Indian geography 9

VOLCANOES

Overview

• A volcano is an opening in a planet’s surface or crust that allows hot magma, ash and gases to escape from below the surface
• Volcanoes erupt enormous quantities of toxic gases and water vapour, and can cause significant changes in global climate patterns
• The magma from volcanoes, upon cooling, solidifies into igneous rocks like basalt and granite
• Volcanism is mainly responsible for the formation of the earth’s atmosphere
• Active volcanoes are those that have erupted within the Holocene period (last 10,000 years)
• Dormant volcanoes are those that have not erupted in recent times, but might potentially erupt in the future.
• Extinct volcanoes are those that are not likely to erupt again, because the volcano no longer has a supply of lava. It is difficult to differentiate extinct volcanoes from dormant ones since many volcanoes that lie inactive for tens of thousands of years suddenly erupt without warning
  
• The explosiveness of a volcanic eruption is measured by the Volcanic Eruption Index (VEI). The index goes from 0 to 8, with 0 representing non-explosive eruptions and 8 representing mega-colossal eruptions from supervolcanoes
  

Occurrence of volcanoes

The 1991 eruption of Mt Pinatubo (Philippines) was the world's largest in living memory. The eruption sent an ash plume 19 km into the atmosphere and caused global temperatures to drop by 0.5 C

• Most volcanic activity occurs in the oceans, continuously forming new sea floor
• The most active volcanic belt is the Ring of Fire, which occurs along the boundaries of the Pacific Ocean
• In addition to the Earth, volcanoes occur on other planets as well
• Jupiter’s moon Io is the most volcanically active object in the solar system
• The tallest mountain in the solar system, the Olympus Mons on Mars (21 km tall), was built by volcanic activity

Volcanoes and plate tectonics

• Volcanoes are generally found tectonic plates are diverging or converging, but not where two tectonic plates slide past each other
• Divergent boundaries: At mid ocean ridges, tectonic plates diverge from one another. The release of pressure due to thinning of the crust leads to volcanism. Examples: deep sea vents, Iceland
  
• Convergent boundaries: when two tectonic plates, one subsides over the other, creating subduction zones. Water released from the subducting plate lowers the melting temperature of the other, creating magma. Examples: Mt. Etna, Pacific Ring of Fire
  
• Hotspots: Hotspots are not located at the boundaries of tectonic plates but above mantle plumes (narrow stream of hot mantle convecting up towards surface). The temperature of the plume causes crust to melt and form venting pipes. Examples: Hawaii, Yellowstone Caldera
  

Effects of volcanoes

• Earthquakes, hot springs, geysers etc often accompany volcanic activity
• Volcanoes typically emit large quantities of water vapour, carbon dioxide and sulphur dioxide
• Large explosive volcanic eruptions inject these gases into the stratosphere to heights of 16-32 km
• Conversion of the sulphur dioxide into sulphuric acid increases the earth’s albedo, increasing the reflection of radiation from the sun
• This leads to significant and protracted global cooling
• Gas emissions from volcanoes results in acid rain
• Volcanic activity releases about 0.13-0.23 giga tonnes of carbon dioxide every year (about 1% of amount released by human activity)
  

Decade volcanoes

• Decade Volcanoes are those volcanoes that have been identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) as being worthy of particular study
• Decade Volcanoes are bring particular attention due to their history of large destructive eruptions and their proximity to population areas
• They are named Decade Volcanoes because they were initiated as part of the UN International Decade for Natural Disaster Reduction (the 1990s)
• The Decade Volcanoes project encourages studies and public awareness activities with the aim of better understanding the volcanoes and the dangers they represent
• There are 16 recognised Decade Volcanoes. See list given
  

FEATURES OF VOLCANOES

Composition of Lava

Volcanoes release enormous quantities of gases into the atmosphere in an effect called Volcanic Injection

• Lava is the name given to magma once it has escaped to the surface
• Felsic lava: If the magma erupted contains a high percentage of silica (> 63%), the lava is called felsic lava
  • Felsic lava tends to be highly viscous and are erupted as domes or short stubbly flows.
  • They tend to form stratovolcanoes or volcanic domes
• Intermediate lava: silica content 52-63%
  • Generally occur at subduction zones
• Mafic lava: silica content 52-45%
  • These lavas have higher content of Magnesium and iron
  • Less viscous but much hotter than felsic lavas
  • They occur in mid ocean ridges, shield volcanoes and continental flood basalts
• Ultramafic lava: silica content less than 45%
  • Ultramafic lava flows are very rare
  • They have not erupted in millions of years
  • Ultramafic lavas were the hottest lavas

