Photo courtesy USGS The 1980 eruption of Mount Saint Helens, in
there is a major volcanic eruption in the world, you'll see a
slew of newspaper articles and nightly news stories covering
the catastrophe, all stressing a familiar set of words --
violent, raging, awesome. When faced with a spewing volcano,
people today share many of the same feelings volcano-observers
have had throughout human history: We are in awe of the
destructive power of nature, and we are unsettled by the
thought that a peaceful mountain can suddenly become an
unstoppable destructive force!
While scientists have cleared up much of the mystery
surrounding volcanoes, our knowledge has not made volcanoes
any less amazing. In this article, we'll take a look at the
powerful, violent forces that create eruptions, and see how
these eruptions build volcanic structures like islands.
When people think of volcanoes, the first image that comes
to mind is probably a tall, conical mountain with orange lava
spewing out the top. There are certainly many volcanoes of
this type. But the term volcano actually describes a
much wider range of geological phenomena.
You & the Volcano Have you ever visited a volcano?
Generally speaking, a volcano is any place on a planet
where some material from the inside of the planet makes its
way through to the planet's surface. One way is "material
spewing from the top of a mountain", but there are other forms
Photo courtesy USGS Pahoehoe lava flowing from a volcano in
The first question this raises is: what exactly is this
"material from the inside"? On our planet, it's magma,
fluid molten rock. This material is partially liquid,
partially solid and partially gaseous. To understand where it
comes from, we need to consider the structure of planet Earth.
Graphic courtesy USGS
The earth is composed of many layers, roughly divided into
three mega-layers: the core, the mantle and the
We all live on the rigid outer crust, which is 3 to 6
miles (5 to 10 km) thick under the oceans and 20 to 44 miles
(32 to 70)thick under the land. This may seem fairly thick
to us, but compared to the rest of the planet, it's very
thin -- like the outer skin on an apple.
Directly under the outer crust is the mantle, the
largest layer of the earth. The mantle is extremely hot, but
for the most part, it stays in solid form because the
pressure deep inside the planet is so great that the
material can't melt. In certain circumstances, however, the
mantle material does melt, forming magma that makes its way
through the outer crust.
In the 1960s, scientists developed a revolutionary theory
called plate tectonics. Plate tectonics holds that the
lithosphere, a layer of rigid material composed of the
outer crust and the very top of the mantle, is divided into
seven large plates and several more smaller plates. These
plates drift very slowly over the mantle below, which is
lubricated by a soft layer called the asthenosphere.
The activity at the boundary between some of these plates is
the primary catalyst for magma production.
Graphic courtesy NASA The blue lines mark plate boundaries, the red
triangles mark active volcanoes and the yellow dots show
Where the different plates meet, they typically interact in
one of four ways:
If the two plates are moving away from each other, an
ocean ridge or continental ridge forms,
depending on whether the plates meet under the ocean or on
land. As the two plates separate, the mantle rock from the
asthenosphere layer below flows up into the void between the
plates. Because the pressure is not as great at this level,
the mantle rock will melt, forming magma. As the magma flows
out, it cools, hardening to form new crust. This fills in
the gap created by the plates diverging. This sort of magma
production is called spreading center volcanism.
At the point where two plates collide, one plate may be
pushed under the other plate, so that it sinks into the
mantle. This process, called subduction, typically
forms a trench, a very deep ditch, usually in the
ocean floor. As the rigid lithosphere pushes down into the
hot, high-pressure mantle, it heats up. Many scientists
believe that the sinking lithosphere layer can't melt at
this depth, but that the heat and pressure forces the water
(the surface water and water from hydrated minerals) out of
the plate and into the mantle layer above. The increased
water content lowers the melting point of the mantle rock in
this wedge, causing it to melt into magma. This sort of
magma production is called subduction zone volcanism.
If the plates collide and neither plate can subduct
under the other, the crust material will just "crumple,"
pushing up mountains. This process does not produce
volcanoes. This kind of boundary can develop later into a
Some plates move against each other rather than push or
pull apart. These transform plate boundaries rarely
produce volcanic activity.
here for a great diagram of plate boundaries.
Magma can also push up under the middle of a lithosphere
plate, though this is much less common than magma production
around plate boundaries. This interplate volcanic
activity is caused by unusually hot mantle material forming in
the lower mantle and pushing up into the upper mantle. The
mantle material, which forms a plume shape that is from 500 to
1000 km wide, wells up to create a hot spot under a
particular point on the earth. Because of the unusual heat of
this mantle material, it melts, forming magma just under the
earth's crust. The hot spot itself is stationary; but as a
continental plate moves over the spot, the magma will create a
string of volcanoes, which die out once they move past the hot
spot. The Hawaii volcanoes were created by such a hot spot,
which appears to be at least 70 million years old.
