Secondary School Level

Outline: Volcanoes posing hazards to mankind. After eruption lava fires out. What happens after volcanic outburst? Classifying volcanoes on the bases of eruption.

Volcanic outbursts have always posed extreme hazards and death-traps to civilizations. The word volcano is derived from the name “Vulcans”. Vulcans is a volcanic island in the Aeolian Islands of Italy. The study of volcanoes is called volcanology. However, a volcano is an opening in an earth's crust.

This opening allows hot magma, volcanic ash and gases to escape out from below the surface of the earth. Molten rock is called magma when it is below the surface of the earth. After it erupts from on opening, it is called Java. Lava is red-hot when it fires out of a vent. Originating several miles underneath the ground, the rising lava contains crystals, dissolved gases and solid pieces of rocks. Mostly this liquid is made up of oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, potassium, and manganese.

Due to the tremendous pressure of different gases, molten rock forces its way skyward to break through a weak spot in the earth's crust. When an eruption occurs, it explodes into the air and the molten rock pours out. The lava starts flowing. Some of the finer materials may be carried far off by wind and fall to the ground miles away. Even finer dust particles are thrown high into the atmosphere. These particles are also carried around the biosphere (Earth) by high altitude winds before being fallen somewhere. Gradually, the colour of the lava changes to a dark grey, black or even red as it cools and hardens. After cooling, the liquid magma solidifies to form igneous rock.

Volcanoes are generally found where there is stretching and thinning of the earth's crust. For example, the Wells Gray Clearwater volcanic field in Eastern African Rift and the Rio Grande Rift in Northern America. A popular way of classifying volcanoes is by their frequency of eruption. Those volcanoes which erupt regularly are called active volcanoes. On the other hand those that have erupted in historical times but are now quiet are called dormant volcanoes. However, those volcanoes which have not erupted in historical times are called extinct volcanoes. Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting; or showing signs of unrest such as unusual earthquakes. There is no real accord among volcanologists on how to define an “active” volcano. The life span of a volcano can vary from months to several million years. Many volcanoes have erupted in past few thousand years but are currently not showing signs of eruption.