The processes by which molten rock material, or magma, rises from the
interior of the earth on to or towards its surface, and by which
associated gases are released into the atmosphere are called Volcanism.
The study of these processes, and of the structures, deposits, and
landforms they create is called volcano logy.
and gases exploit weak zones in the earth's outermost layer, the
lithosphere, in order to reach the surface. Such weaknesses are found
primarily along the boundaries between the earth's tectonic plates, and
this is where most volcanism occurs. Where magma and gases do reach the
surface, through vents or fissures in the earth's crust, they form
geological structures known as volcanoes, of which there are several
types. The classic picture of a volcano, exemplified by Mount Fuji in
Japan or Mount Mayon in the Philippines, is of a conical structure with
(crater) at the top, from
which (in the case of active volcanoes) ash, steam, gases,
molten rock, and solid fragments erupt, often explosively.
In fact, volcanoes of this type, though not uncommon,
account for less than 1 percent of the earth's volcanic
least 80 per cent of volcanism takes place through lengthy vertical
fissures in the earth's crust. Such fissure volcanism occurs
predominantly along the constructive boundaries between the plates into
which the lithosphere is divided. Oceanic ridges where new lithosphere
is continuously being created and the plates pushed apart mark
constructive boundaries. Indeed, it is the rising, cooling magma
produced by fissure volcanism that makes the new ocean floor. Most of
the world's volcanism therefore takes place unseen, beneath the oceans.
or continental, volcanism is much less important than sub-oceanic
volcanism in terms of the volume of magma ejected, but much more is
known about it because it is visible and directly affects human beings.
It has been known since ancient times that volcanic activity ranges from
violent explosions to the gentle extrusion of magma, which becomes known
as lava when it is on the earth's surface.
volcanism is mostly associated with oceanic ridges, but it also occurs
on land, and in some cases has led to spectacular results. Fissure
volcanoes emit large volumes of very fluid material, which spreads out
to cover large areas; successive eruptions can build up great plains or
plateaux. Today, fissure volcanoes are probably best seen in Iceland,
which straddles the Mid-Atlantic Ridge. However, fissure volcanism on
land is most associated with the past, with the great plateaux to be
found on most continents. Plateau basalts, flood basalts, or
ignimbrites, as they are called, have formed, among others, the Deccan
Plateau of west-central India; the Paraná Basin of southern Brazil,
Argentina, and Uruguay; the Columbia Plateau of the north-western United
States; the Drakensberg Plateau of South Africa; and the central plateau
of the North Island, New Zealand.
majority of surface volcanic activity, however, is associated with
more-or-less circular vents, or clusters of vents, in the earth's crust,
rather than fissures. These vents give rise to central volcanoes, of
which there are two basic types. The steep-sided conical volcano
mentioned above is occasionally constructed entirely from solid
material, or tephra, which ranges in size from ash and cinders to rocks
and boulders. The tephra have been ejected explosively in an eruption,
or series of eruptions, and have fallen back to the ground in the
immediate vicinity of the crater, the external outlet of the vent. A
well-known example of such a volcano is Parícutin, in Mexico, which
first erupted in a field on February 20, 1943, and within six days had
built a cinder cone 150 m (492 ft) high; by the end of the year the cone
was more than 336 m (1,100 ft) high.
very few conical volcanoes eject only tephra in every eruption, to
become cinder-cone volcanoes. Lava is likely to be extruded in some
eruptions, in which case the resulting volcanic structure will comprise
alternating layers of tephra and lava. Such volcanoes are called
composite volcanoes, or strato-volcanoes. Most of the world's largest
and best-known volcanoes, including Stromboli and Vesuvius in Italy,
Popocatépetl in Mexico, Cotopaxi in Ecuador, and Kilimanjaro in
Tanzania, as well as Fuji and Mayon, are of this type. Although most
conical and near-conical volcanoes generally have a single central vent,
this does not preclude volcanic material sometimes emerging from
secondary, often temporary, vents on the flanks of a volcano.
other main type of central volcano is the shield volcano. These are very
large structures, which can be up to many tens of kilometres in
diameter, and which have relatively gentle slopes, generally of less
than 12°. They are usually formed by hundreds of outpourings of fluid,
basaltic lava. Shield volcanoes often have more than one vent, as well
as fissures along their sides. This is particularly true of the largest
shield volcanoes, notably those of the Hawaiian Islands in the North
Pacific. The Hawaiian Islands are a complex of shield volcanoes rising
from the ocean floor; Mauna Loa, on the island of Hawaii, is one of the
most recently formed. It is considered to be the most massive mountain
on earth, rising more than 10,000 m (32,800 ft) from the seabed. In
Europe, Mount Etna is a shield volcano.
