Vulcanicity: Definition, Types and Effects of Vulcanicity

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Vulcanicity Definition

Vulcanicity refers to the rising of molten magma or other gaseous materials beneath the earth’s crust on the surface or within the earth’s crust. Molten magma naturally escapes through cracks and fissures, hence vulcanicity is associated with areas that have experienced mountain building processes of compression and tension.

Types of vulcanicity

Vulcanicity can be analyzed in two forms based on its mode of formation and origin

  1. Intrusive vulcanicity
  2. Extrusive vulcanicity

Intrusive vulcanicity

This involves the solidification of molten magma within the earth’s crust; however, it may become visible at the Earth’s surface due to erosion and weathering. When molten rock is forced up through the crust, it may solidify in a number of Forms. However, only a little quantity of magma reaches the surface, the remaining is intruded into the crust where it solidifies and produces intrusive features. Intrusive activity has little and most times no impact at all on the surface until overlying rocks are later worn away, leaving landforms produced by their exposure on the surface. Magma usually cools slowly during intrusive activity while it allows the mineral crystals to grow. When this happens, it produces a kind of coarsely crystalline rock, for instance, granite.

Types of intrusive landforms

  • Dike
  • Sill
  • Batholith
  • Laccolith
  • Loppolith
  • Phacolith
  • Dike ridges
  • Volcanic necks

Dike- this is a discordant volcanic intrusion that forms when magma solidifies within vertical faults. Dikes get cooled in the same position and develop into a wall-like structure or walls of hard rocks cutting the country-rock discordantly. Examples of dikes include Cleveland Dike in England, Luanite Dike in Scotland, and the western Maharashtra area. However, these are seen as the feeders for the eruptions that led to the development of the Deccan traps.

Sill- sill unlike dike is a sheet of solidified magma that lies accordantly to the bending plains of sedimentary rocks. Examples of sills include Northumberland, The Great Whin Sill that outcrops across a large part of North-East England, Salisbury Crag in Zimbabwe, and the Great Karoos in South Africa. Because they are much more resistant than the surrounding rocks they form escarpments. For example, and Peel Crags on Hadrians Wall.

Batholith– this is a dome-sloped mass of igneous rock formed deep down in the crust. This magnetic intrusion is often composed of granite. it is much larger than Laccolith, it covers large areas and may extend hundreds of square kilometers. The formation of batholith has been attributed to the sinking in a resultant melting and incorporation of large blocks of surrounding area rocks into the molten magma which may later solidify.

Laccolith– this appears like a huge blister and is produced as a result of viscous magma forming the covering of sedimentary rock into a dome. This is by local accumulation of molten magma below. These are large dome-shaped intrusive bodies; they resemble the surface volcanic domes of the composite volcanoes, only these are located at deeper depths. they can be regarded as the localized source of lava that finds its way to the surface. Many isolated conical hills for example Trapin Law, Eldon Hills in Scotland, The Karnataka plateau, and the Henry Mountains in Utah, USA are formed in this way.

Lapolith– this occurs when intrusive magma forms into a saucer-shaped mass. An example of this is in Scotland and England.

Phacoliths– these are solidified masses of rocks that occupy both synclines and anticlines of bedding planes.

Volcanic Necks– These results from the solidification of lava in the vents of volcanoes. They resist erosion better than volcanic cones themselves. Examples include Dumbarton Rock, Scotland, Devils Towers Wyoming in the USA.

Dike Ridges– they are steep-sided ridges of volcanic origin. They are usually larger than volcanic rocks for example Crazy Mountains of Montana, the Great dike of Rhodesia.

Extrusive vulcanicity

This refers to the solidification of molten magma directly on the surface of the earth’s crust to form distinctive features. When this occurs, there is usually a large number of rock fragments, volcanic dust, and fine volcanic ashes ejecting from the vent with the lava. However, a large amount of gaseous materials is equally ejected. In the ejection process, the basaltic lava flows out first followed by the andesite lava, and lastly the rock fragments. The lava that flows out during volcanic eruptions when it cools changes into igneous rocks. The cooling may occur either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava. The lava that cools within the crustal portions takes on different forms. These forms are called intrusive forms.

Almost all the volcanoes in the world are found along the border of the continents and the oceans. The main belt extends from New Zealand in the Pacific to the southern tip of South America covering the Philippines, Japan, Alaska, and the western coast of North America and this belt is known as Fiery Ring. Out of 850 active volcanoes of the world, this belt simply accounts for 75% of the total. Moreover, traces of volcanic activities are also manifested in the submarine ridges of the oceans. For example, the land forming activities in the submarine ridges of the Mid-Atlantic are still active.

Types of Volcanic Eruption

These are listed in ascending order of magnitude (from the calmest to most explosive):

  • Icelandic
  • Hawaiian
  • Krakatoan
  • Pelan
  • Plinian
  • Strombolian
  • Vulcanian
  • Vesuvian

Effects of Vulcanicity

  1. Volcanic eruptions can be very dangerous, it can lead to the loss of lives and destruction of property. This may be due to lava flow, the emission of hot ashes, dust, or poisonous gases which are released.
  2. A large portion of agricultural land is also affected when this occurs; however, volcanic materials including ashes and granite could affect soil fertility in areas where volcanic activities take place.
  3. In volcanic areas where the original landscape has been altered, human settlement and agricultural activities are discouraged.
  4. Volcanic mountains create a shadow effect on leeward sides leading to aridity.
  5. Volcanic features like mountains usually stand as barriers to the development of transport, communication, and other human activity.

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