We frequently hear about earthquakes that have violently shaken some part or other of a distant country, often causing considerable damage. Fortunately we rarely feel the tremors in Britain although in 1984 some parts of the country did. Also, from time to time we hear of volcanic eruptions that are taking place in which huge quantities of molten lava or equally huge quantities of cinders and ash, together with enormous clouds of steam and gases are thrown out of the earth's surface. Hundreds of such eruptions have taken place in different parts of the world in historic times but we only remember the names of the really spectacular ones. The most recent big eruption occurred in Washington state in the USA when, in 1980, Mount St Helens suddenly erupted, although a few hours before this happened an earthquake shook the surrounding area.
It is estimated that the explosion that took place inside the volcano was equal to 10 million tonnes of TNT and that this blasted away 6 km2 of rock from the top of the volcano. The rock that was blasted away disintegrated into small pieces which formed a gigantic mass of ash and dust, some of which reached a height of over 18 km as the explosion and hot gases towered into the atmosphere. The enormous heat melted millions of tonnes of ice and snow and the water from this mixed with the ash forming mudflows which swept across the countryside blocking rivers, burying homes and smashing down forests. Very little lava was ejected from the volcano. An enormous cloud of ash was caught by the westerly winds and blown right across the USA and out over the Atlantic. So large was the cloud, which stayed in the atmosphere for many months, that it was thought to have had a profound affect on the weather in parts of the northern hemisphere. A photograph taken at the time of the explosion clearly shows a large part of the volcano being blasted away.
Movements within the earth usually produce several phenomena which often form a chain reaction. This was shown in the Mount St Helens eruption. Slow movement along tectonic plates eventually resulted in a sudden movement (the earthquake) which was followed by the explosion and eruption. This in turn gave rise to mudflows and flooding by some rivers, forest fires and the production of a huge cloud of ash which affected subsequent weather patterns.
Helgafell eruption
Quite a different volcanic eruption occurred on the island of Heimaey in Iceland
in 1973. The Helgafell volcano on this island had been dormant (had no erupted)
for a very long time, but in 1973 great rumbles were heard which were followed
by the opening of a long fissure (deep crack) in the surface of the island.
Large quantities of molten lava and black ash poured out of the fissure and
spread out over much of the island. The fissure extended beneath the sea and
the molten lava caused the water to boil. This eruption was a lava eruption.
The lava flowed freely. This type of lava is called basalt. It does not build
steep-sided conical mountains commonly associated with volcanoes. The Helgafell
eruption, like others in Iceland before it, resulted from the slow movement
apart of the North American and Eurasian plates. The Helgafell volcano is located
in a zone of weakness known as the Mid-Atlantic Ridge while Mount St Helens
is located in another zone of weakness that rings the Pacific Ocean. This zone
is called the Pacific Ring of Fire. The two zones are quite different.
We will now look at other aspects of vulcanicity.
This refers to all the various ways by which molten rock and gases are forced into the earth's crust and on to its surface. Volcanicity therefore includes volcanic eruptions (the formation of volcanoes and lava plateaus and geysers), and the formation of volcanic features such as batholiths, sills and dykes, etc., in the crust.
Rocks below the crust have a very high temperature, but the great pressure exerted on them by the crust, keeps the rocks in a semi-solid state. Friction along rock surfaces at the boundaries of tectonic plates raises the temperature, and this, plus a reduction in pressure caused by faulting and folding associated with the movement of tectonic plates, causes these rocks to become molten and semi-fluid. Such rocks are called magma. As the magma rises, it forces its way into the cracks of the crust. The magma may stay in the crust where it forms batholiths, sills and dykes, or it may reach the surface either quietly, or with great violence. If the magma contains a lot of gases, especially steam, then as the magma approaches the surface, the pressure on the gases is reduced. This causes the gases to expand rapidly which gives rise to violent explosions. When magma reaches the surface it loses its gases and is called lava.
This is a very large mass of magma which accumulates in the crust. Sometimes it forms the root or core of a mountain. Batholiths are made of granite and they form surface features only after they have been exposed by denudation. Batholits are exposed at the suface in south-west England where they form Dartmoor, Bodmin Moor, Land's End and the Scilly Isles. These are the cores of an ancient mountain chain the tops of which have long since been removed by erosion.
