A volcano is a vent or fissure in the earth’s crust through which hot lava and volcanic gases are thrown out. Volcanic eruptions may be either explosive or quiet.
The earth’s upper mantle (asthenosphere), under the crust, is nearly molten. Magma originates at this depth. They migrate upwards, often along faults. In a volcano, the magma rises through a chimney-like opening called ”vent” and reaches the surface as lava. At the surface pressure in the rising magma falls, as a result, dissolved gases are separated out. Such a magma is called ”Lava”. Read composition of lava
In addition to the emission of gases and molten lavas, vast quantities of fragmental materials (pyroclasts) are also produced during volcanic eruptions. This material accumulates around the vent. Conical hill-like masses formed in this way, are called ”volcanic cones”. Volcanoes often have side vents as well. The smaller cones formed around these sides vents, are called ”parasitic cones”.
A circular depression found at the top of volcanic cones is called a ”crater”. A greatly enlarges crater is called ”caldera”. A caldera is a gigantic depression the diameter of which maybe 16 km or more. Most of the calderas may have resulted either due to blowing of the summit of the volcanic cone by explosive action, or it may be due to subsidence. The subsidence is caused when the magma chamber lying below the volcano is partially emptied during eruption and the can then sinks into the void.
Read full article about Types of calderas
Types of volcanoes:
Volcanoes commonly don’t erupt continuously for long periods. Mostly they show intermittent activity. Depending on the variation in the frequency of their activity, volcanoes are divided into three groups: (i) active volcanoes, (ii) dormant volcanoes, and (iii) extinct volcanoes.
(i). Active Volcanoes: An active volcano is one, which erupts very often. They mostly occur at crustal plate boundaries. The volcanoes that occur along a great arc around the Pacific Ocean from Chile to East Indies, are examples of active volcanoes.
(ii). Dormant Volcanoes: The volcanoes, which show eruptions after long intervals of time, are called ”dormant volcanoes”. During the dormant period they appear quite inactive.
(iii). Extinct Volcanoes: An ancient volcano that has not shown any volcanic activity for a very long time in geological history, is called an ”extinct volcano”.
Three types of material are thrown out from a volcano: (i) gases, (ii) molten lava, and (iii) solid rock fragments.
- Gaseous Products: The most important constituent of volcanic gases is steam. It forms nearly 90% of the total gas content. The other chief gases, in the order of abundance, are carbon dioxide, nitrogen, and sulfur dioxide, and smaller amounts of hydrogen. The density of magma and molten lava is reduced by the presence of dissolved gases.
- Liquid Products: Liquid emissions from a volcano are known as ”Lavas”. Lavas of acidic composition are more viscous and mobile than highly fluid basic lavas. Fluid lava results in calm eruption as they allow the dissolved gases to escape freely. In viscous lava, the gases do not escape freely. They frequently build up internal pressure to produce a violent eruption.
- Solid Products: Besides gases and liquid lava, volcanoes eject solid rock fragments of various sizes. These rock fragments are thrown out by the escaping gases during violent eruptions. When fragments of very viscous lava are blown off into the air, they solidify quickly and fall to the ground as pyroclasts. The solid rock fragments produced during volcanic eruptions are called ”pyroclasts”. Depending on the size, the pyroclasts are divided into the following groups.
- Volcanic Blocks: Bigger angular fragments of dead lava are called ”volcanic blocks”. They are formed when large pieces of solidified lava are blown off during an explosion.
- Volcanic Bombs: Bigger and somewhat rounded or spindle-shaped fragments are called ”volcanic bombs”. They may be up to the size of a football. Volcanic bombs are clots of live lava which generally solidify before reaching the ground.
- Lapilli or Cinders: The rock fragments up to the size of gravels or peas, are called ”lapilli” or ”cinders”. Normally their size ranges between 4-32 mm.
- Volcanic Ash: Particles that range in size between 0.25 to 4.0 millimeters, is called ”volcanic ash”.
- Dust: The dust includes very fine particles. their sie is less than 0.25 mm.
The volcanic dust and ash on consolidation form a rock which is called ”tuff”. When hot as, usually glassy shreds, falls, it sticks together to form a ”welded tuff”. The larger fragment, such as bombs or blocks of older rocks commonly accumulate near the volcanic cone. On consolidation, they produce a rock called ”agglomerate” or ”volcanic breccia”.
In lavas where gases are released during consolidation, small bubbles may be entrapped into the rock. These gas cavities are called ”vesicles”. In viscous acidic lavas, the vesicles may be so abundant that the rock takes the form of a light spongy mass, which is called ”pumice”. The ”pumice is so light that it can float on the water. The term ”scoria” is applied to those bombs and lapilli which are spongy like pumice but they are merely rugged and have knife-sharp edges. (Scoria is basaltic lava ejected as fragments from a volcano, typically with a frothy texture).
Successive eruptions from a single vent result in the accumulation of volcanic material around it. Conical mountains formed in this way are called ”Volcanic cones”. Volcanic Cones are of three types: (i) cinder cones, (ii) lava cones, (iii) composite cones.
(i). Cinder Cones: These are steep-sided volcanic cones, which are built up of fragmentary materials, mostly cinders. Such cones are usually very symmetrical and have circular craters. Cinder cones are small in size and their height seldom exceeds 300 meters. They often occur as parasitic cones on larger volcanoes.
(2). Lava Cones: Lava cones are composed mainly of solidified lava flows. They are much flatter than cinder cones. They have a slope of few degrees at their flanks and nearly ten degrees near their summit. Volcanoes having such types of ones are called ”shield volcanoes”. They derive their name from the shape of the cones, which is more or less like a shield. The Hawaiian volcanoes are important examples of shield volcanoes. Their cones are formed due to the solidification of highly fluid and mobile basic lavas.
(3). Composite Cones: These cones are composed of alternate layers of pyroclastic material and solidified lava-flows. Composite cones usually have a steep summit and rather gently sloping flanks. The contributing magma has an andesitic to rhyolitic composition and marked stratification are called ”strat-volcanoes”. The examples of strat-volcanoes” are Vesuvius (Italy) and Mount Fuji (Japan).