Theories of Geography Part 5 – Vulcanism

Volcanoes & Volcanism

Introduction to Volcanoes

A volcano is simply an opening in the Earth’s surface in which eruptions of dust, gas, and magma occur; they form on land and on the ocean floor. The driving force behind eruptions is pressure from deep beneath the Earth’s surface as hot, molten rock up wells from the mantle. The results of this activity are a number of geological features, including the build-up of debris that forms a mound or cone, which we commonly imagine when talking about a volcano.

Vulcan is Greek God of beneficial and hindering fire. Vulcanization is another term derived from the name of Vulcan God. Vulcanization is adding sulfur or other curatives to rubber or other polymers to make them more durable. The rubber so produced is called Vulcanite or ebonite. Vulcano is the name of an island near Sicily, the largest island in the Mediterranean Sea and an autonomous region of Italy.

An opening or vent through which the magma, molten rocks, ashes, gases and other volatiles erupt on the surface of Earth is called a Volcano.  The most known types of Volcanoes are conical mountains which spit law and poisonous gases. But there are other types of Volcanoes.  The Volcanoes can be divided in the basis of Type of Eruption, Material erupted & Periodicity of eruption.

Types of Volcanoes by Volcanic Eruption

There are three major categories of the volcanic eruptions. The magmatic eruptions involve the decompression of gas within the magma. This decompression of the gas propels it outward. In Phretomagmatic eruptions involves compression of the gas within the magma. Another is Phreatic eruption which involves superheating of steam via contact with Magma. In Phreatic eruption, there is no magmatic release and they cause the granulation of the rocks. Apart from this there are other types of eruptions which sometimes don’t seem to be Volcanic Eruptions. The following graphic shows this classification. Magmatic Eruptions When Magma, the mixture of rocks, volatiles and solids erupts in a fissure, it is called magmatic eruption.

Pheratic Eruptions

These eruptions usually occur with extremely loud explosions. The explosions are mostly accompanied by carbon dioxide or hydrogen sulfide gas emissions which prove fatal to the organisms around. This eruption is also known as steam-blast eruption and most common example is 1979 explosion in the Java Island, which killed more than 100 people.

Other Types of Eruptions

Effusive eruption causes the lava to flow on ground slowly, and it travels slowly away from the site of eruption.

Sub aerial eruptions occur on the surface in contrast with the submarine or subglacial eruption.

Limnic Eruptions occur below the bed of lakes and is called Lake Overturn. The gases (mostly CO2) suddenly erupt from the bed of the lake making the water and environment poisonous killing animals. The lake tsunamis are caused by Limnic disruptions sometimes. Lake Monoun & Lake Nyos in Cameroon have suffered this kind of eruptions in near past.

Types of Volcanoes by Periodicity of Eruption

There are three kinds of Volcanoes on the basis of frequency of eruption viz. Active, Dormant and Extinct.

1. Active volcanoes

Active Volcanoes erupt frequently and mostly located around Ring of Fire. The Mount Stromboli is an active volcano and it produces so much of Gas clouds that it is called Light house of Mediterranean. Other examples are Eyjafjallajökull in island, which erupted in 2010, Mount St. Helens located in Washington USA, Mt. Etna located in Sicily.

2. Dormant Volcano

Dormant Volcanoes are those who are not extinct but not erupted in recent history. Mount Kilimanjaro, located in Tanzania which is also the highest mountain in Africa is known to be a dormant Volcano. The dormant volcanoes may erupt in future.

3. Extinct Volcano

Extinct or inactive volcanoes have not worked in distant geological past. In most cases the crater of the Volcano is filled with water making it a lake.

