vulcanicity or vulcanism 1

Vulcanicity is formed when molten rock or magma forces its way into planes of weakness or vent of the earth’s crust to escape quietly or explosively to the surface.

The magma, while moving through the vent to the earth’s surface may cool and solidify within the crust or before it gets to the surface of the earth as Plutonic rocks, leading to intrusive land forms.

On the other hand, when the magma reaches the surface of the earth, it cools and solidifies as volcanic rocks, and results in extrusive land forms. Rocks, formed by either plutonic or volcanic activity are called igneous rocks.



The main intrusive land form of vulcanicity includes Sills, Dyke, Batholiths, Laccoliths, Lopoliths and Phacoliths.

  1. Sills: Sill, results when an intrusion of magma is made horizontally along the bedding planes of sedimentary rocks.
  2. Dykes: Dyke, results when an intrusion of magma is made vertically within the sedimentary layers.


Characteristics of Dyke

  • A dyke is a rock form lying vertical or inclined across the bedding planes (discordant).
  • It is a narrow wall-like feature that is rarely prominent in the landscape.
  • At times, when exposed a dyke forms shallow trenches.
  • A dyke is a resistant rock.
  • It may be circular in shape e.g. Ring dyke in Jos, and very extensive in length.
  • Sometimes, a dyke gives rise to waterfalls or rapids and constitutes oil trap or water trap e.g. Clevel and Dyke of York Shire, Ring dyke in Jos.


Formation of Dyke: A dyke is formed as cracks and joins. This molten magma then cools off and becomes solidified to form vertical and narrow walls of igneous rock with shallow trenches formed when exposed to denudation.



  1. Laccoliths: A laccolith is a large mass of igneous mound with a dome-shaped upper surface and a level base.
  2. Lopoliths: A lopolith is a saucer-shaped igneous intrusion with a shallow base.
  3. Phacoliths: A phacolith is a lens-shaped mass of igneous rocks occupying the top of an anticline or the bottom of a syncline.
  4. Batholiths: A batholith is a huge mass of igneous rocks which form a massive and resistant upland when eventually exposed to the earth surface.

Characteristics of Batholith

  1. Batholith is the most spectacular of the igneous rocks.
  2. It has a large class ranging from hundreds of kilometres in extent.
  3. It performs longitudinal ridges in the core of Fold Mountains.
  4. Batholiths may become exposed as a result of denudation.
  5. Batholiths can be found in Wicklow Mountains in Ireland, Upland Britanny in France and along the Rockies in U.S.A.


Formation of Batholith

  1. Batholith owes its formation from the volcanic origin.
  2. They are usually formed at the foot of mountains with massive and resistant upland.
  3. Its formation is as a result of large mass of magma rising upwards which accumulates in the crust.



  1. Ejection of hot lava, ashes etc.
  2. Increases surrounding temperature.
  3. Submergence/displacement of settlement.
  4. Destruction of lives and properties.
  5. Air pollution.
  6. Water pollution.
  7. Causation of floods and landslides.
  8. Displacement of settlement results in resettlement.
  9. Formation of fertile soil for agriculture.
  10. Depletion of natural forest products.
  11. Results in the formation of precious stones and minerals.



When molten magma emerges at the surface of the earth, it is called Lava. The magma could get to the surface of the earth either through a vent (hole) or a fissure (crack). If lava emerges through a vent, it usually builds up a volcano and if through a fissure; it may build up lava plain or a lava plateau.


Some major extrusive land forms of vulcanicity include, Composite cone, Lava domes or Shield volcanoes, Ash and Cinder cones.

  1. Composite Cones: Composite cones are also called Straso-volcanoes.


Characteristics of Composite Cone

  • Composite cones are the highest form of volcanoes.
  • It is the most common type of volcano.
  • It is conical in shape.
  • It is made of alternate layers of ash and lava that build up conelets around the major Etna in Sicily, Mount Fuji, Mount Chimborazo and Vesuvius.


Formation of Composite Cone

Composite cone is made up of alternate layers of ash and lava. The lava pours out forming layers on top of the ash. In minor eruption, lava moves out from the sides and conelets are built while new eruption form new layers which grow gradually with increasing cano blows off or sinks, e.g. Chimborazo, while a large crater called Caldera may finally emerge at the point of Volcano.


  1. Lava Domes: Lava domes are also called Shield volcanoes. They have gently-rising slope and broad flattened top. Shield volcanoes are formed from basic lava; hence, they can flow over a large area before solidifying. Examples are found in Cameroun Mountain.
  2. Ash and Cinder Cones: Cinder cones are formed from less fluid acid lava, containing cinder, ash, rock fragments and lava bombs. The lava flows are viscous and they solidify after a short distance. At times, the lava closes the vent and this result in an explosion which blows the vent open. After the explosion, A Crater which is a depression on the top of the cone is formed. If the water occupies the crater, it becomes a Crater Lake. When the crater becomes an enlarged depression, it is called Caldera. Examples include Mt. Kenya, Mt. Kilimanjaro in East Africa.



There are three main types of volcanoes. These are active, dormant and extinct volcanoes.

  1. Active Volcanoes: These are volcanoes known for their frequent eruptions; especially in their early stage e.g. Cameroun Mountain which has erupted four times within a period of 50 years.
  2. Dormant Volcanoes: These are volcanoes that are known to have erupted and have shown signs of possible eruption in the future. Such a volcano is described as dormant.
  3. Extinct Volcanoes: These are volcanoes that have not erupted in historic times. The most popular disastrous volcanic eruptions were those of Mt. Vesuvius, Mt. Krakatoa and Mt. Pelee.

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