So what happens to the magma formed by these processes? We
saw that the magma produced at ocean ridges just hardens to
form new crust material, and so doesn't produce spewing land
volcanoes. There are a few continental ridge areas, where the
magma does spew out onto land; but most land volcanoes are
produced by subduction zone volcanism and hot spot volcanism.
When the solid rock changes form to a more liquid rock
material, it becomes less dense than the surrounding solid
rock. Because of this difference in density, the magma pushes
upward with great force (for the same reason the helium in a
balloon pushes up through the denser surrounding air and
oil pushes upward through denser surrounding water). As it
pushes up, its intense heat melts some more rock, adding to
the magma mixture.
The magma keeps moving through the crust unless its upward
pressure is exceeded by the downward pressure of the
surrounding solid rock. At this point, the magma collects in
magma chambers below the surface of the earth. If the
magma pressure rises to a high enough level, or a crack opens
up in the crust, the molten rock will spew out at the earth's
Photo courtesy USGS Flowing lava on Kilauea Volcano in
If this happens, the flowing magma (now called lava)
forms a volcano. The structure of the volcano, and the
intensity of the volcanic eruption, is dependent on a number
of factors, primarily the composition of the magma. In the
next section, we'll look at some different magma types and see
how they erupt.
Magma Volcanoes vary a great deal in their
destructive power. Some volcanoes explode violently,
destroying everything in a mile radius within minutes, while
other volcanoes seep out lava so slowly that you can safely
walk all around them. The severity of the eruption depends
mostly on the composition of the magma.
Photo courtesy USGS Gas vents from Kilauea Volcano in
You & the Volcano Have you ever visited a volcano?
first question to address is: why does the magma erupt at all?
The erupting force generally comes from internal gas pressure.
The material that forms magma contains a lot of dissolved
gases -- gases that have been suspended in the magma
solution. The gases are kept in this dissolved state as long
as the confining pressure of the surrounding rock is
greater than the vapor pressure of the gas. When this
balance shifts and vapor pressure becomes greater than the
confining pressure, the dissolved gas is allowed to expand,
and forms small gas bubbles, called vesicles, in the
magma. This happens if one of two things occurs:
The confining pressure decreases, due to decompression
from the magma rising from a higher pressure point to a
lower pressure point.
The vapor pressure increases because the magma cools,
initiating a crystallization process that enriches the gas
content of the magma.
In either case, what you get
is magma filled with tiny gas bubbles, which have a much lower
density than the surrounding magma, and so push out to escape.
This is the same thing that happens when you open a bottle of
soda, particularly after shaking it up. When you decompress
the soda (by opening the bottle), the tiny gas bubbles push
out and escape. If you shake the bottle up first, the bubbles
are all mixed up in the soda so they push a lot of the soda
out with them. This is true for volcanoes as well. As the
bubbles escape, they push the magma out, causing a spewing
The nature of this eruption depends mainly on the gas
content and the viscosity of the magma material.
Viscosity is just the ability to resist flow -- essentially,
it is the opposite of fluidity. If the magma has a high
viscosity, meaning it resists flow very well, the gas bubbles
will have a hard time escaping from the magma, and so will
push more material up, causing a bigger eruption. If the magma
has a lower viscosity, the gas bubbles will be able to escape
from the magma more easily, so the lava won't erupt as
Of course, this is balanced with gas content -- if the
magma contains more gas bubbles, it will erupt more violently,
and if it contains less gas, it will erupt more calmly. Both
factors are determined by the composition of the magma.
Generally, viscosity is determined by the proportion of
silicon in the magma, because of the metal's reaction
to oxygen, an element found in most magmas. Gas content varies
depending on what sort of material melted to form the magma.
As a general rule, the most explosive eruptions come from
magmas that have high gas levels and high viscosity, while the
most subdued eruptions come from magmas with low gas levels
and low viscosity. Volcanic eruptions don't often fall into
easy categories, however. Most eruptions occur in several
stages, with varying degrees of destructiveness.
If the viscosity and the gas pressure are low enough, lava
will flow slowly onto the earth's surface when the volcano
erupts, with minimal explosion. While these effusive
lava flows can reap considerable damage on wildlife and
manmade structures, they are not particularly dangerous to
people because they move so slowly -- you have plenty of time
to get out of the way.
