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
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.
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DIAGRAM OF DYKE
SILL
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjOjpZ1Dc8nUHD-Kt4dlMYJqsABqGWxbQTYQ3UrpgkF3zjULE1cnwNKMfUKq517F2UdrTYGb7KY7zF4L_CsL4lppUKZlbehHVKagyIAThBVpi5RNUG6s4WH1OS9aeSDE5rmvBNCHm1pDiI/s400/Picture5.gif)
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).
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiKhfMN13d2pkSRHzoXCj2AO_1RN729ufgaar8gKjQlvK7JWAhcA2KjwZAT7I19D6kmYdEhTZvhhB0TWA8cZltCZa3TGiF9MrH7_x9zeQ0IbrgF7wvN6qCy1X8QIVL7CHiDQpHAht8dlFM/s400/Picture9.jpg)
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.
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjgnixROL35u5zQ7_2AjxmStTFnhXAilS8dqK8xx04p0e7lZ4Cy7U9B42fbchJsgrYRI1nz5X3IommzOG1lTerhGHhUvF9Y1OseKTvpByVXkwnA3IF-7-BpwiJEckM6XqrHkEZcpnoadSk/s400/Picture12.jpg)
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.
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.
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