Volcanoes: Definition, Types, & Facts

Volcanoes: Definition, Types, & Facts

Volcanoes referred to landforms created by the accumulation of solidified lava and volcanic detritus near the vent.

In simple words, a volcano is a rupture in the crust of a planetary-mass object, like Earth, by which hot lava, gases, and volcanic ash escape from the magma chamber below the surface. Volcanoes are most often found on Earth at the place where tectonic plates are diverging or converging. The most concentration of volcanoes is found underwater.

Magma

In the deep of the earth, it is too hot so some rocks start melting slowly and become a thick liquid, that substance is termed magma. As the magma is lighter than the solid rock around it, it rises and collects in the magma chamber.

Lava

The magma pushes through the vents and fissures to the surface of the earth, that erupted magma is called lava (molten rock). These form volcanic landforms. The volcanic landforms are the result of repeated volcanic activities. For example, The large lava erupts of Mauna Loa volcano in Hawaii. It is an example of a huge shield volcano that is built up by repeated eruptions of fluid lava.

Volcanic Gases

The concentration of volcanic gases can vary from one to another. generally, water vapour is the most abundant volcanic gas, along with Carbon dioxide and sulfur dioxide. Other principal gases like hydrogen sulfide, hydrogen chloride, and hydrogen fluoride are also found. Other gases like trace gases are also found in volcanic eruptions.

Tephra

Inside the volcano when magma blows apart by the rapid expansion of hot volcanic gases then the tephra forms. It is a fragmental material produced by a volcanic eruption.

Types of Volcanos

When the magma erupts as lava on the earth’s surface, different types of volcanoes formed on the basis of viscosity, magma, amount of gas in the magma, the composition found in magma, and the way of magma by which reached the surface.

1. Shield Volcano
2. Composite volcano or stratovolcano
3. Volcanic Dome
4. Cinder Cone
5. Caldera
6. Fissure Cone

Shield Volcano:

Shield volcanos form gentle slopes or shield low shapes consisting of shallow slopes built by many lava flows.

General characters:

• Most shield volcanoes are the eruption of basalt lava flows, tephra (pulverized rock and clastic materials ejected violently during eruption) ejection.
• In these volcanoes the silica content of magma is low
• These are low viscous, with low gas content, and can flow a great distance from the vent.
• Example: Mauna Kea and Mauna Loa, are the world’s largest active volcanoes, which rise over 9 km above the seafloor in Hawaii.

Composite volcano or stratovolcano:

Composite volcanos are tall Cone-shaped mountains with steep sides. These are built of alternating layers of lava flows, tephra, and pyroclastic deposits. As these are created from multiple structures during different kinds of eruption, so also known as composite volcanos.

General characters:

• In these volcanoes the silica content of magma is intermediate.
• Viscosity is intermediate it does not flow easily.
• Gas content is high.
• The most common rock types of stratovolcanoes are Andesite. (it is named after the Andes mountains)
• These produce explosive eruptions due to gas in viscous magma.
• Examples: Mt. Fuji, in Japan, Mayon Volcano in the Philippines, Mount Vesuvius, and Stromboli in Italy.

Volcanic Dome:

It is a type of volcano with roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano.

General characters:

• The silica content of magma is high
• The viscosity is high, with high gaseous content.
• The rock-type formation is Rhyolite
• These are highly explosive.
• Example: Mt. Lessen, USA

Cinder Cone:

These are also called ash cones. It is a cone-shaped small (less than 450m or 1500 ft.) truncated top (bowl-shaped crater at the top) form that is built up of cinders. Cinders accumulate during moderately explosive eruptions. These are made up of tephra and scoria.

General characters:

• the silica content of magma is low in cinder cones
• Viscosity is also low in these types
• Low gas content
• formation of rocks is basalt Pyroclastic rock fragments.
• Example: springer Ville, AZ.

Caldera:

After a very large explosive eruption, the magma chamber empties, and the roof of the magma chamber can collapse as the result of a large explosion forming a depression or bowl on the surface having very steep walls. These form caldera that may fill with rainwater such as crater lakes. these calderas can be tens of miles across. Example: Taal Lake in the Philippines.

Fissure Cone: These are linear volcanic vents, by which the lava erupts. These vents are often a few meters wide and maybe many kilometers long.

General Characters:

• These vents can cause large flood basalts which run first in lava channels, and later in lava tubes.
• After some time the eruption built up spatter cones.
• These are non-explosive
• Example: Holuhraun Iceland.

