Volcanicity is the ejection of solid, liquid or gaseous materials from some deep-seated reservoir of molten magma beneath the earth crust to the earth surface. Volcanicity is defined as the rising of molten magma or other gaseous materials beneath to the surface of the earth or within the earth crust. Volcalnicity can be referred to as all various processes associated with the ejection of magma and relates the material from the depth below onto the earth crust.



Volcanicity is formed when molten magma or mobile rock forces its way into the place of weakness or vent of the earth crust to escape quickly or explosively to the surface. Molten magma naturally escape through areas that have experienced mountain building process of compression and tension. The magma while moving through the vent to the earth surface may cool and solidify within the earth crust before getting to the surface of the earth as plutonic rock leading to intrusive land form.

On the other hand, when the magma reaches the surface of the earth, it cools and solidifies as volcanic rock and results in extensive land form. Rocks formed by either volcanic activities or plutonic activities are referred to as igneous rocks.



The intrusive land form of volcanicity include:

  1. Dyke
  2. Sill
  3. Batholith
  4. Phacolith
  5. Lopolith
  6. Laccolith
  7. Volcanic rock and dyke neck


This is a discordant volcanic intrusion which is formed when magma solidifies within vertical faults. Dyke results when an intension of magma is made vertical within the sedimentary layers. Example of dyke include, dyke in England, deviant in scotland.



  1. A dyke is a rock form lying vertical or inclined across the bedding planes (discordant)
  2. It is a narrow wall – like features that is rarely prominent in the landscape.
  3. at times when exposed, a dyke forms shallow trenches
  4. A dyke is a resistance rock
  5. It may be circular in shape e.g. ring dyke in Jos and very extrusive in length
  6. Sometimes, adyke gives rise to water polls or rapids and constitutes oil tap or water tap e.g. clave land dyke of Yorkohire, ring dyke in Jos.


Sill unlike dyke as a sheet of solidified magma which lies cordantly to the bedding of sedimentary rocks, sill, it react when an intrusion of magma is horizontally along the bedding place of sedimentary rock.

Examples are: Nerthumberland in England and little and great Farrons in South Africa.


It is a huge mass igneous rock usually granite which after removal of the overlaying rock forms a massive and resistant upland region, such as Wick low mountain of Ireland. The upland of Britain, France and the main ranges of Malaysia.

Formation of Batholiths have been attributed to the shrinking and the resistant melting and the incorporation of large block of surrounding areas rocks into the molten magma which may later solidify.


Laccolith is a large baster or igneous mould with a dome-shaped upper surface and a level base fed by a pipe-like conduct from below (this is by local accumulation of molten magma). If arches up overlying strata of sedimentary rocks e.g. the laccolith of the Henry mountains in vtah USA.


Lopolith is another variety of igneous intrusion with a saucer shape. Example are the Bashreld lopolith of Tranovaal, South Africa examples area also found in England and Scotland and Sierra Leone peninsula.



Phacolith is a lens-shaped mass of igneous rocks occupying the crest of an anticline or the bottom of a syncline and being Jed by a conduct from beneath. An example of phacolith is London Hill in Shropshire England.


They result from solidification of larva invents of volcanoes. They resist erosion better than volcanic cones themselves e.g. Dumberyan Rocks, Scotland Devils, Tower in Wyoming USA.


They are steep-sided sharp ridges of volcanic origin. They are usually larger than volcanic rocks e.g. (i) crazy mountains of Montana (ii) the great Dyke of Rhodesia.



Extrusive landforms are determined by the nature and composition of the larva and other ejected materials that reach the surface of the earth. When molten magma immerge at the surface of the earth, it is called larva. The magma could get to the surface of the earth through a vent (hole) or fissures (cracks). If the larva immerged through a vent, it usually build up a volcano and if through a fissure, it may build up a larva plane or larva plateau. Some major extrusive landform of volcanicity include:

  1. Composite cone
  2. Larva dome/shield volcanoes
  • Ash and cinder cone
  1. Caldera



  1. COMPOSITE CONES: They are also called strato – volcanoes. These hence, formed some of the grand-outer mountains of the earth. They are built of alternating layers of lava flows, volcanic ashes and cinders through several vents on parasitic cone. Volcanoes begin earth emption with a great explosion and extremely viscous acid larva, domes or plugs. The magma volumes of dissolved gases such as water vapour, carbon monoxide, carbon dioxide, hydrogen sulphate.


These are huge craters of volcanoes often surrounded by the shattered remain of cone. Caldera often result from violent eruption of volcanic cone the extent that the huge crater often several kilometres across. When water accumulates in a Caldera, a lake is formed. Examples of this include Lake Toba in Somalia, Paryen Lake in Pankshro near Jos.


Hot springs are thermal springs which emit hot water quietly without any violent eruption. Water rises up from the rock to the surface without any explosion. Some springs contain dissolved minerals which may have medicinal value.

Hot springs provide tourist attractions e.g. Ikogosi Hot spring in Ekiti state, Nigeria and those in Japan and Hawall.

Such springs are common in Iceland some of them have been harnessed to heat courses, swimming pools and for other domestic purposes.


This is made up of very hot lava with temperature of about 1000C and therefore is in liquid form. They flow for relatively long distance as soon as they poured out of the volcano. They flow out as thin mobile sheets of lava for great distances before solidifying. Examples of this cone is Manna in the pacific.


These are volcanic cone which are made up of mainly ash and cinder e.g (i) Valance defanged in Guantemeta (2) Ikere one in Ekiti, Nigeria.

Ash and cinder cones are typically small volcanoes occurring in groups, and rarely exceeding 300m in height other examples include Mt. Nuume in Italy and Mt. Pricilla in Morocco.


Geysers are not only more violent than springs but emits superheated steams which may spout up to a height of 46m from the earth beneath the phemo memorare aerated with thermal of volcanic region in which the water below is heated beyond the boiling point (1000c). The jet of water usually emitted with an explosion trigger off by gases seeping out of the heated rocks.

The major geyser are found in (i) Iceland (ii) Northern Ireland of Zealand (iii) Yellow stone of National park of USA.


See also

Regions of Earthquake Occurrence


Air and Noise Pollution


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