Photo courtesy USGS An effusive lava flow from Pu`u `O`o Cone on
Kilauea Volcano in
If there is a good deal of pressure, however, a volcano
will begin its eruption with an explosive launch of material
into the air. Typically, this eruption column is
composed of hot gas, ash and pyroclastic rocks --
volcanic material in solid form. There are many sorts of
explosive eruptions, varying significantly in size, shape and
Within these two broad eruption categories, there are
several typical eruption varieties. The most common eruption
Plinian Eruptions: These awesome eruptions can
inflict serious damage on nearby areas -- the eruption that
buried Pompeii and Herculaneam was a Plinian eruption. They
are initiated by magma with very high viscosity and gas
content. The powerful upward thrust of the expanding gases
propels pyroclastic material as high as 30 miles (48 km) in
the air, at hundreds of feet per second. The eruption, which
can last hours or even days, produces a towering, sustained
eruption plume. This dumps a huge amount of
tephra, fallen volcanic material, on surrounding
areas (usually more to one side, depending on how the wind
blows). Additionally, a Plinian eruption can produce
extremely fast moving lava flows that destroy everything in
Photo courtesy NASA A
tall Plinian plume erupts from Klyuchevskaya Volcano in
Hawaiian Eruptions: Generally, these eruptions
are not very destructive or explosive. They don't thrust
much pyroclastic material into the air, producing instead a
relatively sluggish flow of low-viscosity, low-gas-content
lava. This flow can take a couple of different forms. The
most impressive display is the fire fountain, a
fountain of bright orange lava pouring hundreds of feet in
the air, for a few minutes or sometimes several hours. The
more typical eruption style is a steady lava flow from a
central vent, which can produce wide lava lakes,
ponds of lava forming in craters or other depressions. Lava
flows and spatter from fire fountains can certainly destroy
surrounding vegetation or trees, but the flow is usually
slow enough that people have plenty of time to make it to
safety. Hawaiian eruptions are so named because they are
common to Hawaii's volcanoes.
Photo courtesy USGS A
fire fountain erupting from Pu'u O'o Volcano in
Strombolian Eruptions: These eruptions are fairly
impressive but not particularly dangerous. They thrust small
amounts of lava 50 to a few hundred feet (15 to 90 meters)
in the air, in very short bursts. The lava has a fairly high
viscosity, so gas pressure has to build to a high level
before it will thrust the material upward. These regular
explosions can produce impressive booming sounds, but the
eruptions are relatively small. Strombolian eruptions
generally don't produce lava flows, but some lava flow may
follow the eruption. These eruptions produce a small amount
of ashy tephra.
Photo courtesy USGS A
Strombolian eruption spouting from Stromboli Volcano,
off the coast of
Vulcanian Eruptions: Like Strombolian eruptions,
these eruptions are characterized by many short explosions.
Vulcanian eruptive columns are typically larger than
Strombolian columns, however; and they are mostly made up of
ashy pyroclastic material. The explosions are initiated by
high-viscosity, high-gas-content magma in which small
amounts of gas pressure build up and thrust material into
the air. In addition to ashy tephra, Vulcanian eruptions
will also launch football-sized pyroclastic bombs
into the air. Vulcanian eruptions generally aren't
associated with lava flow.
Photo courtesy USGS A
Vulcanian column erupts from Tavurvur Volcano off the
coast of Papua New
Hydrovolcanic Eruptions: When volcanic eruptions
occur near oceans, saturated clouds or other wet areas, the
interaction of water and magma can create a unique sort of
eruptive column. Basically, the hot magma heats the water so
that it becomes steam. This rapid change of state causes an
explosive type of expansion in the water, which breaks apart
the pyroclastic material, creating a fine ash. Hydrovolcanic
eruptions vary considerably. Some are characterized by short
bursts, while others build sustained eruptive columns.
Volcanic eruptions can also melt large amounts of snow,
causing mudslides and major flooding.
Photo courtesy USGS A
hydrovolcanic eruption from Ukinrek Volcano, off the
Fissure Eruptions: Not all eruptions start with
an explosion caused by gas pressure. Fissure eruptions occur
when magma flows up through cracks in the ground and leaks
out onto the surface. These often occur where plate movement
has caused large fractures in the earth's crust, and may
also spring up around the base of a volcano with a central
vent. Fissure eruptions are characterized by a curtain of
fire, a curtain of lava spewing out to a small height
above the ground. Fissure eruptions can produce very heavy
flows, though the lava is generally slow moving.
Photo courtesy USGS A
fissure eruption "curtain of fire" on Kilauea Volcano in
These different eruption types build different sorts of
volcanoes around them. In the next section, we'll look at the
most common types of volcanoes and see how they're formed.
Shapes and Sizes Most land volcanoes have the same
basic structure, but volcano shape and size varies
considerably. There are several elements that these different
volcano types have in common are:
a summit crater - the mouth of the volcano, where
the lava exists
a magma chamber - where the lava wells up
a central vent - leads from the magma chamber to
the summit crater.