Surface Volcanoes and Plate
volcanoes are often associated with the destructive boundaries formed by
tectonic plates, which are moving together. When two plates converge,
the leading edge of one plunges down beneath that of the other towards
the mantle, the semi-molten layer which underlies the lithosphere. This
causes subduction, the re-incorporation of the rocks of the lithosphere
into the mantle. Occasionally the leading edges of converging plates are
both composed of oceanic lithosphere; more commonly one is composed of
oceanic lithosphere, the other of continental crust. Because the latter
is thicker and less dense, it is the oceanic lithosphere, which is
oceanic crust melts as a result of subduction, the magma formed rises
upwards along the plane of subduction to erupt on the earth's
surface—usually on the landward side of the destructive boundary,
which is normally marked by oceanic trenches. Where the magma erupts on
land, it leads to the creation of long mountain chains, notably the
Andes of South America and the Cordillera of North America, which
includes the Cascade Range and the Rocky Mountains. Where the volcanism
caused by subduction occurs in the ocean, long, arc-shaped chains of
volcanic islands are produced, such as Japan and the Philippines.
majority of the earth's subduction zones lie around the edge of the
Pacific Ocean, and so do more than three-quarters of active, dormant, or
extinct surface volcanoes. They form a belt known as the “Ring of
Fire”, along which earthquakes are also common. The Ring of Fire
extends through the Andes, the Cordillera, the Aleutian Islands, the
Kamchatka Peninsula in eastern Siberia, the Kuril Islands, Japan, the
Philippines, Sulawesi, New Guinea, the Solomon Islands, New Caledonia,
and New Zealand.
crater, which has emitted volcanic material, often remains a depression,
even when the volcano is dormant, as a result of lava sinking back into
the volcanic vent. Sometimes the lava sinks so far that the top of the
volcano collapses into the vent, forming a much larger depression known
as a caldera, which can be several kilometres in diameter. Calderas can
also be formed by very violent explosions that “blow the top off”
the volcano in question, for example, Krakatau in Indonesia (see below).
Over time, calderas of dormant or extinct volcanoes can become filled up
with water to form crater lakes. The best known is probably Crater Lake
in Oregon, United States. Some 8 km (5 mi) in diameter, it was formed by
the collapse of a prehistoric composite volcano, Mount Mazama.
Tephra = Volcanic Materials.
most active or potentially active volcanoes lies a magma chamber
containing molten rock. The magma it contains probably originated in the
asthenosphere, the mobile layer immediately below the earth's
lithosphere; the chamber is a “staging post” on its way to the
surface. When the magma reaches the surface, however, it can be in
liquid, solid, or gaseous form.
magmas contain dissolved gases, such as carbon dioxide and sulphur
dioxide, which are released during the severe pressure reduction that
occurs as the magma rises towards the surface. The release can be very
sudden, taking place with an explosive force that can shatter the magma
and send it skyward as tephra, and molten or semi-molten fragments that
cool to a greater or lesser extent as they fall back to the ground.
Tephra range in size from very small dust and ash particles, which can
be carried vast distances by wind, to boulders weighing over 100 tons.
Associated with intense eruptions, these boulders can be thrown several
kilometres from the vent. In some less intense eruptions, volcanic
fragments are not blown upwards. Instead, mixed in deadly combination
with hot gases, they flow along the ground as a nude ardent (French,
“glowing cloud”), which smothers and destroys everything in its
volcanoes never erupt explosively but quietly extrude magma on to the
ground. Such eruptions are caused by extremely fluid basaltic magma,
which contains little silica or gases. They are mostly associated with
fissure volcanism and shield volcanoes, such as those of Hawaii. The
more silica that magma contains the more viscous (sticky and
slow-moving) it becomes. It is harder for gases to escape from viscous
magma, so increasing viscosity is usually associated with more explosive
Types of Eruptions.
