---
The point within the Earth where the earthquake originates is called the focus. The place on the surface of the Earth, immediately above the focus, which vibrates, is called the epicentre.
VOLCANOES
The word volcano comes from Vulcan, the blacksmith of the Roman gods. His home was said to be in a volcano in Sicily – called Vulcano.
Volcanoes are a natural hazard. They are formed when magma a mixture of gases, ash and hot melted rock, gushes up from inside the earth and breaks through a crack or weak spot in the Earth’s crust, usually at the edges of plates. These are called plate margins or boundaries, the same places as earthquakes occur. The magma lies in a chamber far below the volcano’s vent. Pressure builds up and this forces the magma to blast its way through and escape though the vent, with clouds of gas and ash exploding into the air. The ash is formed because gas dissolved in the magma escapes with such force that it blasts the hot rock into billions of tiny pieces. These fragments of rock are called pyroclastics. They range from lava blocks as big as houses to powdery dust fine enough to float right round the world in the upper atmosphere. Volcanic bombs are lava fragments that were ejected while partially molten, and larger than 64 mm in diameter. Many acquired rounded aerodynamic shapes while they were travelling through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs and ‘cow-dung’ bombs.
----
Once the magma reaches the surface, it is called lava. Lava is red when it is hot and it turns grey or black as it cools and hardens on the Earth’s surface and solidifies into rock. Very powerful explosive eruptions can hurl huge blocks several miles from the volcano, but the biggest fragments usually land nearest to the vent and the smaller ones furthest away. A deep, steep-walled crater forms around the main vent.
Cross-section of a volcano
In the largest volcanic eruptions, the crater of the volcano may collapse into its empty magma chamber to form a steep-sided circular caldera which may fill with water to form a lake. The empty caldera of Ngorongoro Crater in Tanzania is now a wildlife reserve.
When plates collide, one is forced under the other and melts into magma which then rises through the adjacent plate and may burst on to the surface to form a chain of volcanoes.
Chain of volcanoes
There are many different types of lava. Some are so sticky that they hardly flow at all. Others are so runny that they flow a great distance very quickly – up to 60mph. Runny lava quickly cools to form a skin that is wrinkled by the lava still flowing beneath. In Hawaii, this is called pahoehoe lava, which cools to form basalt rock.
Types of Volcano
A volcano’s shape depends on the kind of lava or ash that it produces. Where tectonic plates are colliding, the lava is very thick and sticky. It cools to form tall, cone-shaped volcanoes, like Mount Fuji in Japan. Where plates pull apart, the lava is thinner and runnier, and spreads out to form huge, flat volcanoes. A cone volcano can be very dangerous. Sometimes magma solidifies in the vent to form a plug. There is great pressure from the gases inside and the cone may explode.
Composite (or Strato) volcano
--
Many of the world’s tallest volcanoes are built of alternating layers of lava and volcanic ash. This is the reason they are called ‘composite’. The lava is quite sticky, so does not flow far before it solidifies. Between eruptions they are so quiet they seem extinct, but when they erupt, they do so in an explosive way because of the thick magma.
Cinder cone
--
A cinder volcano also has a steep conical shape, but is made mostly of volcanic ash or ‘scoria’. They get their name because they are built from lava fragments called ‘cinders’. Each time the volcano erupts, more layers of ash are added. An example of a cinder cone is Paracutìn in Mexico. It was born on 20th February 1943 in a corn field and grew to 300 feet in 5 days.
Shield volcano
This kind of volcano forms where eruptions give runny lava that flows out across a wide area instead of making a cone. Shield volcanoes often have many side vents, and the volcano has broad sloping sides like a shield. Famous shield volcanoes can be found in Hawaii – e.g. Mauna Lau and Kilauea.
--
Fissure (flood basalt)
---
Sometimes runny lava floods out over vast areas from widening splits in the Earth’s crust. It cools to form giant basalt plateaux, such as India’s Deccan. A plastic skin often forms over fast-flowing, runny lava. The skin is dragged into picturesque folds by the still liquid lava running beneath it.
