as shield volcanoes owe their shape to the
highly fluid nature of the extruded lavas, so
too do composite cones reflect the nature of
the erupted material.
Composite cones are produced when relatively
viscous lavas of andesitic composition are
extruded.
Composite cone (stratovolcano)
St. Augustine volcano, Alaska. Composite cone.
A composite cone may extrude viscous lava for long
periods. Then, suddenly, the eruptive style
changes and the volcano violently effects
pyroclastic material. Most of the ejected
pyroclastic material falls near the summit,
building a steep-sided mound of cinders. In time,
this debris becomes covered by lava.
Occasionally, both activities occur simultaneously,
and the resulting structure consists of alternating
layers of lava and pyroclastics.