Volcano Formation

A volcano is simply an opening or tear in the earth's outer crust through which lava and rock material can escape.
The shape of a volcano is dependent on factors like geographical positioning (tectonic boundaries) and types of rock found at that location, and as such the shape can often influence the characteristics of the volcano. Each formation is covered below with examples of well-known volcanoes of that type.
Dependent on the eruption, vocanoes can be both beautiful and deadly with some ash plumes reaching tens of kilometres in height.


Fig 1. Redoubt Volcano, Alaska (1989)
Fig 1. Redoubt Volcano, Alaska (1989)

Contents:
- Plate Tectonics
- How does Lava Contribute to the Formaton of a Volcano?
- Types of Volcanoes
- Changes in Volcano Formation
- Useful Links & Further Reading
- References


Plate Tectonics


plate-tectonics-map-usgs.jpg
Fig 2. Overview of different plates

Plate boundaries: Three types of plate boundaries exist, characterised by the way the plates move relative to each other. They are associated with different types of surface phenomena. They are:
  • Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
  • Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e.g. Mid-Atlantic Ridge) and active zones of rifting such as Africa's Great Rift Valley are both divergent boundaries.
  • Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America.
The tectonic plates sink into the mantel and the molten rock melts into magma. The magma then rises to the earths surface through cracks - becoming lava in the process - causing a volcanic eruption. The volcano is built up gradually over many years by the many layers of lava.



How does lava contribute to the formation of a volcano?


The lava that is created is usually the main factor that contributes towards volcano morphology. The lava is created through the melting of molten rock in the mantel, which seeks towards the earth's surface through cracks. The viscosity and explosivity of the lava are the main elements that give the volcano its overall form. For example lava that has a high viscosity will generally form more of a bulky structure whereas lava with a low vicosity will normally result in a smooth, dome shaped formation.
For example:

The Cinder-Cone formation
Fig 3. Cinder cone volcano
Fig 3. Cinder cone volcano

Fig 4. Paracutin,Mexico
Fig 4. Paracutin,Mexico

A cinder cone volcano (as shown in images above) is formed by a gas charged lava that explodes through the central vent into small fragments in the air which solidfy and fall as cinders around the vent. This is what creates the circular or oval cone ('cinder' 'cone'). An example of a cinder cone is a volcano in Paracutin, Mexico (above image) which was 'born' on February 20th 1943 in a corn field and grew to 100 feet in 5 days. Eruptions occured regularly, hence the rapid increase in height.

The Lava Dome formation
Fig 5. Lava dome spines
Fig 5. Lava dome spines
Fig 6. Mount St Helens - Lava dome growth rate
Fig 6. Mount St Helens - Lava dome growth rate

The Lava dome formation (as shown in the images above) is formed by the highly viscous lava that cools and hardens quickly, creating a bulge rather than a smooth flow. The frequent eruptions cause this bulk to get larger and larger and creates a mount over the vent. The lava domes can also create what are known as 'spines' on the surface due to the general upward force of the magma above (Image 1).

The image below shows how the silica and water content affect the type of lava and therefore the type of volcano:


Fig 7. Diagram showing difference in volcano formation due to lava differences
Fig 7. Diagram showing difference in volcano formation due to lava differences



Types of Volcanoes

The Shield Volcano
Fig 8. Shield Volcano
Fig 8. Shield Volcano

The Shield volcano is (as shown in the image above) is formed by the low viscous lava that travels far and wide to create a smooth dome structured volcano. The size of this type of volcano is dependent on number of eruptions that occur: more eruptions means more lava that solidifies thus resulting in a built up volcano.

The Fissure Volcano
Fig 9. Fissure volcano
Fig 9. Fissure volcano

Taking their name from the giant cracks in the earth’s crust from which they erupt, they are sometimes known as ‘linear volcanoes’ on account of them generally occurring along fault lines. They throw out large amounts of lava which then cools and solidifies maintaining the generally flat shape. Fissure volcanoes are commonly found in Iceland and Hawaii, astride the Mid-Atlantic Ridge.

The Caldera Volcano

Fig 10. Mt. Teide
Fig 10. Mt. Teide

A collapsed volcano after a large eruption that has subsided back into itself. This occurs because the chamber of magma empties during the eruption and leaves a large void under the volcano. Mt. Teide on the island of Tenerife is one example of an active caldera.

The Composite Volcano
Fig 11. Mt Fuji
Fig 11. Mt Fuji

Volcanoes with explosive - often pyroclastic - eruptions constructed from deposit upon deposit of lava. Often called 'stratovolcanoes' after the Latin ‘stratum’ meaning layer. The slow-flowing, highly viscous lava hardens rapidly giving steep sides. Mt. Fuji, Krakatoa and Vesuvius are three well-known examples.

The image below gives a good impression of the internal structure of each of the six volcano types above:

Fig 12. Diagram showing internal structure of differnt tyes of volcanoes
Fig 12. Diagram showing internal structure of differnt tyes of volcanoes



Changes in Volcano Formation


The shape a volcano takes is almost always temporary, although it will take many years for the shape to change significantly. The change in shape is due to the change in the rocks the volcano is made from.
There are igneous rocks, metamorphic rocks and sedimentary rocks.
  • Igneous rocks are formed by magma, when the magma erupts and cools it forms volcanoes.
  • Metamorphic rocks are rocks that have changed from one rock to another due to lots of heat and pressure.
  • Sedimentary rocks are formed from sediments that have been compressed.

Metamorphism is the term used to describe the change in the rocks mineral assemblage and texture. There are various factors effecting how the rocks change, these being chemical composition, temperature, pressure, heat, and time.
The rocks that volcanoes are made from can be weathered, uplifted, transported, deposited or tilted. These actions often run in cycles and once a volcano is formed and unformed it may eventually form again.


Useful Links & Further Reading


National Geographic Volcano Photos

Student Site Information

Different Types of Volcanoes

BBC Science and Nature Animated Guide

Further reading on ‘Magma Flow Instability and Cyclic Activity’


References



Christopherson, R.W. (2008) ‘Geosystems: An Introduction to Physical Geography’ New Jersey, Pearson Education

Francis, P (2004) ‘Volcanoes’ Oxford, Oxford University Press

Voight, B et al. (1999) ‘Magma Flow Instability and Cyclic Activity at Soufriere Hills Volcano, Montserrat, British West Indies’ ScienceMagazine, 19th Feb 1999 pg 1138-1142 [online] http://www.sciencemag.org/cgi/content/abstract/283/5405/1138 Accessed 29th October