Hot Spots
Sometimes volcanoes form when a crustal plate moves over a ‘hot spot’ in the Earth’s mantle. Molten magma bores upwards from a fixed position deep below the ocean floor and breaks through the drifting plates to form shield volcanoes, like the Hawaiian Islands. The sea hides most of the dormant volcano Mauna Kea, which is in Hawaii. Only its peak peeps out above the surface of the water. Mauna Kea will eventually become extinct as the Pacific plate moves on, away from the hot spot zone.
--
Kinds of volcanoes
There are three kinds:-
(i) Dormant or ‘sleeping’ volcanoes
These are the ones that are quiet for a long time and then suddenly erupt again. Two examples are Mount Fujiyama in Japan, which last erupted in 1707, and Mount Rainier in the USA, which has not erupted for over 100 years. Both these volcanoes still have lava bubbling in the crater and steam rising from them. Nobody knows when they might erupt again.
Mount Fujiyama, Japan:
(ii) Active volcanoes
These are the ones that still erupt. They occur mainly at the edges of the Earth’s plates, where new crust is being formed and old crust is being destroyed. Of the 500 active volcanoes in the world, 20-30 erupt each year, such as Mount Pinatubo in the Philippines and Mount St. Helens, a volcanic peak in the Cascade Mountains of Washington State, USA.
Mount Pinatubo
After lying dormant for 600 years, Mount Pinatubo erupted suddenly in June 1991, spurting clouds of steam and ash 14,000 metres in the air. This showered an island 90 km away with so much grit and ash that the airport had to be closed. Huge clouds of ash were thrown into the air and blocked out the sun for days. The ash slowly settled and buried fields and villages for miles around. Over 100 metres of ash lay in drifts on the upper slopes of the volcano. Torrential rains followed, causing mud flows that cascaded down the river valleys and swept away roads, bridges and several villages. At least 400 people were killed and another 400,000 were left homeless. With no breathing masks to protect themselves from the gritty ash and fumes, many of the survivors developed pneumonia. Everyone suffered from badly inflamed eyes.
PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS is the name given to the effects of breathing the poisonous gases given off by volcanoes.
Mount Erebus an active volcano in Antarctica:
--
Mount St. Helens
ON 18th May 1980, a huge explosion ripped Mount St, Helens apart, releasing clouds of ash and dust which are still in the atmosphere. Geologists knew that the volcano could erupt at any time. It had been dormant since 1857, but a series of small earthquakes during the 1970’s suggested that magma was rising into the mountain. There were other warning signs too – the side of the mountain was bulging and steam and gas were escaping. At 8.32 a.m., an earthquake broke the bulging side loose, causing the worst landslide ever recorded. Rock and lava plummeted down the mountainside and clouds of hot gases and ash plunged the valley into darkness. The eruption killed 63 people, flattened forests and destroyed wildlife. The volcano continued to erupt violently for four days and there were smaller eruptions for several months.
Last Gasp Tree Removal Zone
---
Mount Etna
Mount Etna in Sicily is still an ‘active’ volcano, and has been active for 2,500 years. Long streams of red-hot lava flow down the mountainside burning everything in its path. It erupts slowly and the local people usually have time to escape, but in 1669 there was an eruption which killed 20,000 people.
(iii) Extinct volcanoes
An extinct volcano is one which has not erupted for thousands of years. Mount Egmont in New Zealand, Mount Kilimanjaro in Tanzania, Africa, and Mount Teide on the Canary Island of Tenerife are all examples of extinct volcanoes.
Mount Teide, Tenerife and Volcanic rocks, Tenerife
Mount Vesuvius
The most famous eruption of all time shook Mount Vesuvius, near Naples in Italy in AD 79. It had been dormant for a long time and when it erupted on 24th August, the people living in the two Roman towns of Pompeii and Herculaneum were caught completely unawares. Hot ash fell on Pompeii for hours until it was buried several metres deep. Many people escaped, coughing and stumbling through the darkness of the ash cloud. Over 2,000 people in the town were overwhelmed by a sudden blast of ash and gas.
Both towns were virtually forgotten until excavations began in the 18th century. The digs have since unearthed a priceless geological and archaeological treasure – two Roman towns frozen in the moments they were destroyed.