Some Notable Terms Related to Volcanoes

  • Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air. The Tephra is all the volcanic material such as Ash, Plumes, Volcanic Bombs, Volcanic Blocks, lapilli etc.
  • Volcanic Bomb: Pieces of Viscous lava often 2.5 inch size are ejected from the volcanoes. They are viscous rounded shaped half semisolid pieces called Volcanic Bombs. They are either round or spindle shaped or ribbon shaped. Sometimes referred to as Volcanic Blocks, however, Volcanic blocks are thought almost same size, are solid. The smaller particles less than 2.5 inch are called Lapilli. The pieces of rocks that erupt violently are also called ballistic fragments.
  • Lapilli: Lapilli mean “little stones.” These are round to angular rock fragments, measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state.
  • Volcanic Ash: The Ash from the Volcanoes is hard and abrasive type which is made up of rock particles, minerals and Volcanic glass fragments. The cloud made by the Volcanic Ash is called Ash Cloud. When this ash falls on the ground, it is called Volcanic Ash Fall. The clouds are called Avalanches sometimes.
  • Pillow lava: Interconnected, sack-like bodies of lava formed underwater.
  • Pyroclastic Rocks: It is the fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
  • Cinder Cone: A cone shape hill of volcanic fragments that accumulate around and downwind from a volcanic vent is a cinder cone. There is usually a bowl-shaped crater at the top. As the gas-filled lava erupts into the air, the lava fragments and forms cinders.
  • Repose: The time lag between the volcanic eruptions is called repose.
  • Volcanic Explosivity Index: Volcanic Explosivity Index is a scale that measures the Volume of Volcanic Products, Height of Plume and other observations to decide which volcano is more explosive. Highest Magnitude is 8.


Other Important Points for prelims

  • There are more than 1500 active Volcanoes in the word.
  • The Crater Lake in Oregaon USA was formed when a Volcano lost its top in eruption thousands of years ago.
  • The Volcanic Ash is mostly acidic.
  • The Olympus Mons is the tallest known Volcano on Planet Mars.
  • Italy’s Stromboli Volcano is erupting for more than 2500 years.
  • The Mount St. Helense had erupted in 1980, which caused the ash travel across entire US.
  • The 1883 eruption of Indonesia’s Krakota eruption was so loud that blasts were heared 3000 miles away.
  • Mauna Kea in Hawaii is the tallest Volcano on earth. The meaning of its name is White Mountain as it is snow capped. Its height is 4205 meter from Sea Level; however, if it measured from its oceanic base, it is the higher than mount Everest (over 10000 meters).

Pacific Ring of Fire

Pacific Ring of Fire is a horse-shoe shaped 40,000 kilometer area with 75% of Earth’s active and dormant volcanoes. It is the area with large number of Volcanic Eruptions and Earth quakes. The most active Volcanoes are located in Chile, Mexico, United States, Canada, Russian Far East, Japan, Philippines, Indonesia, New Zealand, & Antarctica.


Basics of Magma

Magma is a mixture of molten or semi-molten rock, volatiles and solids. Besides molten rock it may contain suspended crystals and dissolved gases. The two most abundant elements in earth’s crust and mantle are oxygen and silicon which combine to make Silica i.e SiO2.

Types of Magma

The classification of the Magmas is done primarily on the basis of Silica content. On this basis there are four types of Magmas as mentioned below:

Magma Type Silica Content Fe-Mg Content Temperature Eruption Viscosity
Ultramafic or Picritic Less than 45% 8-32% High up to 1500°C Gentle Low
Mafic or Basaltic Around  50% Less than 10% Up to 1300°C Gentle Low
Andesitic Around 60% Around 3% Up to 1000°C Explosive Medium
Felsic / Rhyolitic Around 70% Around 2% Below 900°C Explosive High

From the above table we may note down the following observations:

  • Increasing silica content is the basis of classifying the Magma from Picritic to Felsic.
  • Increasing Silica content implies a lower temperature of the Magma.
  • Increasing silica content implies an explosive eruption behaviour of Magma
  • Increasing silica content implies an increasing viscosity of Magma.

Magma often collects in magma chambers that may feed a volcano or turn into a pluton. Magma is capable of intrusion into adjacent rocks, giving rise to Sills and Dikes, and extrusion onto the surface as lava, and explosive ejection as Tephra to form pyroclastic rocks. The Tephra is all the volcanic material such as Ash, Plumes, Volcanic Bombs, Volcanic Blocks, lapilli etc.

Gases in Magma

  • The gases are dissolved in magma at high pressure beneath the layers. The gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth; very much similar to the carbonated beverages which are bottled at high pressure.
  • Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma is: Mostly H2O (water vapour) & some CO2 (carbon dioxide) Minor amounts of Sulphur, Chlorine, and Fluorine gases
  • The amount of gas in magma is also related to the chemical composition of the magma.
  • Rhyolitic magmas usually have higher gas contents than basaltic magmas. That is also the reason that the Rhyolitic Magma is more explosive than the Basaltic Magma.