The biggest variation in volcano
structure is the edifice, the structure surrounding the
central vent. The edifice is built up by the volcanic material
spewed out when the volcano erupts. Consequently, its
composition, shape and structure are all determined by the
nature of the volcanic material and the nature of the
eruption. The three main volcano shapes are:
Stratovolcanoes: These are the most familiar type
of volcanoes, and generally have the most destructive
history of eruptions. They are characterized by a fairly
symmetrical mountain edifice, which curves steeply near the
relatively small summit crater at the top. They are usually
built by Plinian eruptions that launch a great deal of
pyroclastic material. As the lava, ash and other material
spews out, it rapidly builds the edifice around the vent.
Stratovolcanoes tend to have highly infrequent eruptions --
hundreds of years apart -- and typically form in subduction
Photo courtesy USFWS Kanaga Volcano, a stratovolcano in
Scoria cone volcanoes: These relatively small
cones are the most common volcano type. They are
characterized by steep slopes on both sides of the edifice,
which lead up to a very wide summit crater. This edifice is
composed of ashy tephra, usually spewed out by Strombolian
eruptions. Unlike stratovolcanoes, many Scoria cone
volcanoes have only one eruption event.
Photo courtesy USGS Sunset Crater, a scoria cone volcano in
Shield volcanoes: These wide, relatively short
volcanoes occur when low-viscosity lava flows out with
minimal explosiveness, such as in Hawaiian eruptions. The
lava disperses out over a wide surface area -- sometimes
hundreds of kilometers -- building up a shield-shaped dome.
Near the summit, the edifice gets a little steeper, giving
the volcano a slightly raised center. Many shield volcanoes
erupt with great frequency (every few years or so).
Photo courtesy USGS Mauna Loa, a shield volcano in
You & the Volcano Have you ever visited a volcano?
can also produce other interesting structures, such as
calderas and lava domes. Calderas, large
crater-shaped basins, form when eruptions drain a magma
chamber and the volcano edifice collapses into the empty
space. These often fill up with water, creating round lakes,
such as Crater Lake in Oregon. Lava domes form when
most of the gas vesicles escape during an initial eruption,
and the remaining viscous lava lacks the necessary pressure to
spew out and so it flows out very slowly at the summit crater.
This creates a domed plug at the top of the volcano, which may
continue to grow over time.
Photo courtesy USGS The caldera at Kaguyak Volcano, in Alaska, is
about 1.5 miles (2.5 km) in
There are a startling number of volcanoes on earth -- more
than 500 "active" volcanoes in the world, about as many
"dormant" volcanoes, and many volcanoes that have been deemed
"extinct." As it turns out, these determinations are largely
based on subjective interpretation or somewhat arbitrary
standards. The traditional criteria for this determination was
the date of the last eruption. If the last eruption fell
within historic times -- the period people have been recording
history -- the volcano was deemed active. If the last eruption
occurred before historic times but within 10,000 years, the
volcano was considered "dormant" because it likely had the
potential to erupt again. Volcanoes that had not erupted in
more than 10,000 years were considered extinct, because it
seemed unlikely they would erupt again.
Photo courtesy USGS A
1990 eruption of Redoubt Volcano in
is certainly an inexact standard. For one thing, "historic
times" is fairly vague, and varies from culture to culture.
Additionally, different volcano types have widely varying
eruption frequencies. Scientists generally use a more sensible
criteria these days, though it's based mostly on subjective
assessment. If the volcano is erupting or demonstrating
activity in the form of earthquakes or gaseous emissions, it
is considered active. If the volcano is not showing any signs
of activity, but has erupted within the last 10,000 years and
has the potential to erupt again, it is considered dormant. If
it has not erupted in 10,000 years or has clearly exhausted
any magma supply, the volcano is considered extinct.
Of the 500 or so active volcanoes, around 10 are erupting
on any given day. For the most part, these eruptions are small
and well-contained, so they don't threaten life and limb. From
time to time, however, we get a major eruption that either
takes lives or, more often, devours property. And while not as
catastrophic as life-threatening eruptions, these destructive
events can certainly take a heavy financial toll on the
There have been, in recorded history, dozens of extremely
catastrophic volcanic eruptions -- one may even have wiped out
an entire civilization. In fact, in just the last 200 years
there have been 19 eruptions that have killed more than 1,000
people. Volcanic activity has certainly played a significant
and destructive role in our history, and will continue to do
so in the future.
This is only half the story, however. As destructive as it
is, volcanic activity is one of the most important,
constructive geological processes on Earth. After all, as we
saw when we looked at plate tectonics, volcanoes are
constantly rebuilding the ocean floor. As with most natural
forces, volcanoes have a dual nature. They can wreak horrible
devastation, but they are also a crucial element of the
earth's ongoing regeneration. They are certainly one of the
most amazing, awe-inspiring phenomena on the planet.