Archaeologists have made plaster casts of the bodies of the people of Pompeii and there is a permanent reminder of what happened that day. As they people fled from the erupting volcano, ash and pumice set around their bodies like wet cement. In time, the soft body parts decayed and the ash and pumice turned to solid rock. The shapes of the dead Romans’ bodies were left as hollows in the rock; only the hard bones remained inside the hollows. In 1860 the Italian king appointed Giuseppe Fiorelli as director of excavations. He invented a method for removing the skeletons from the body hollows and filling the space with wet plaster of Paris. After the plaster hardened, a true copy of the bodies could be dug out of the volcanic rock. Many of these casts show people grimacing, trying to hide, or huddling together in terror.
Herculaneum
When Mount Vesuvius erupted on 24th August, the ash cloud that covered Pompeii missed Herculaneum Less than 3cm of debris had fallen on the town when it was blasted by the great surge of hot ash and gas. Early excavations uncovered very few bodies which was quite puzzling and archaeologists decided that most of the people must have escaped in boats beforehand. But in the 1980’2, several hundred skeletons were found huddled beneath massive brick arches that once stood on the shoreline. A lot of Herculaneans must have taken shelter there, only to be overcome by the deadly ash and gas in the air.
Because the skeletons from Herculaneum were buried in waterlogged ground, they had no surrounding body shape – as the bodies decayed, the wet ash got closer and closer until it was packed tightly around the bones.
Effect of Volcanoes on the environment
When Mount St. Helens in the Cascade Mountains erupted on 18th May 1980, a huge explosion ripped the mountain apart, releasing clouds of ash and dust which are still in the atmosphere. The volcano continued to erupt violently for four days and there were smaller eruptions for several months. After the eruption, the countryside resembled a desert of ash and charred remains. Yet five years later, wildlife returned to the area. Sadly the effect of the 30 million tonnes of ash may have caused changes in the weather worldwide.
Large ash eruptions are often followed by landslides or mud flows. The ash that piles up near the crater may collapse and bring part of the mountain down with it. Heavy rain often adds to the problem, creating a wet slurry which turns into a mud flow. In mountains both earthquakes and volcanic eruptions might trigger avalanches. If they occur by the sea, they can cause giant water waves. These are often called ‘tidal waves’ but because they are not actually created by tides, scientists prefer to use the Japanese name tsunamis. Tsunamis can travel for thousands of miles across oceans and when they break on faraway shores, the huge walls of water can cause horrendous damage.
Dark days, severe winds and heavy falls of rain or even mud may affect the local area for months after a volcanic eruption. If the gas and dust are lifted high into the atmosphere, they may travel great distances around the world. When this happens, the climate of the whole planet can be altered. The volcanic material filters out some of the sunlight, reducing temperatures down below. The high-flung particles also affect our views of the Sun and Moon by scattering certain frequencies and allowing other wavelengths through. This can cause spectacular sunrises and sunsets, with the effect of haloes or a strange-coloured glow. Two big eruptions in 1783 caused problems for polar explorers who came across unusually thick pack ice. In the longer term, volcanic particles may cause global cooling, mass extinctions or even ice ages.
The gases erupted by volcanoes include carbon dioxide and sulphur dioxide. Carbon dioxide is a greenhouse gas which means that it helps to keep the earth warm. In the past, the amount of carbon dioxide released by volcanoes helped to maintain the balance of gases in the atmosphere, but this balance is now being upset. Burning fossil fuels and cutting down and burning trees both produce large amounts of carbon dioxide. If too much carbon dioxide is trapped, temperatures around the world could go up. This is called ‘global warming’.
There is a hole in the ozone layer over the Antarctic Ocean and the sulphur particles that Mount Pinatubo threw into the atmosphere may cause even more damage to this layer, again affecting the world’s temperatures.
The sulphur dioxide produced by volcanoes contributes to the problem of acid rain. When it mixes with the water vapour in the air, it forms a very weak acid. This acid then falls to the ground in rain, snow or dust and can kill trees, destroy buildings and pollute ponds, rivers and lakes and poison any life in them.
