VOLCANISM (VULCANICITY).

MEANING OF VOLCANISM  (VULCANICITY).

Vulcanicity or volcanism refers to range of process by which molten materials and gases are either intruded (injected) into the earth’s crust or extruded (ejected) onto the surface.

A distinction can be made between volcanism and vulcanicity . Volcanism is a broader term which includes both extrusive and intrusive igneous activities, while volcanicity refers to the extrusive activities in which the materials are  forced out onto the surface. The features formed by extrusive vulcanicity  includes volcanoes. Volcanoes are the hills (mounds) formed due to the accumulation of lava.

Intrusive vulcanicity is the process in which the materials are injected into the earth’s crust forming different intrusive features, both plutonic and hypabysal features. When a molten materials are still inside the earth’s crust they are referred to as magma but on reaching the surfaces they are referred to as a lava.

Magma and lava can either be acidic, if there is a lot of silica or basic if there is less silica, with large proportion of metallic materials. Acidic lava is viscous while basic lava is less viscous (runny), hence tends to flow to great distances and occupy  a large area.

ORIGIN OF MAGMA.

Magma originated from the interior of the earth where it is kept as a molten rock due to the influence of very high temperature. When a crack or fault (fissure) or hole (vent) is formed, magma is forced out through these lines of weakness due to high pressure under the ground. The movement of the molten materials leads to the production of both intrusive and extrusive features.

Let us see origin of magma.

FEATURES ASSOCIATED WITH VULCANICITY.

A. INTRUSIVE VULCANIC FEATURES.

These features occurred inside the earth crust. There are divided into hypabysal and plutonic.

Hypabysal are the features which are formed inside but near the surface.

Plutonic are the features which are deep down within he crust.

Example of intrusive vulcanic features are;-  dyke, sill, laccoliths (rock cistern), lopolith, phacolith, and batholith

  DYKE

This is a wall of rock, which cut across the bedding plane of the main rock. Its formed when magma solidifying along the vertical or slanting fissure (fault). Its therefore a discordant feature since it does not follow the nature of the rock strata. for example there are many dykes found in west and south of Blantyre along Tyolo scarp in Malawi.     

DIAGRAM OF DYKE

    SILL

This is a rock sheet formed when magma solidifies horizontal along the bedding plane. its concordant with rock strata.  it can be of any thickness and can extend for many kilometers. sills are the cause of many water falls in the river valley. Kinkon falls in guinea are good examples.

3.      LACCOLITHS (ROCK CISTERN).

This is a dome-shaped(cake-like) mass of rock with flat bottom formed of viscous  magma which on accumulating forces they overlying rock strata to bend up. It look like a mushroom. The Fonjey massif and Ambreny massif in Madagascar are the best examples of laccoliths.

Diagram of laccoliths.

      LOPOLITH

This is a saucer-shaped mass of rock formed in the geosynclines. The saucer-like shape may be due to the increased weight of the deposits. The Bushveld basin in the Transvaal in south Africa is formed of several lopoliths.

Diagram of lopolith.

PHACOLITH.

This is a lens-shaped strip of igneous rock formed when magma solidifies along the anticlines or synclines. Examples can be drawn from the cordon hills in UK .

6.      BATHOLITH.

This is a large mass of solidified rock which is formed when magma cools plutonically of mountain ranges. It is situated deep and granite is the best example of the rock which forms batholiths. Batholiths can be exposed to the surface through denudation. Example Chailu massif in Gabon.

   B.  EXTRUSIVE  VOLCANIC FEATURES.

These are the features which associated with ejection of molten materials onto the surfaces (out side of the earth surface). The molten materials that are ejected onto the surface are called Lava.

it can be violent eruption or peaceful eruptions.

Peaceful eruption is take place without involving much force this occur through fissures.

Violent eruption are the ones that takes place without involving much force and most of peaceful eruptions takes place through fissure (crack or fault).

Also, there are central features and minor features volcanic features.

Central features are the ones which are formed when the lava comes out through a vent (hole). It can be associated with either violent eruption or peaceful eruption.

The central features which are formed due to violent eruption are ash and cinder cone (scoria cone), composite cone (strato-volcano), volcanic plug (plug dome volcano), crater and caldera.

 And the central features which are formed due to peaceful eruption are acid lava cone (cumulo dume volcano) and shield volcano.

      ASH AND CINDER CONE

Is cone shaped accumulation of rock fragment around the vent. The slope of the cone are always concave due to the spreading tendency of lava at the base of the cone .

ASH          

COMPOSITE CONE (STRATO-VOLCANO)

Is a large cone with alternative layers of pyroclasts  (fragment) such that ash and cinder on one hand and lava on the other hand. The cone has steep slopes is the most common volcano, examples  Kilimanjaro.

Diagram of composite cone.

VOLCANIC PLUG (PLUG DOME VOLCANO).

Is a rigid cylindrical plug formed when viscous lava is forced out by very explosive eruptions. The plug is extruded  amid clouds of hot blowing ash and cinders.

Volcanic plug

CRATER.

Is a depression formed on the summit of the cone after the plug has been blown off the top of the cone.

Diagram of crater.

CALDERA.

Is a large crater (large rounded depression) formed when the upper part of the volcano is either bombarded away by violent eruption or subsides into the crust or in the volcanic cone.

Diagram of caldera.

   SHIELD VOLCANO.

Is an extensive cone with gently sloping sides which is formed when basic (basalt) lava is poured on the surface and spread to occupy a large area. Lava tends to come through vent.

Diagram of shield Volcano.

Minor volcanic feature

Are the features which are start taking place when volcanic eruption is approaching its end. It includes the features like solfatara, fumerole, mofette, hot spring and  geysers.

 Geysers

 Is the force full emission of hot water from the ground to high level in the atmosphere. When the ejected water contains fine material forms mud volcano, which later can lead to the formation of very fertile soil. Geysers are very common in Iceland and New Zealand, It is tourist attraction because fountain water thrown into the sky.

DIAGRAM OF GEYSERS

Hot spring

Is an out flow of super-heated water from the ground water from the ground, which contains some mineral substances in solution or in suspension. They are common in Iceland as well as in some of the African countries such as Kenya. They attract tourist. 

DIAGRAM OF HOTSPRING

   Fumerole: volcano which emit steam, mud and several other gasses than sulphur. Example kibo in Kilimanjaro

Solfatara : is a volcano which emits sulphur gas.

Moffete.  volcano which produce carbon dioxide gas, example java in Indonesia.