Acid
Lava Volcanoes: These volcanoes are found at destructive
plate boundaries. The lava from this type of volcano is highly viscous with a
high melting point, light in colour, of low density and has a high percent of
silica. They flow slowly and seldom travel for far before solidifying. The
viscous lava extrudes like thick toothpaste and cools rapidly producing very
steep sided cones. As a result of the lava being very viscous the vent is
usually blocked by the outpouring resulting in a loud explosion throwing out
many volcanic bombs and pyroclasts. These types of volcanoes usually produce a
spine of plug, eg. Mount Etna, Italy.
Basic Lava Volcanoes: These volcanoes are very hot reaching
about 1000oc and are highly fluid. The lava from this
type of volcano is a dark colour and rich in iron and magnesium but poor in
silica. Basic lava volcanoes are found at constructive plate boundaries. Due to
high fluidity of this lava, they flow readily at great speeds. They affect
extensive areas spreading out like thin sheets over a great distance before they
solidifies. The volcano which results from these types of lava is very gently
sloping with a wide diameter, eg. Mauna Loa in Hawaii.
Formation and Structure of a Volcano
A volcano is an opening in the earth’s crust, through which molten rock, ashes, steam, etc are ejected. Volcanoes are formed when
molten rock (magma) from the interior of the earth reaches the surface though a
vent. As the magma arrives at the surface it is then called lava. Next
the lava and ash ejected from the vent then collects around the vent to form a
conical hill or mountain (Figure: 1). Volcanoes are
conical in appearance with a funnel-shaped depression or crater usually found
to the top. Its structure is built up when lava is solidified around a vent. In
the lower part of the funnel is a conduit or vent through which the materials
are ejected. (Figure 1)
Figure 1: Typical structure of a volcano.
Volcanoes can be formed at convergent or divergent plate margins. At convergent margins, the oceanic plate with sediments and water subduct into the mantle and forms magma which then rises to the surface through the overriding plate forming explosive and violent volcanoes.
At divergent boundaries, the rising currents in the mantel pushes the plates apart and releases pressure that melt the areas of the plates. The magma then rises to the surface and creates a volcano that erupts quietly.
At divergent boundaries, the rising currents in the mantel pushes the plates apart and releases pressure that melt the areas of the plates. The magma then rises to the surface and creates a volcano that erupts quietly.
Distribution of Volcanoes
Volcanoes are located
near the margins of continents lining the Pacific sea board and the western
side of North, Central and South America. This area in the Pacific Ocean
accounts for more than half of the circle that makes up the area known as the
‘Ring of Fire’ (the area marking the boundary of the Pacific plate). Volcanoes
occur in narrow linear belts and are mostly found along convergent plate
margins (destructive boundaries). Significant folding and fracturing occurs at
this type of boundary and because of these channels or cracks are formed where
the magma can escape to the Earth’s Surface. An exception to the general
distribution of volcanoes is those found in the middle of the Pacific plate
(the Hawiian islands) which are formed due to hotspot activity. The next most important area is the mid-world belt running from the West Indies and through the Mediterranean. In the centre of the Atlantic is the Mid-Atlantic Ridge, including Iceland, and there are a few minor areas including the Great Rift Valley of East Africa.
How a Volcano Erupts
Volcanoes erupts along
lines of weakness where folding and fracturing of the earth’s crust occurs.
Magma from the earth’s mantle forms chamber below the earth’s surface. As the
magma rises gases dissolved in the molten material expands and bubble off. The resulting
froth exerts tremendous outward pressure that sends the magma upwards. As the
magma comes in contact with the ground water the volcano become like a ‘pressure
cooker.’ The froth and gasses push through the cracks in the volcano and comes
to the surface. The pressure suddenly releases and the volcano explodes with a
combination of ash and molten rock.
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Figure 2: A volcanic eruption.
Intrusive Volcanic Features
Magma from the earth's interior pushes its way to the surface through cracks and lines of weakness in the earth's crust. However, magma does not always reach to the earth's surface. It pushes into the narrow joints in the rocks and widens it from centimetres to tens of metres across, more magma then flows into the cracks and cools and hardens in the earth's interior forming 'intrusive volcanic features' (Figure: 3) such as sills, dykes, batholiths and plugs. These features are formed depending on how the magma travelled and accumulated underground. The features are then exposed after years of erosion.
Sills are formed by magma flowing horizontally between two rock layers, and roughly parallel to the surface, eg. Great Whin SIll in norther England.
Dykes (or Dikes)
Dykes are vertical sheets of igneous rocks formed when magma moves directly upwards towards the surface, eg. Moule `a Chique headland in St. Lucia
Plugs are vertical pipe of rock which is formed when molten material in the vent of a volcano cools and hardens.
Batholiths
These intrusive features are very large. They form when a large underground reservoir of molten rocks cools and hardens below the surface. Examples of batholiths are found in Tobago, Virgin Gorda and Tortola.
