| Any place on the surface of the earth, which spews | | | | slow progress, and these blocks fall down. Very |
| magma, is called volcano. In a more defined sense, it | | | | viscous lava does not flow and forms a plug in the |
| means any mountain spewing fire, which originates | | | | vent of the crater. This sometimes leads to fierce |
| from the magma. Magma is a molten rock from the | | | | explosions, which sometimes tear apart and hurl part |
| lower layers of the earth's crust that penetrates to | | | | of this plug. When gases and hot slabs of lava cannot |
| the earth's surface in the form of lava. An eruption, | | | | escape vertically through the plug, they will push |
| which is the hurling of the magma and gases, passes | | | | through the sides of the volcano and tumble down as |
| through a volcanic vent or fissure. | | | | clouds of molten mass. They reach high speeds and |
| Deep underneath the volcanic vent, in the depth of | | | | may cover and so destroy large areas. |
| 100 to 300 kilometres, we find the magma reservoir, | | | | Mixed eruptions and composite volcanoes (also called |
| which is connected to the upper earth's mantle. The | | | | stratovolcanoes) are quite common. They owe their |
| material rising from great depths collects there and | | | | name to their structure: the alternating layers of tuff |
| melts. Neighbouring rock is also melting. When the | | | | and lava form a symmetrical conic mountain. When |
| pressure in the reservoir is higher than the pressure | | | | the volcano grows over a certain height limit, its walls |
| of the rock above it, an eruption will occur. A new | | | | cannot support the pressure of the magma in the |
| volcano may form, or a dormant volcano becomes | | | | volcanic vent and the side wall is torn apart. In cases, |
| active. | | | | where the top of the mountain is blown up, it is |
| At the top of the volcanic vent is the crater in the | | | | replaced by a caldera, a large funnel-shaped crater. |
| form of a bowl or a funnel. Released gases, magma | | | | New eruptions form a new cone, which is the case |
| in the form of molten lava, or loose products push | | | | of Vesuvius, for example, or a volcano in the Bay of |
| upwards and are hurled into the atmosphere in an | | | | Naples. |
| enormous cloud. Volcanic bombs (boulders of lava, | | | | During a central eruption the lava rises together with |
| which can reach the size of a building), cinders, lapilli | | | | other material through the volcanic vent, which |
| (small, barely several centimetres large fragments of | | | | reached great depths. Deposits of coarser material |
| lava), sand, and ash are the loose volcanic products. | | | | around the volcanic vent form the cone, which may |
| The products of the eruptions form a wall, steep | | | | grow several metres in a few days. Central eruptions |
| inside and sloping outside, which grows upwards into | | | | may occur in any of the regions that have active |
| a cone. Lava products are called the pyroclastic rock | | | | volcanoes. |
| (from Greek: pyros = fire, klasis = to break). | | | | In the case of an existing volcano, repeated opening |
| In addition to eruptions through the volcanic vent, we | | | | of the volcanic vent may often result in explosive |
| may encounter eruptions alongside fissures. In | | | | ejection of boulders filling the vent, together with ash |
| general, these are primary eruptions, often | | | | and lava. During this type of eruptions, large part of |
| accompanied by earthquakes. When a fissure forms, | | | | the volcano may be blown away. |
| tremendous amounts of lava and loose products may | | | | The characteristics of the various eruptions maybe |
| be expelled. Analogously to the vent eruptions, the | | | | determined in accordance with the location type. |
| material is expelled to one place. This may lead to the | | | | These types are Icelandic, Hawaiian, Stromboli, |
| formation of a lined-up volcanic crest. | | | | volcanic, and Pelee. |
| The type of eruption depends on the aggregate | | | | The so-called linear volcanoes are quite common in |
| state of the magma at the exit. There are three | | | | Iceland. Their main characteristic is the outflow of |
| states: volcanic gases, flowing lava, and solid material. | | | | thin, fluid lava, which rises from deep fissures. It does |
| When the magma cools, it releases gases which are | | | | not form volcanic cones. An example of the Icelandic |
| explosively expelled. It is not possible to obtain | | | | type is the eruption of the fissure Laki in Iceland in |
| appropriate samples, the composition can be | | | | 1783 (25 kilometres long). |
| determined only by smell, the colour of the flames, | | | | The Hawaiian type corresponds to the shield volcano. |
| and similar factors, or by taking samples from lava | | | | These volcanoes produce few gases, with lava |
| lakes or fumaroles. The composition of the gases is | | | | pouring slowly out of the volcano. There are no |
| different depending on the type of eruption. | | | | explosions and no pyroclastic clouds. Mauna Loa |
| However, they do contain almost always water, | | | | (4170) and Mauna Kea (4206) in Hawaii are good |
| carbon dioxide, hydrogen, hydrogen chloride, | | | | examples. |
| hydrogen sulphide, hydrogen fluoride, nitrogen, | | | | The characteristics of the Stromboli type is an almost |
| ammoniac, and various rare gases. | | | | endless, moderate activity, with low explosiveness, |
| The lava flow depends on its viscosity. When a mass | | | | during which hot chunks of lava are accompanied by |
| cools down by 50 degrees Celsius, the viscosity at | | | | clouds of vapour and small amounts of ash. |
| constant composition will increase tenfold. There is a | | | | Volcanic type is the type with mixed volcanic |
| ropey lava, which flows as a narrow fluid stream, and | | | | eruptions. The type was named in 1889 by Mercalli, |
| lava bombs, which form when the upper layer of the | | | | following his studies of the island volcano Vulcano |
| lava is congealing and congealed, while the lower layer | | | | (Aeolian Islands). Today, Vesuvius provides a better |
| Is still liquid. The layer opens and the flowing lava | | | | example of that type. The characteristics of this |
| penetrates the fissure. Following additional thickening, | | | | type are violent explosions, accompanied by dark, |
| the lava crust breaks into blocks in the course of its | | | | pine-shaped clouds of ash. |