volcano is capable of erupting in several ways, but certain types of
eruptions tend to be associated with particular volcanoes. This is
reflected in the classification of volcanic eruptions, with each
category bearing the name of a typical volcano. Fissure and shield-type
eruptions are usually categorized as Icelandic and Hawaiian
respectively. More explosive eruptions are categorized, on a scale of
increasing viscosity of magma, as Strombolian, Vulcanian (after Vulcano
in the Lipari Islands, Italy), Vesuvian, Plinian, and Peléean (after
Mount Pelée, Martinique). Vesuvian, Plinian (a more violent form of
Vesuvian), and Peléean are the most paroxysmal in character, expelling
large amounts of ash as well as lava blocks. Peléean eruptions are
particularly associated with nuées ardentes. The May 8, 1902, eruption
of Mount Pelée destroyed the city of Saint-Pierre and killed about
30,000 people; the majority died from the effects of nuées ardentes and
most violent eruptions tend to be associated with destructive plate
boundaries. The two greatest volcanic explosions in recorded history—Krakatau
(or Krakatoa) and Mount Tambora—occurred at the juncture of the Asian
and Australian plates. Tambora, on the northern coast of Sumbawa island,
erupted in 1815, blowing off its top half and killing an estimated
50,000 islanders. The volcanic island of Krakatau, lying between Java
and Sumatra in Indonesia, erupted in 1883, destroying two-thirds of its
land area. The resulting tidal waves caused the deaths of tens of
thousands of people throughout South East Asia. The noise the explosion
created travelled more than 4,830 km (3,000 mi), while the millions of
tons of ash it threw into the earth's atmosphere caused spectacular
sunsets around the world for more than a year.
contrast to explosive eruptions, which have killed many thousands of
people throughout history, Icelandic and Hawaiian, and to some extent
Strombolian, types are seldom a hazard. The lava can flow rapidly, but
it often flows slowly enough to enable people to get out of the way;
property often suffers, however. Occasionally, it has proved possible to
divert flowing lava away from buildings by digging culverts, building
retaining walls, or even blowing the lava up, but such methods are
seldom completely successful.
usually emerges from the earth at temperatures of 800° to 1,200° C
(1,472° to 2,192° F). It then cools as it flows, solidifying from the
outside inwards until it becomes completely solid in the form of a lava
flow. Depending largely on the viscosity of the original magma, lava
flows have different forms and surface textures. The three main types
are referred to as pahoehoe, aa, and block.
lava comes from very thin and mobile magma. On reaching the ground the
magma quickly forms a thin, plastic surface layer that gets dragged into
rope-like folds by the molten lava continuing to flow beneath it. The
second type, aa lava, is produced from a less mobile magma, which, as it
cools, acquires a thick, hard skin. This skin is broken up by the lava
flowing beneath to form a fragmented, jagged surface. Block lava is also
fragmented, but its surface is smoother. Not all of the gas within the
magma always escapes to the atmosphere during an eruption. Some can
remain, trapped in cavities known as vesicles. These vesicles can
persist even after the magma has become solid lava. Pumice is a highly
vesicular lava; indeed, some has so many vesicles that it is light
enough to float on water.
even tephra can fuse together on the ground, to form what is known as a
tuff. The material from a nuée ardente may also consolidate to form an
ignimbrite. Tuffs and ignimbrites are therefore composite rocks made up
of a wide variety of volcanic fragments.
formed from magma that has cooled and solidified are known as igneous
rocks. A lava flow on the earth's surface is an igneous rock, but there
are also other forms. Some magma does not reach the surface at all.
Instead it is diverted into natural underground cavities, or breaks off
blocks of the surrounding (country) rock to make its own niches.
Occasionally, magma may simply be so hot that parts of the country rock
melt and flow away.
that enters subsurface openings usually solidifies there to form
intrusions, often of great size. A sill is a horizontal ledge-like
intrusion lying between beds of layered rock. Examples include the
Salisbury Crags in Edinburgh, Scotland, and the Palisades sill along the
west bank of the Hudson River, near New York. A laccolith also lies
between rock beds. It is formed when the pressure of the original magma
pushes the overlying rock upwards to form a central dome, creating a
mushroom-shaped intrusion. A lopolith is a saucer-shaped intrusion
formed when magma enters between beds of folded, layered rock; a
phacolith is shaped like an upturned saucer.
a volcano becomes extinct, or merely dormant, any magma remaining in the
vent may solidify to form a volcanic plug. If the surrounding rock then
erodes away, the plug will become exposed to form a conspicuous
landscape feature. The Castle Rock in Edinburgh, Scotland, is a volcanic
plug. If volcanism takes place through a vertical fissure rather than a
cylindrical vent, magma solidifying in the fissure leaves a vertical,
sheet-like intrusion known as a dyke. Probably the most impressive
example is the mineral-rich Great Dyke in Zimbabwe, which runs some 480
km (298 mi) roughly north-south through the centre of the country.
volcanic activity occurs along tectonic plate boundaries because that is
where the lithosphere is weakest. However, some volcanism occurs away
from plate margins, for reasons that are sometimes clear and sometimes
not. There are volcanoes in the vicinity of the East African Rift
Valley, for example, notably Kilimanjaro. This is understandable because
the Rift Valley is a zone in which the continent has begun to split
apart and where, in the future, even larger amounts of magma may be
expected to rise.
