Volcano Notes - Geography

Volcano

A volcano is a landform at the end of a conduit or pipe that rises from below the crust and travels to the Earth's surface. Magma rises up and collects in a magma chambers deep below resulting in eruptions that are effusive or explosive and form mountains. An idealized cross-section of a volcano is shown in Fig. a. In fact, magma from a volcano called lava (molten rock), lava gases and tephra (pulverized rock and fragmented material ejected violently during an eruption) travels through the mantle to the surface and forms a volcanic landform.  

A cinder cone is a small cone-shaped hill usually less than 450 meters (1,500 ft) high with a small top made of cinder that was deposited during minor explosive eruptions. Cinder cones are made of tephra and scoria. Another distinctive landform is a large basin-shaped crater called a caldera (Spanish for 'kettle'). It forms when summit material on a volcanic mountain collapses inward following an eruption or other loss of magma, forming a caldera that may fill with rainwater such as a crater lake. 

   Fig.  a

Composite Volcanoes

Composite volcanoes are built from alternating layers of mixed volcanic ash and lava flows, which form characteristically high, steep-sided cones (Fig. a).

Types of Volcanic Eruptions

Fig. b

There are several classifications of volcanoes based on the type of eruption. The most commonly used classification of volcanic eruptions was given by Lacroix in 1908. According to Lacroix, the four major types of eruptions are (i) Hawaiian, (ii) Strombolian (iii) Vulcanian and (iv) Pelian.

(i) Hawaiian Eruption

It is an eruption in which a large quantities of extremely liquid basic lava is ejected from a fissure or a central vent to form a characteristic shield. Explosive activity is almost absent in this type of volcano.

(ii) Strombolian Eruption

In this type of volcanic eruption, the basic lava (basaltic) is less fluid than in the Hawaiian type. As a result, eruptions are more common and more segmental material is ejected. It is named after a volcano on the island of Stromboli off northern Sicily.

(iii) Vulcanian

In this type of eruption, the surface of the lava solidifies rapidly due to its high viscosity. The solidification results in a build-up of pressure beneath the lava crust and a continuous series of violent eruptions, during which large quantities of pyroclastic material are violently ejected from the vent. Volcanic ash can be widely distributed by the wind.

(iv) Pelean Eruption

The lava at Pelion is extremely viscous. It usually erupts in violent forms. One of its main features is the formation of Nus Andantes (shining clouds). This type of volcanic eruption is named after Mount Pelly (West Indies), where extremely violent eruptions have occurred, including the 1902 eruption.

Distribution of volcanoes in the world

 

Fig. c

There is a close relationship between plate margins and the distribution of volcanoes in the world. In fact, volcanoes are closely concentrated along plate margins. The main concentration of volcanoes is in the following regions (Fig. c).

  1. Along subduction boundaries at continental plate–oceanic plate convergence (Mount St. Helens) or oceanic plate–oceanic plate convergence (Philippines and Japan)

  2. Along ocean floor spreading centers (Iceland) and rift zones on continental plates (Rift Valley in East Africa).

  3. At hot spots, where separate masses of magma lift up the crust (Hawaiian Islands).

Volcanoes on periodic basis can be divided into the following three categories: (i) active volcanoes, (ii) dormant volcanoes, and (iii) extinct volcanoes

Fig. d

   (i) Active volcano

 The volcanoes which continuously emit lava, gases, ash, cinder, pumice etc. are known as active volcanoes. There are currently about 600 active volcanoes in the world, most of which are located in the Pacific Ocean.

  (ii) Dormant volcano

  A volcano which, although not extinct, is not known to have erupted within historical time. The best example of a dormant volcano is the Vesuvius volcano (Italy), which first erupted in 79 AD. It remained dormant for more than 1550 years and then suddenly erupted with great force in 1631 AD. Subsequent eruptions occurred in 1803, 1872, 1906, 1927, 1928, 1929. Kilimanjaro (Tanzania) is also an example of a dormant volcano.

  (iii) Extinct volcano

  A volcano that was active in a specific geological past and whose remains occur in an area where there is no longer any volcanic activity is known as an extinct volcano. The crater fills with water and turns it into a lake. Arthur's Seat (Edinburgh -vthe capital of Scotland), Aconcagua (Andes), Suleiman in Elburz (Iran) and Demavand are some examples of extinct volcanoes (Fig. d).

  

Volcanoes and Society

Volcanic eruptions are the most popular sight of all geological phenomena. For centuries, they have caused despair and terror for those living in the volcanic belt. Volcanoes have been a source of terrible destruction throughout the history of mankind. However, they obtained valuable mineral resources that sustain life. The resources provided by volcanoes include rich volcanic soil, geothermal heat, metallic ores (gold, silver, copper, molybdenum, beryllium, iron, lead, zinc, etc.), and the air we breathe and which We drink water. Industrial materials such as building stone, pumice stone, pumice stone, slate, dikes are the result of volcanic activity. Precious gems like diamond, gems are of volcanic origin. Some of our most spectacular landscapes like Laki, Eyjafjöll and Katla (Iceland), Kilimanjaro (Tanzania), Mount Fuji (Japan), Lake Titicaca (Bolivia-Peru border), Pacific Islands, Yellowstone Park (America) etc. are due to volcanoes. Volcanic eruption also brings climate change.

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