When a sheet of magma lies along a bedding plane it forms a structure called a sill. Some sills form ridge-like escarpments when exposed by erosion. The Great Whin Sill in northern England was quickly recognised by the Romans as being useful for defence and on it they built Hadrian's Wall, large stretches of which remain to this day. The point at which a river crosses a sill is sometimes marked by a waterfall. High Force in Durham county is formed by the River Tees where it crosses the Great Whin Sill.
When a mass of magma cuts across bedding planes, it forms a wall-like feature called a dyke. Sometimes the rocks on either side of a dyke are more resistant to erosion. When this happens, the dyke forms a depression. Dykes sometimes occur in swarms, as in Arran and parts of western Scotland and northern Ireland.
Magma sometimes reaches the surface through a vent (hole), or a fissure (crack) in the surface rocks. When magma emerges at the surface it is called lava. If lava emerges via a vent, it usually builds up a volcano, which is a cone-shaped mound. If it emerges from a fissure, it may build up a lava plain, or a lava plateau. Volcanic eruptions also take place on some ocean floors.
The mound of a volcano is called the cone and this may consist of lava, or a mixture of lava and rocks torn from the crust by the molten magma, or it may consist of ash and cinders (small fragments of lava).The shape and size of the cone largely depend on the nature of the cone's material, and the type of eruption. The channel through which the lava rises is called the pipe, and the exit of the pipe, which is usually a shallow depression, is called the crater. Kilimanjaro has a well-formed, almost circular crater.
Lava is blown to great heights when it is violently ejected, and it breaks into small fragments which fall back to earth and build up a cone.Good examples of ash and cinder cones are Volcano De Fuego (Guatemala) and Paricutin (Mexico).
Lava cones
The slope of a volcanic cone depends on whether the lava forming it was fluid
or viscous when it was molten. A fluid lava builds a gentlysloping cone, e.g.
Helgafell volcano in Iceland. The most famous of all lava cones of this type
is Mauna Loa in Hawaii. This reaches a height of 9100 m on a base of diameter
400 km on the sea bed.
Mont Pelée eruption
Viscous lava produces a steeply-sloping cone. Sometimes the lava is
so viscous that when it is forced out of the volcano it forms a
spine or plug dome that
may completely block the vent. Mont Pelée in Martinique in the West
Indies was an excellent example of a spine or plug dome. A few weeks after
Mont Pelée started to erupt in 1902, violent explosions took place
and molten lava forced its way into the base of the water-filled crater.
The crater
lake boiled and its water spilled over the rim of the crater, swept down
one side of the volcano turning into a mudflow as it went. Later clouds
of ash
and gases burst from the volcano followed by an immense outpouring of frothy
lava which advanced down the slopes at over 150 km per hour. Within minutes
St Pierre, the port and capital of Martinique was overwhelmed causing the
deaths of over 30 000 people. In the autumn of 1902 the plug of lava in
the volcano's
pipe was forced out to form a plug dome over 300 m high. By the middle
of the following year the spine had broken into pieces and disappeared.
There
are
several plug domes in the Hoggar Mountains in Algeria.
This type of cone is formed of alternate layers of lava and ash. The volcano begins each eruption with great violence forming a layer of ash. As the eruption proceeds, the violence ceases and lava pours out forming a layer on top of the ash. Lava often escapes from the sides of the cone where it builds up small conelets. Mount Kilimanjaro, in Tanzania with a height of 5895 m, and Vesuvius, Etna and Stromboli (Video1 , Video2, Video3, Video4), all in Italy, are examples of composite cones.
A composite cone sometimes has its top blown off by violent volcanic explosions and the top disintegrates into a mass of rocks and ash, leaving the crater greatly enlarged. The huge crater-like depression is now called a caldera. A caldera may also form through subsidence. After a major eruption the supply of magma is depleted causing a huge chasm to form beneath the volcano. The weight of the cone sometimes causes faults to develop and in time the whole cone collapses into the chasm beneath. Some calderas are formed in part by subsidence.
A caldera may become the site of a lake, e.g. Crater Lake in the USA. Radio-carbon dating of the carbonised wood from trees killed by the eruption that produced the crater now occupied by Crater Lake, puts the eruption that blew off the top of the volcano at about 7000 years ago. The outer slopes of the caldera walls bear evidence that the volcano supported glaciers and from this evidence it has been calculated that the cone must have been about 3500 m high. This means that more than 65 km2 of volcanic cone must have been destroyed in the eruption.