On the basis of periodicity, the volcanos may be divided into:

1. Active Volcanos: These volcanos constantly eject lava, gases, ashes, cinder, pumices, etc. At present, about 600 volcanos in the world are in this category. Most active volcanos are found in the Pacific Ocean.
2. Dormant Volcanos: These are not extinct but did not erupt within the historic time. The Vesuvius volcano in Italy is an example of a Dormant Volcano, it erupted first in 79 A.D. and after that, it erupted with great force in 1631 A.D. it subsequently erupted in 1803, 1872, 1906, 1927, 1928, and 1929. Another example of a Dormant volcano is Kilimanjaro (Tanzania).
3. Extinct Volcanos: A volcano that was active in a distinct geological past and there is no longer volcanic activity is called an extinct volcano. Example- Crater converting into a lake, Arthur’s seat (Edinburgh- Capital of Scotland), Aconcagua (Andes), Sulaimon, and Demavend in Elburz (Iran).

Types of Volcanic eruptions

On the basis of the mode of eruption, various classifications of volcanoes are given. The most common classification of volcanic eruption is given by Lacroix in 1908. According to him, volcanic eruptions are of four types mainly:

• Hawaiian eruption
• Strombolian eruption
• Vulcanian eruption
• Pelean eruption
• Plinian eruption

Types of volcanic eruptions

Hawaiian eruption: In this type of eruption, large quantities of extremely fluid basic lava flow out from a fissure or a central vent and forms a typical shield. Usually, explosive activity is not found in this type of eruption.

Strombolian eruption: The basic lava is less fluid than the Hawaiian type of eruption. In this type, the explosion is more common. The name Strombolian is the name of the volcano on the island of Stromboli in north Sicily.

Vulcanian eruption: The lava surface rapidly solidifies as having high viscosity. in A result of fast solidification, it builds up pressure beneath the lava crust so there is continuous series of violent explosions during which large quantities of pyroclastic materials are violently ejected from the vent. It consisting ash, which gets distributed widely by wind.

Pelean eruption: in this type of eruption the lava is extremely viscous, it mostly erupts in violent forms. The most common silent feature of this eruption is the formation of “nuee ardentes” i.e. glowing clouds. The eruption is named after Mt. Pelee (West Indies), including 1902, where extremely violent eruptions occurred.

Plinian eruption: It is intensively violent, with the gases boiling out, and the gas-rich magma generates enormous and continuous blasts.

Distribution of Volcanos in the world

The volcanos are closely concentrated along the margin of the plate. The main volcanic regions are as follows:

• It is found along subduction (boundaries at the continental plate- oceanic plate convergence (Mount Saint Helens) or oceanic plate- oceanic plate convergence (the Philippines and Japan)
  • the subduction zones are places where the collision of two plates, usually an oceanic plate and a continental plate takes place.
• Along the seafloor (Mid-ocean ridge- MOR) in which the seafloor mountain system is formed by plate tectonic, here the volcanoes are caused by divergent tectonic plates.
• In the Pacific Ring of Fire, the volcanoes are caused by a convergent tectonic plate.
• The volcanic concentration is also in areas of rifting on continental plates i.e. the rift valley in East Africa, The Wells Gray- Clearwater volcanic field, and the Rio Grande Rift in North America.
• The hot spots, where the individual plumes of magma rise the crust as in Hawaii Island.
  • Hot spots: In terms of geology, hot spots are volcanic regions thought to be fed by the underlying mantle which is anomalously hot in caparison with the surrounding mantle.

Subduction Zone of Volcanoes

Volcanoes and society

Hazardous:

For centuries, they have caused dismay and terror for people living in volcanic belts.

• The volcanos are a source of terrible destruction
• Volcanic gases after reaching the stratosphere form sulphuric acid aerosols which can reflect solar radiation and lower surface temperature.
• The chemical reaction of sulfate aerosol in the stratosphere can also damage the ozone layer, and can also cause acid rain from which the acids such as hydrogen chloride, and hydrogen fluoride can fall to the ground.
• The explosive volcanic eruptions release greenhouse gases.
• In 1601- 1603 the Russian famine may have been caused by a sulfur dioxide eruption from Huaynaputnia.
• The ash of eruptions can affect aircraft and can create a major disruption to air travel.
• The eruption of Mount Tambora in 1815, created global climate anomalies which were known as “year without a Summer” due to the effect on North American and European weather.

Benefits:

• The volcanic eruption has also created some important economic resources as they contain valuable mineral resources, Basalt rich volcanic soils that are most fertile, geothermal heat, metallic ores, etc.
• Industrial materials like building stones, sulfur, pumice, slate, dykes are very useful volcanic eruption materials.
• Many ornamentals stones and precious gems have a volcanic origin.

You can also read:

• Continental Drift Theory
• The Rocks- Igneous Rocks 
• Sedimentary rocks
• Metamorphic rocks