Pyroclastic flows

• Pyroclastic flows are fast moving currents of tephra (hot gas and rock), which travel from volcanoes at speeds up to 700 km/h
• Pyroclastic flows are a devastating result of explosive volcanic eruptions
• The gas can reach temperatures up to 1000 C
• Pyroclastic surges are flows where the proportion of gas is much higher than rock. This makes pyroclastic surges more turbulent and can rise above hills and ridges. Pyroclastic surges are even more devastating than pyroclastic flows and can reach speeds up to 1000 km/h
• Famous pyroclastic flows include the ones that engulfed the towns of Pompeii and Herculaneum in Italy in 79 CE

Calderas

• A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption
• Calderas arise because the emptying of the magma chamber beneath the volcano, with the result that the emptied chamber is unable to support the weight of the volcanic material above it
• Calderas are formed as a result of a large volcanic eruption

TYPES OF VOLCANOES

Shield Volcanoes

Map of major volcanoes around the world

• Shield volcanoes are formed by the eruption of low viscosity lava
• The lava flows a great distance from the vent
• Shield volcanoes do not explode catastrophically
• They are more common in oceans than in continents
• Eg: Hawaii, Iceland

Mud volcanoes

• Mud volcanoes (not strictly volcanoes) are formations created by the geo-excretion of liquids and gases
• Temperatures in mud volcanoes are much cooler than in igneous processes
• Ejected material primarily consists of methane, carbon dioxide and water vapour (acidic)
• Mud volcanoes can reach 10 km in diameter and about 700 m in height

Submarine volcanoes

Barren Island, the only active volcano in India, as seen from the ISS

• Submarine volcanoes are underwater fissures in the earth’s crust from which magma can erupt
• Submarine volcanoes account for over 75% of the world’s magma releases
• Submarine volcanoes are mainly located near ocean ridges, where tectonic plate movement in maximum
• Due to the presence of water, lava from submarine volcanoes cools and solidifies quickly, turning into volcanic glass
• Submarine volcanoes are concentrated in the Ring of Fire in the Pacific Ocean
• The West Mata volcano, in the Pacific Ocean, is currently the deepest erupting submarine volcano (1100 m)

Subglacial volcanoes

• Subglacial volcanoes erupt beneath the surface of glaciers or ice sheets
• The rising lava causes the ice to melt and form a lake
• The rapid melting of ice into water due to lava can lead to glacial lake outburst floods
• Subglacial volcanoes are most common in Iceland and Antarctica

Stratovolcanoes

• Stratovolcanoes are tall conical volcanoes with many layers (strata) of hardened lava and ash
• Stratovolcanoes are characterised by steep slopes and explosive eruptions
• Stratovolcanoes are the most common type of volcanoes found
• They are common in subduction zones, forming chains of volcanoes along tectonic plate boundaries
• Stratovolcano explosions tend to result in destructive pyroclastic flows that have affected civilization through history
• The explosion of Tambora Volcano (Indonesia) in 1815, the most powerful eruption in recorded history, lowered global temperatures by about 3 C.
• Eg: Mt. Vesuvius (Italy), Mt. Fuji (Japan), Mt. St Helens (USA), Mt Pinatubo (Philippines)

Supervolcanoes

Fountain of lava about 10 m high issuing forth from a vent in Hawaii

• Supervolcanoes are volcanoes with ejected material greater than 1000 cubic km, which is millions of times larger than any volcanic event in known history
• Supervolcanoes can produce devastation on an enormous continental scale
• Supervolcanoes occur when magma rises to the crust in hotspots but is enable to break through the crust. Pressure build in the large and growing magma pool until the crust is unable to contain the pressure
• Super volcanic eruptions cause long lasting climate change (esp. global cooling) and directly result in the large scale extinction of species
• There are only seven known supervolcanoes: Yellowstone Caldera (USA), Long Valley Caldera (USA), Valles Caldera (USA), Lake Toba (Indonesia), Lake Taupo (New Zealand), Aira Caldera (Japan), Siberian Traps (Russia)
• Large igneous provinces are also considered supervolcanoes due to the amount of lava released, but they are non-explosive in nature
• There have been no supervolcanic eruptions in the Holocene period (10,000 yrs BP). The last supervolcano eruption was the Lake Taupo (New Zeland) about 26,500 yrs ago