presence of 10,000 or more undersea volcanoes on the Pacific Ocean
floor, however, long defied explanation. Known as seamounts, most,
though far from all, are now extinct. The vast majority appears to be
scattered randomly around the ocean floor, but some clearly form linear
chains, for example, the Hawaiian-Emperor chain. Their presence away
from destructive boundaries has now been explained. Within the earth's
mantle there are thin vertical plumes of hot magma, probably rising from
the core, which remain fixed in position as the tectonic plates move
overhead. These plumes create “hot spots” in the lithosphere above,
where volcanic activity occurs. The site of such volcanism moves as the
plates move. The Hawaiian-Emperor hot spot, for example, is now at the
Hawaiian end of the chain; the volcanic islands in the chain get
progressively older with distance from the island of Hawaii.
not all the volcanic hot spots resulting from mantle plumes lie beneath
the oceans. An example of a continental hot spot is Yellowstone National
Park in the United States. There are no volcanic eruptions at
Yellowstone today, but there is abundant heat, which has generated hot
water and rising water-jets known as geysers.
Volcanism as Hazard.
millions of people in the world are at risk from volcanic eruptions,
especially explosive ones. Some of those millions actually live on
volcanic slopes. Why do they do it, when the danger is so great? The
primary reason is that the soils generated by the breakdown of volcanic
products from previous eruptions are highly fertile, and have thus long
attracted high populations. Many volcanic danger zones are ancient
centres of civilization and continue to be areas of dense settlement.
Volcanoes will therefore continue to take their toll, as, for example,
Mount Pinatubo did in 1991. Located north of Manila, it erupted in June
and July of that year, throwing millions of tons of ash into the air,
which combined with tropical rainfall to produce massive mudslides. An
estimated 550 people died and 650,000 lost their livelihood. The
Pinatubo eruption also highlights the dangers of thinking that a volcano
is inactive or extinct. Mount Pinatubo had not previously erupted for
more than 600 years. More than three million people continue to live in
the Naples area, although it is known that Vesuvius will certainly erupt
again suddenly one day. The last significant eruption was in 1906, but
by the mid-1990s there were early indications that Vesuvius is
volcano is a fissure or vent through which molten rock material, or
magma, and gases from the interior of the earth erupt on to its surface,
and the landform, which is produced as a result of this eruption. The
word “volcano” derives from Vulcano, one of the volcanic Lipari
Islands in the Mediterranean Sea, and the place where, according to
Roman mythology, Vulcan, the god of fire, kept his forge. The processes
that create volcanoes and other volcanic structures are called
landforms, volcanoes are formed by the deposition of the magma that
flows or is ejected, normally from one or several circular vents, as
molten or solid material. Molten magma is known as lava when it reaches
the earth's surface; the solid material—classified as dust, ash,
cinders, and bombs depending on size and shape, is called tephra.
Volcanoes which form round, circular vents are known as central
volcanoes; the basin-like mouth of the vent is known as the crater. Most
volcanoes tend to be conical in shape; some, however, are much larger
structures with very gentle slopes. Often covering many square
kilometres, they are known as shield volcanoes.
volcanoes are much more active than others. A few may be said to be in a
state of permanent eruption, at least during the geological present.
Stromboli, in the Lipari Islands, has been constantly active since
ancient times; Izalco, in El Salvador, has been active since it first
erupted in 1770. Other constantly active volcanoes are found in a belt,
called the Ring of Fire, that encircles the Pacific Ocean.
other volcanoes, such as Vesuvius, in Italy, continue in a state of
moderate activity and then become quiescent, or dormant, for periods
ranging from months to centuries. The eruption that succeeds prolonged
dormancy is usually violent. This was the case with the 1980 eruption,
after 123 years of quiescence, of Mount St Helens in Washington State,
United States. The massive eruption of Mount Pinatubo in the Philippines
during June 1991 came after more than 600 years of dormancy.
a long period after it has ceased to erupt either lava or tephra, a
volcano continues to emit acid gases and vapour in what is called the
fumarolic stage. After this phase, hot springs may arise from the
volcano. Examples of this type of activity include the geysers of
Yellowstone National Park in the United States, and of the central area
of the North Island of New Zealand. Eventually, the last traces of
volcanic heat may disappear; springs of cold water may issue from the
volcano and from the ground in its vicinity.
becoming inactive, a volcano is progressively reduced in size as a
result of weathering and erosion. Finally, the cone may be obliterated,
leaving only a volcanic pipe—a chimney filled with lava or tephra, and
extending from the earth's surface down to the former magma reservoir
under the volcano. The diamond-rich mines of South Africa are found in