In the past there were many popular notions about volcanoes, the houses of fire. Some people believed they were abode of gods while others felt they were house of demons and evil spirits. Even today they are considered as sacred places. For instance, in Japan, temples can be seen in the proximity of volcanoes.

Volcano is a fissure / vent in the crust of the earth that spews magma. Density of magma within the earth is lower than the solid rocks above it and around it. Hence, it moves upward due to gravity. Dissolved gases of magma approaching the surface are released. As a result, lava flows out of the vent. A huge stream blast occurs if molten rock suddenly blends with ground water in same quantity during a volcanic eruption. Root zone of volcanoes lie in upper mantle. Generally volcanoes are found near mountain chains. Volcano's life span can spread over million years. Volcanoes releasing only gases (carbon dioxide and sulphur) are called active but not eruptive. An erupting volcano is a source of lava /ashes/blocks/cinders.

Magma

Magma is molten /partly molten rock within the earth. When it emits and appears on the earth surface it is called lava. Lava solidifies faster than magma. Magma cooled and solidified within the earth is referred to as intrusive igneous rock. Different types of lava including vesicular, amygdaloidal, glassy, crystalline, even grained and porphyritic are found Vesicular lava has cavities of spherical shape due to gas bubbles. If these cavities contain minerals, lava is referred to as amygdaloidal. Pumice and scoriae are example of these lavas.

Viscosity of magma varies from that of honey to that of almost solid material. Boiling of gases depend on magma viscosity. Lava fountains form if viscosity is low. Magma is a mixture of viscous, hot liquid and solid crystals because all minerals do not melt at same temperature. Both yield strength and viscosity of magma change as temperature changes.

Basaltic / basic magma: Silica content (less than 50%) and viscosity are low. Temperatures are high. Volcanic gases make only 0.5% to 1.0% of the total weight. 90% of volcanoes emit basaltic magma. The highly fluid magma moving very fast covers long distance prior to solidification. Basaltic magma is more fluid than dacitic magma.

Dacitic / acidic magma: Both silica content (more than 50%) and viscosity are high. They flow very slowly and do not travel long distance before solidification. Temperatures are low. Volcanic gases form 5% to 6% of the total weight. Acidic magma is generally a feature of explosive eruption.

Active, Dead and Dormant Volcanoes

Active volcano is a volcano that erupted during historic time and it may erupt in future. There are hundreds of active volcanoes in the world. The volcanoes that never erupt are called extinct/dead volcanoes. Dormant / sleeping volcanoes are active but presently not erupting. Repose time is a period between two eruptions of a dormant volcano.

Types of Volcanic Eruptions

There are six types of volcanic eruptions, viz. fissure/Icelandic, Hawaiian, Strombolian, vulcanian, Pelean and Plinian. Fissure eruptions are least explosive while Plinian eruptions are highly explosive.

Icelandic eruptions: Usually hot and fluid lava from as long as 25 kilometer fissures flows. Magnificent lave fountains are created due to gases. Krafla volcano in North Iceland is an example Icelandic eruption. Lava plateau forms during these types of eruptions.

Hawaiian eruptions: Lava flows from fissures at the summit of mountain/cone. High lava fountains and rivers of lava are characteristic to the eruption taking place at weak zones. Their upward growth is faster than on their sides. They form dome mountains with gentle slopes. These mountains are called shield volcanoes. Kilauea volcano (Hawaii) is an example of the eruption.

Strombolian eruptions: Gases are released at very short interval (a quarter of an hour /less than one hour) with explosion. Incandescent lava is ejected into the air. It creates steep, conical mountain because thick lava flows are very viscous. Source of lava is pipe like conduit, not a fissure. Stromboli volcano in Italy is called Lighthouse of the Mediterranean.

Vulcanian eruptions: These eruptions are very explosive. Volcanic gases and steam dominate dark ash clouds. Sluggish but thick lava flows. Steep stratovolcano / composite cones form with alternate layers of lava and ash. Vulcano in Italy is a vulcanian eruption.

Pelean eruptions: Very fast avalanches of hot ashes called glowing clouds (nuees ardentens) are peculiar feature of these highly destructive eruptions. Temperatures are very high.

Plinian eruption: During this highly explosive eruption, volcanic ash in the form of high clouds is released continuously. Ash and pumice gather at ground, while fine ash and volcanic gases reach stratosphere. Combustion is absent. At the summit, a circular basin, caldera, is formed.

Death toll during Pelean and Plinian eruptions is very high unlike Icelandic and Hawaiian eruptions.

Rift, Hot Spot and Subduction Volcanoes

Rift volcanoes: Volcanoes that fill crevices/spaces left open by tectonic plates moving away from each other with lava are called rift volcanoes. Examples of rift volcanoes can be seen in the Eastern African rift valley and Iceland. Many such volcanoes erupt under sea along the mid-ocean ridges.

Subduction volcanoes: Volcanoes that erupt inland from a subduction zone where two converging tectonic plates generally over ride each other are called subduction volcanoes. Thousands of subduction volcanoes erupt near converging plate edges. They are found in the Mediterranean-Indonesian volcano belt and the Ring of Fire.

Hot spot volcanoes are effusive. Streams of molten lava are generated.

Explosive and Effusive Eruptions

Explosive eruption: Magma with high gaseous content is thick and viscous. As soon as pressure is relived gases explode, magma is torn into fragments (hot ash, large fragments). If ash, rock particles and gases are denser than air, these products descend rapidly like an avalanche which is called ash flow/pyroclastic flow/nuees ardentes. If they are lighter than air, they rise like a dark cloud.

Effusive eruptions: Magma with low gaseous content is less viscous. Lava fountains form during these less explosive eruptions. If gases are released before lava arrived at surface, slow and quiet lava flow will occur.

Some volcanoes are more effusive, while others are more explosive.  Submarine volcanoes are generally effusive in nature in deep sea. But they are violent in shallow water.  

Lava Flows

Lava flow is one outpouring of lava. Surface of the lava flow is usually irregular and cindery. But they are fine grained / glassy if cooling is very rapid. Pahoehoe is a lava flow with smooth ropy surface. These flows are result of volcanic eruptions marked by low viscosity, high temperature, and low emission rate. AA is a sharp, thick, and loose heap of lava blocks. Their surface is rough.

The Ring of Fire is an arc of volcanoes around the Pacific Ocean. Four fifth volcanoes of the world erupt in the Mediterranean-Indonesian belt and the Ring of Fire. Mauna Loa, the largest volcano of the world, is located in Hawaii. The Hawaiian volcanoes erupt in deep water. Lava flows quietly. Before cooling and solidification, lava moves large distance.

Volcanic eruption, the catastrophic phenomenon, may cause gigantic tidal waves. It produces ash, gases and pyroclastic fragments that can scatter several kilometers away from the source. But volcanic activity recycles air, crock crust and water. Fertile and rich soil is produced. Geysers and hot water springs, the result of volcanic eruptions, are known for their therapeutic value. Hot ground water could be used to generate geothermal power.