- Four types of volcanism can be identified.
- Exhalative Volcanism (vapour or fumes)
- Effusive Volcanism (lava outpouring)
- Explosive Volcanism (violent ejection solid material)
- Subaqueous Volcanism
Exhalative Volcanism (vapour or fumes)
This includes the discharge of material in gaseous forms, such as
- steam, fumes and
- Hydrochloric acid
- Ammonium chloride
- Sulphur dioxide
- Carbon dioxide
- Carbon monoxide.
- Hydrogen sulphide
- These gases may escape through vents which are in the form of hot springs, geysers, fumaroles and solfataras.
- This kind of volcanism indicates the volcano is reaching its extinction.
- Associated landforms are called sinter mounds, cones of precipitated minerals and mud volcanoes.
Effusive Volcanism (Lava outpouring)
- Effusive: relating to or denoting igneous rocks poured out as lava and later solidified.
- This type of activity refers to abundant outpourings of basaltic lava from a vent or fissure.
- The Deccan traps, which are composed of such lavas today, cover an area of 5,00,000 square km. The original extent of the formation must have been at least 14 lakh square km.
- Columnar structure is sometimes developed in fine-grained plateau basalts (Deccan Traps near Mumbai).
Explosive Volcanism (Violent ejection of solid material)
Volcano (William Crochot, via Wikimedia Commons)
- This type of activity results in fragmentation and ejection of solid material through vents.
- Volcanic eject that settles out of air or water is sometimes called pyroclastic sediments.
- Tephra: all fragmented ejects from the volcanoes.
- Ash: The finest sand-sized tephra
- Lapilli: These are gravel-sized particles either in the molten or solid state.
- Blocks: Cobble or boulder-sized solid ejecta.
- Bombs: a lump of lava thrown out by a volcano.
- Tuff: Layers of volcanic dust and ashes.
- Smaller particles like lapilli and ash travel through air for many kilometres.
- The heavier particles like bombs and blocks fall in the vicinity of the vent.
- This type of volcanic activity takes places below the surface of the water.
- When lava is in contact with water, it consolidates to produce a structure like that of a heap of pillows
- Highly viscous lavas erupted at lesser depths develop glassy margins on pillows. The related volcanic product is hyaloclastite. Most hyaloclastites identified are in Iceland.
Eruptive Volcanism Types
- Hawaiian eruptions are a type of volcanic eruption, named after the Hawaiian volcanoes.
- They are the calmest types characterised by the effusive eruption of very fluid basalt-type lavas from craters, lava lakes, fissures with little-ejected material (low gaseous content).
- A single flow spreads widely over open slopes or flows down the valleys as lava rivers.
- Steady production of small amounts of lava builds up the large, broad form of a shield volcano.
- Eruptions are not centralised at the main summit as with other volcanic types and often occur at vents around the summit and from fissure vents radiating out of the centre.
- The Icelandic type is characterized by effusions of molten basaltic lava that flow from long, parallel fissures.
- Such outpourings often build lava plateaus. E.g. Deccan Traps, Siberian Traps.
- Strombolian eruptions are a type of volcanic eruption, named after Stromboli (Lipari Islands, Italy).
- Stromboli Volcano (lighthouse of the Mediterranean) has been erupting continuously for centuries.
- Strombolian eruptions are driven by the continuous formation of large gas bubbles within the magma.
- Upon reaching the surface, the bubbles burst with a loud pop, throwing magma in the air.
- Because of the high gas pressures associated with the magma, episodic explosive eruptions occur (erupts once in every few minutes – fountain like eruption).
- The greatest volcanic explosion known to humans is perhaps that of Krakatoa (Plinian) eruption in 1883.
- The explosion could be heard in Perth, Australia, almost 3,000 miles away.
- More than 36,000 people died, mostly from the tsunamis that followed the explosion.
- At present, Krakatoa (Krakatau or Krakatoa Archipelago) is a group of four small volcanic islands in the Sunda Strait, between Java and Sumatra.
- Three of the four islands are the remnants of the previous volcanic structure destroyed in 1883 eruption.
- The fourth island, Anak Krakatau (meaning Child of Krakatoa) emerged in the 1920s from the caldera formed in 1883. It is the current location of eruptive activity.
- In recent times, Anak Krakatau has become increasingly active with Strombolian type eruptions.