LIST OF VOLCANOES

See here for a full list of volcanoes

List of Decade Volcanoes

S. No.	Volcano	Classification	Location	Notes
1	Avachinsky-Koryaksky	Active Stratovolcano	Kamchatka Peninsula, Russia
2	Colima’s Volcano	Active Stratovolcano	Mexico	One of the most active volcanoes in North America
3	Mount Etna	Active Stratovolcano	Sicily, Italy	Largest active volcano in Europe (3300 m) One of the most active volcanoes in the world Last eruption 2008
4	Galeras	Active Stratovolcano	Colombia
5	Mauna Loa	Active Shield volcano	Hawaii, USA	Largest volcano on Earth in terms of volume and area covered One of five volcanoes that constitute island of Hawaii Eruptions tend to be non-explosive
6	Mount Merapi	Active Stratovolcano	Indonesia
7	Mount Nyiragongo	Active Stratovolcano	Congo	Famous for its crater that contains a lake of lava
8	Mount Rainier	Dormant stratovolcano	USA	Has 26 glaciers and 35 sq miles of permanent snow fields and glaciers World’s largest volcanic glacier cave network
9	Sakurajima	Active stratovolcano	Japan	Eruption in 1914 cause former island to be connected to Osumi peninsula
10	Santa Maria	Active stratovolcano	Guatemala
11	Santorini	Dormant	Greece	Forms an archipelago of volcanic islands Site of one of largest eruptions in recorded history (3600 yrs ago destroyed Minoan civilization)
12	Taal volcano	Active stratovolcano	Philippines	Currently giving signs of activity since June 2009 Currently on a Level 1 alert
13	Mount Teide	Active stratovolcano	Canary Islands, Spain	Third largest volcano in the world
14	Ulawun	Active stratovolcano	Papua New Guinea
15	Mount Unzen	Active stratovolcano	Japan	Group of several overlapping volcanoes
16	Mount Vesuvius	Active stratovolcano	Italy	Most densely populated volcanic region in the world Famous for 79 AD eruption that destroyed cities of Pompeii and Herculaneum

List of supervolcanoes

Volcano	Location	Notes
Lake Taupo	North Island, New Zealand	Latest known supervolcanic eruption (26,500 yrs BP). Ejected 1170 cu km of material Last major eruption in 180 CE (ejected 100 cu km) Eruption of 180 CE was noticed as far away as China and Rome
Lake Toba	Sumatra, Indonesia	Largest volcanic lake in the world Supervolcanic eruption 74,000 yrs ago of VEI 8 (2800 cu km) Believed to be largest eruption on Earth in last 25 million yrs ago Eruption deposited ash layer 15 cm thick all over India, some parts up to 6 m thick Caused major extinctions of plant and animal species, including severely endangering human species
Whakamaru	North Island, New Zeland
Yellowstone Caldera	Wyoming, USA	Last eruption 640,000 years ago
Island Park Caldera	Idaho, USA	Last eruption 2.1 million years ago
Kilgore Tuff	Idaho, USA	Last eruption 4.5 million years ago
Blacktail Creek	Idaho, USA	Last eruption 6.6 million years ago
La Garita Caldera	Colarado, USA	Last eruption 28 million years ago Largest known explosive eruption in history of Earth (5000 cu km)

List of volcanoes in India

Volcano	Classification	Location	Notes
Barren Island	Active stratovolcano	Andaman Islands	Only active volcano in India Last eruption in July 2009
Baratang	Mud volcano	Andaman Islands	Last eruption in 2005
Narcondam	Potentially active stratovolcano	Andaman Islands	Thought to have been inactive, but recently mud and smoke activity in June 2005 Recent activity possibly related to 2004 Indian Ocean Earthquake Narcondam is famous for the Narcondam Hornbill, an endangered species Narcondam island is the eastern-most point of the Andaman & Nicobar Islands. It is claimed by Burma
Deccan Traps	Large Igneous Province	Deccan Plateau	One of the largest volcanic features on Earth Multiple layers of basalt more than 2 km thick Technically it may classified as a supervolcano