Life after a volcano
Even though an active volcano may erupt at any time several farms and villages are built on the slopes of volcanoes because the soil is very fertile. In India the ancient basalt lava flows from volcanic eruptions in the past have eroded into the rich black soil of the Deccan Plateau where cotton is now grown. Farmers in Indonesia grow rice in volcanic soil and there are vineyards and orange and lemon groves on the slopes of Mount Etna in Italy.
For the landowner and farmer, an eruption that produces less than 20 cm of ash is a blessing. The ash is full of nutrients which enrich the soil, but too much fertilizer would cause problems.
The worst case for a farmer would be when the land is over-run by lava flows, because thick lava flows can take months to cool. It might take decades before mosses and lichens spread slowly across the barren landscape, and then flowering plants and trees follow.
Alternative energy
Geothermal energy, or energy from the Earth is another vital resource of volcanic areas. It would take 20,000 times the world’s coal supply to produce the heat that exists in the upper 11 kms of the Earth’s crust. Today, many geothermal power stations have been built in Iceland, New Zealand and Japan, where groundwater and magma provide hot water and steam. As supplies of fossil fuels run out geothermal energy is a relatively clean and almost endless resource of the future.
Water seeps through layers of porous rock, like limestone, until it meets a layer of impermeable rock, such as granite. The water collects in the tiny spaces between the grains of the rock, and is heated by the hot rocks of the volcano to produce steam. This steam is piped to power stations where it is used to turn the turbines of an electricity generator. In Iceland, hot ground water is piped into cities where it is used to heat homes and provide heat for greenhouses in Iceland, shown
below :-
--
GOLD
The heat from volcanoes makes metals in the crust melt and flow through cracks in the rocks, where they cool and solidify. Gold is often mined from the remains of old volcanoes.
VOLCANOES UNDER THE SEA
More volcanoes are found under the sea than on land because the oceanic crust is thinner than the continental crust. Volcanoes usually occur along the edges of the plates, but some are found in places where the mantle is so hot that it melts a hole in the thin crust above. These are called ‘hot spots’
The new island of Surtsey
In the 1960’s, a new island gradually formed off the coast of Iceland. It took about four years for the volcanic rock to pile above the sea.
The new island was named Surtsey after Surt, the Nordic God of Fire.
CAN VOLCANOES BE PREVENTED?
We cannot stop volcanoes erupting and we cannot stop people from living near them and taking advantage of the fertile soils to grow their crops.
Scientists try to give warnings about future eruptions so that people can be evacuated. They study movements in the rocks beneath the Earth’s surface. Rising magma creates a series of tiny earthquakes which can be used as early warnings. The volcano shape can also be studied. As the magma chamber fills up, it begins to bulge or swell, which shows that the volcano is about to erupt. Even though we have this information, predicting eruptions is still dangerous and not always reliable, as we found out from the deaths of volcanologists Maurice and Katia Krafft at Mount Unzen in Japan in 1991.
Laser beams are used to predict volcanic eruptions; a swelling of the volcano caused by a build up of magma can be detected by a change in the length of the laser beam.
When Mount Etna erupted in 1983, dynamite was used to control the flow of lava and divert it into an inactive crater of the volcano.
Planes have also been used to bomb lava flows to divert the course of the lava away from towns.
Sources :-
Eyewitness Science Guides – How the Earth Works – by John Farndon
Dorling Kindersley Eyewitness Guides – EARTH – by Susanna Van Rose
Dorling Kindersley Eyewitness Guides – VOLCANO – by Susanna Van Rose
Volcanoes and Earthquakes – Ladybird Discovery series
Horrible Histories – Disaster! The Story of Major Catastrophes. (Watts Books)
Natural Disasters – Volcanoes (Watts Books)
Collins Children’s Encyclopedia
The Children’s Factfinder – Colour Library Direct
Internet
School sheets
VOLCANIC PLUGS
When the vent of a volcano is plugged by thick lava, it may explode- or die out altogether. If it dies out, the rest of the volcano may be slowly worn away to leave nothing but the hard plug. Sugar loaf mountain, in Rio de Janeiro is a plug, so is the Devil’s Tower is Wyoming, USA.