For two centuries Krakatoa remained dormant and then in 1883 it began to erupt. The explosions became increasingly violent. Dense clouds of volcanic gases and ash reached a height of 26 km. Heavy rains turned into liquid mud. The eruption reached a climax in August 1883 when a series of stupendous explosions occurred which could be heard in parts of Australia 4800 km away. A glowing cloud of white hot ash rose 80 km into the air, and much of this was carried around the world several times in the upper layers of the atmosphere. The explosion caused two-thirds of Krakatoa island to disappear. It simply sank into a deep submarine depression formed by the eruption. All that remained was a submarine caldera.
Volcanic activity - are volcanoes always active?
Volcanoes usually pass through three stages in their life cycle. Eruptions are
frequent in the early stage. This is when the volcano is active. Later, eruptions
become very infrequent, when the volcano is said to be dormant (sleeping).
This is followed by a long period of inactivity. Volcanoes which have not erupted
in historic times are said to be extinct.
Like all landforms, a volcano is attacked by weathering and erosion,
and by the time it is extinct, most, if not all of the volcano may
have been removed.
Eruption of lava from fissures usually takes place very quietly. The lava spreads out over the surrounding countryside, and successive outpourings cause layer upon layer of lava to form. These may completely cover up the features of the surrounding region. At times the layers of lava are hundreds of metres thick and they form high, fairly level features called lava plateaus. Subsequent erosion by rivers sometimes results in the original surface becoming exposed on the floors of deep valleys.
The largest lava plateau in Britain is in Antrim (Northern Ireland). It has an area of about 150 000 km2.
The Colmbia and Snake Plateau in the USA is over 500 000 km2 whilst the Deccan lava plateau is almost 650 000 km2. When rivers cross a lava plateau they often carve out deep valleys. Sometimes, depending on the nature of the lava, the action of weathering breaks down the lava into fertile soil. This has happened in the north-western part of the Deccan Plateau, India.
Most magma contains gases which are under great pressure. In some cases there is a sudden decrease in pressure in the rising magma, and this causes the gases in it to expand very rapidly. This sudden expansion can cause violent explosions. Water vapour is often one of the gases, and it may have originated in the magma or from water in the crust with which the magma came into contact. Many of the gases burn with a fierce heat and some of them, such as sulphur gases, form a dense cloud which rolls down the side of the volcano killing everything in its path. When eruptions are violent, the lava explodes into small pieces which are blown to great heights. The sizes of the pieces vary from grains to small chunks of rock. The latter are called volcanic bombs. If the explosions are particularly violent, the fine dust can reach such great heights that it gets carried along by the air currents of the upper atmosphere. When Krakatoa exploded in 1883, some of its dust passed around the world causing vivid sunsets in many countries.
Emissions of gases and steam periodically take place from dormant volcanoes. Similar emissions of gases and steam take place in some volcanic regions where active lava eruptions have long since ceased. Superheated water may flow quietly, as in hot springs, or it may be thrown out with great force and accompanied by steam, as in geysers. Thus a geyser differs from a hot spring in that its water is ejected explosively. Geysers often form natural fountains. Hot springs and geysers are common in Iceland, the North Island of New Zealand and the Yellowstone National Park of the USA.
Most people would agree that volcanic eruptions cause nothing but destruction. They can destroy settlements and farm land, burn down forests and cause flooding, and they can kill people. In this respect they would appear to be utterly destructive. And yet people who live in some regions of volcanic activity do not quit the regions for long. This happens in regions where lavas weather to give fertile soils. There are many such regions in the humid tropics, in Indonesia and the West Indies where high daily temperatures and abundant rainfall work together to turn some lavas into rich soils.
Volcanic activity sometimes results in the formation of precious stones and minerals which occur in some igneous and metamorphic rocks, e.g. diamonds of Kimberley in South Africa, copper deposits of Butte in the USA, and nickel deposits of Sudbury in Canada.
We have seen that volcanic activity sometimes results in the formation of hot springs and geysers. In Iceland and New Zealand the superheated water is used for heating buildings. In Iceland a power station has been built which taps the steam to drive turbines for generating electricity. This type of energy development is called geothermal energy.
Volcano:
Volcano, 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 volcanism or vulcanism.
As 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.
Many 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.
For 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.
After 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 volcanic pipes.