Formation of Magma

Outer core is the ONLY part of earth which is liquid, but outer core is NOT the source of Magma, because it does not have the right chemical composition.  For instance, the outer core is mostly Iron, but magmas are silicate liquids. Magma originates in the lower part of the Earth’s crust and in the upper portion of the mantle. There, high temperatures and pressure cause some rocks to melt and form magma. Since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present.

Then, magma does not occur everywhere below us. There are only some specific places where volcanoes exist. This means that Magma is formed under some special conditions, which exist in some limited area.

Another point is that in the ocean basins, magmas are not likely to come from melting of the oceanic crust, since most magmas erupted in the ocean basins are basaltic.

To produce basaltic magmas by melting of the basaltic oceanic crust would require nearly 100% melting, which cannot happen. In the continents, both basaltic and rhyolitic magmas are erupted and intruded. Basaltic magmas are not likely to have come from the continental crust, since the average composition is more siliceous, but more siliceous magmas (andesitic – rhyolitic) could come from melting of the continental crust. Basaltic magmas must come from the underlying mantle. Thus, with the exception of the continents, magmas are most likely to originate in the mantle from melting of mantle peridotite, (a rock made up of olivine, pyroxene, and garnet) — evidence comes from pieces brought up by erupting volcanoes.

Does Geothermal Gradient causes melting of Rocks?

Temperature increases with depth or pressure in the Earth along the geothermal gradient.  The normal geothermal gradient is somewhat higher beneath the oceans than beneath the continents, at least at shallow levels.  But when we observe the normal geothermal gradients, we find that under the normal conditions, the geothermal gradient is not high enough to melt rocks, which is why that with the exception of the outer core, most of the Earth is solid. Thus, the geothermal gradient is not a very substantial factor contributing in the formation of the Magma.

Does Radioactive Heat Cause Melting of Rocks?

The radioactive elements such as Uranium, Thorium etc, keep decaying below. During radioactive decay, sub-atomic particles are released by the decaying isotope and move outward until they collide with other atomic particles.  Upon collision, the kinetic energy of the moving particles is converted to heat. If this heat cannot be conducted away, then the temperature will rise.  Most of the heat within the Earth is generated by radioactive decay, and this is the general reason why temperature increases with depth in the Earth.  But again this is not enough to prove the melting of the rocks. We should know that most the radioactive isotopes are concentrated in the crust.  Although there are areas in the continental crust where high concentrations of radioactive elements have locally raised the temperature, at least high enough to cause metamorphism, but it is more unlikely that areas of high concentration develop within the mantle.  Thus, concentrations of radioactive elements are not likely to cause melting.

Does decrease in Pressure cause rock melt?

There are two things. First is that very high pressures in mantle rocks prevent atoms within minerals from breaking chemical bonds and moving freely from one another to form magma. Therefore, most rocks within the mantle do not melt even though their temperature may be greater than that necessary to melt the same rocks at the lower pressures of the Earth’s surface. However, if something occurs that the pressure on mantle rock is decreased; the atoms may move freely from one another.

This would result in the partial melting of the already very hot solid rock. This process is called pressure-release melting. It is a scientifically proved theory and is found to be common along divergent plate margins, and within mantle plumes.

Does addition of Water causes melting?

The addition of small amounts water to peridotite will result in a decrease in its melting temperature. This is largely due to the electrically polarized nature of a water molecule, as there is an unequal distribution of electrons around the water molecule. The electrical polarization causes a decrease in cation-anion bond strengths within minerals, and so at very high temperatures the bonds may be broken so that atoms may move freely from one another to form a magma. This process also results in partial melting of the mantle rock. This type of melting occurs within subduction zones as water is ‘squeezed’ from the subducted oceanic lithosphere into the overlying ultramafic mantle wedge.

How Magma is finally formed?

The initial composition of the magma depends upon the composition of the source rock and the degree of partial melting.   In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas, while melting of crustal sources yields more siliceous magmas. In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma. Then, the transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.  This is called magmatic differentiation and includes some processes such as assimilation, mixing, and fractional crystallization.


January 12, 2018

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