2018 Sunda Strait tsunami
- The eruption of Anak Krakatoa in December 2018 and subsequent collapse of the southwest sector of the volcano, including the summit, triggered the tsunami that has killed more than 400 people.
- While Indonesia possessed a tsunami warning system for tsunamis caused by earthquakes, there were none in place for volcanic tsunamis, and hence there were no early warnings.
- In Vulcanian eruptions, intermediate viscous magma within the volcano makes it difficult for gases to escape.
- This leads to the build-up of high gas pressure, eventually resulting in an explosive eruption.
- They are also more explosive than their Strombolian counterparts, with eruptive columns often reaching between 5 and 10 km high.
- The molten lava is explosively ejected as a great cauliflower cloud of dark tephra. Bombs, blocks, lapilli and other ejecta fall in the surrounding area.
- After each eruption cycle, the volcano is dormant for decades or centuries.
- Plinian eruptions (or Vesuvian) are a type of volcanic eruption, named after the historical eruption of Mount Vesuvius in 79 AD that buried the Roman town of Pompeii.
- In Plinian eruptions, dissolved volatile gases stored in the magma are channelled to the top through a narrow conduit (pipe-like structure).
- The gases erupt into a massive column of the gas plume that reaches up 2 to 45 km into the atmosphere.
- As it reaches higher the plume expands and becomes less dense and convection and thermal expansion of volcanic ash drive it even further up into the stratosphere.
- At the top of the plume, powerful prevailing winds drive the plume in a direction away from the volcano.
- Mount Vesuvius is a stratovolcano in Bay of Naples, Italy.
- It is best known for its Plinian type eruption in AD 79 that led to the destruction of the Roman cities of Pompeii, Herculaneum and others. More than 1,000 people died in the eruption.
- The eruption ejected a cloud of stones, ashes and volcanic gases to a height of more than 30 km.
- Vesuvius has erupted many times since. Today, it is regarded as one of the most dangerous volcanoes in the world because of the population of 3,000,000 people living nearby.
- The eruptions alternated between Plinian and Peléan with most of them being Plinian type.
Mount St. Helens
- Mount St. Helens is an active volcano located in the Cascade Volcanic Arc.
- Mount St. Helens is most notorious for its major 1980 Plinian type eruption that killed more than 50 people.
- Mount Tambora is an active volcano located in Lesser Sunda Islands of Indonesia.
Mount Tambora (Google Maps)
- Tambora is known for its major Plinian type eruption in 1815.
- The 1815 eruption was one of the most powerful in recorded history, with a Volcanic Explosivity Index (VEI) of 7.
- The ash from the eruption column dispersed around the world and lowered global temperatures, in an event sometimes known as the Year Without a Summer in 1816.
- More than 71,000 people died due to famines caused in Europe and America.
Volcanic Explosivity Index (USGS)
Nevado del Ruiz
- Nevado del Ruiz is a volcano located in Colombia.
- The volcano usually generates Vulcanian to Plinian eruptions, which produce destructive lahars.
- In 1985, a small eruption produced an enormous lahar that buried and destroyed the towns causing an estimated 25,000 deaths.
- A lahar is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
- Lahars are extremely destructive as they flow fast and can engulf entire settlements in a matter of minutes.
- Mount Pinatubo is an active volcano located in the Luzon island of the Philippines.
- Pinatubo Plinian type eruption in 1991 brought about dramatic changes in the global environment.
- The amount of sulfuric ash it sent into the stratosphere cooled global ground temperatures by 1°C for the next two years, and ozone depletion temporarily increased substantially.
- The eruption resulted in more than 700 deaths.
- Peléan eruptions are a type of volcanic eruption, named after the volcano Mount Pelée in Martinique.
- In Peléan eruptions, a large amount of gas, dust, ash, and lava fragments are blown out laterally by the collapse of the cinder cone. The sudden burst of lava dome causes the collapse of the cinder cone.
- This type of eruption results in very viscous, gas-rich, acidic lava breaking out laterally and flowing out violently at high speed causing massive destruction on its path.
- Hot gas and lava mixture are not carried skyward to become cold tephra but spreads downslope as a nuce ardente, continuing to cushion the flowing fragments.
- Mount Pelée is a volcano at the northern end of Martinique Island (French overseas department in the Lesser Antilles island arc of the Caribbean).
- The volcano is famous for its Pelean type eruption in 1902 that killed about 30,000 people.
- Most deaths were caused by pyroclastic flows which destroyed the city of Saint-Pierre.