Figure: 3 Intrusive volcanic features
SillsSills are formed by magma flowing horizontally between two rock layers, and roughly parallel to the surface, eg. Great Whin SIll in norther England.
Figure: 4 Sills
Dykes (or Dikes)
Dykes are vertical sheets of igneous rocks formed when magma moves directly upwards towards the surface, eg. Moule `a Chique headland in St. Lucia
Figure: 5 Dykes
PlugsPlugs are vertical pipe of rock which is formed when molten material in the vent of a volcano cools and hardens.
Batholiths
These intrusive features are very large. They form when a large underground reservoir of molten rocks cools and hardens below the surface. Examples of batholiths are found in Tobago, Virgin Gorda and Tortola.
Figure: 6 Batholiths
Extrusive Volcanic Features
Extrusive volcanic features are formed by lava and ash on the surface of the earth's crust. The two types of volcanoes determine the extrusive features that are formed. When the lava exits the vent it builds up and form a volcano (cone-shape mound) it emerges through a crack or fissure in the crust it forms a lava platform or lava flow. Some extrusive features include ash and cinders, spine, crater, crater lakes and caldera. Two types of extrusive features we will be looking at are cones and plateaux.
Ash Cones
Ash cones cones are produced when there are violent eruptions. During an eruptions, molten lava is thrown high in the air. Before it reaches back to the surface, it cools and solidifies forming volcanic ash. A number of eruptions may build many layers of ash to build up a cone. This type of cone is usually symmetrical in shape such as, Mount Fuji in Japan or Paracutin in Mexico.
Lava Cones
A lava cone forms from slowly up-welling lava. Lava is accumulated around the vent forming a broad and gentle slope with a wide crater at the top. For example, Mauna Loa in Hawaii.
Composite Volcanic Cones
Some volcanoes produce both lava and ash during eruptions. During the eruption, ash is produced forming a layer. It is then followed by another eruption of lava to form a next layer. The result is a composite volcano with many layers of lava and ash built up over a period of time (Figure 8 ). Many Caribbean volcanoes are of this type such as Soufrière in St. Vincent. Another example is Vesuvius in Italy.
Plateaux
Basic lava volcanoes produce basaltic lava that pours out from long fissures instead of the central vent. As the lava oozes out of the fissures it forms a sheet that covers a wide area (Figure 9). Each time lava is ejected a new layer is formed. Examples of plateaux are Iceland and the Deecan Plateau in Indian.
Further Reading: http://www.ukgeohazards.info/pages/eng_geol/volcanic_geohazard/eng_geol_volcanic_diag.htm
Sources: Ash Cones
Ash cones cones are produced when there are violent eruptions. During an eruptions, molten lava is thrown high in the air. Before it reaches back to the surface, it cools and solidifies forming volcanic ash. A number of eruptions may build many layers of ash to build up a cone. This type of cone is usually symmetrical in shape such as, Mount Fuji in Japan or Paracutin in Mexico.
Figure 7: An ash cone.
Lava Cones
A lava cone forms from slowly up-welling lava. Lava is accumulated around the vent forming a broad and gentle slope with a wide crater at the top. For example, Mauna Loa in Hawaii.
Composite Volcanic Cones
Some volcanoes produce both lava and ash during eruptions. During the eruption, ash is produced forming a layer. It is then followed by another eruption of lava to form a next layer. The result is a composite volcano with many layers of lava and ash built up over a period of time (Figure 8 ). Many Caribbean volcanoes are of this type such as Soufrière in St. Vincent. Another example is Vesuvius in Italy.
Figure 8: A composite volcanic cone.
Plateaux
Basic lava volcanoes produce basaltic lava that pours out from long fissures instead of the central vent. As the lava oozes out of the fissures it forms a sheet that covers a wide area (Figure 9). Each time lava is ejected a new layer is formed. Examples of plateaux are Iceland and the Deecan Plateau in Indian.
Figure 9: A basalt plateau
Further Reading: http://www.ukgeohazards.info/pages/eng_geol/volcanic_geohazard/eng_geol_volcanic_diag.htm
The Caribbean Environment for CSEC Geography. Fourth Edition (2012) Mark Wilson
New Caribbean Geography with map reading and CXC questions. Fourth Edition. Vohn A.M. Rahil
Geography for CSEC Examinations. (2010) Neil E. Sealey
Characteristics and Features of Igneous Intrusions:
http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/communities/Geography/AS%20Geography/AS%20Revision/Earth%20Systems%20Revision/Characteristics%20and%20formation%20of%20Intrusive%20landforms
New Caribbean Geography with map reading and CXC questions. Fourth Edition. Vohn A.M. Rahil
Geography for CSEC Examinations. (2010) Neil E. Sealey
Characteristics and Features of Igneous Intrusions:
http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/communities/Geography/AS%20Geography/AS%20Revision/Earth%20Systems%20Revision/Characteristics%20and%20formation%20of%20Intrusive%20landforms
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