Stratovolcano & Stratovolcano

A volcano may defined as an opening or a rapture in the planet’s surface that can allow hot magma, gases, and, ash to escape from under the surface. There are many types of volcanoes and these include; lava domes, cryptodomes, volcanic cones, super volcanoes and stratovolcano. A Stratovolcano which is sometimes called a composite volcano is a conical, tall volcano which has several layers (strata) which are hardened tephra, lava, and volcanic ash. Stratovolcano is characterized by a steep a steep profile which has periodic explosive eruptions.

The lava flowing from stratovolcano appears to be viscous and this makes it to cool and becomes hard before spreading in to a large area. This lava is formed from felsic which has very high intermediate levels of silica which has a lesser amount of the less viscous mafic magma. In some cases, stratovolcanoes are referred to as composite volcanoes due to the presence of a composite layered structure which is built from the sequence of outpourings of the materials from eruption.

Stratovolcanoes are the most common volcanic activity which is a contrast to the less common shield volcanoes. The most famous stratovolcano is the Krakatoa which is best remembered for its eruption in 1883. Stratovolcanoes are mainly common in subduction zones, where they form chains along the tectonic plate boundaries where the continental plates are drawn over the oceanic crust or another oceanic plate. This results in to continental arc volcanism found in places such as Central Andes where we find Cascade Range.

Other forms of stratovolcano are found in Japan’s Aleutian Island. The magma forming the stratovolcano rises due to the water trapped in the porous basalt rock and hydrated minerals found on the upper oceanic crust, which is released in to the mantle rock found in the asthenosphere above the sinking oceanic slab. The process of release of water from the hydrated minerals is called dewatering and it takes place at different temperatures and pressures of each mineral as the plates goes to deeper depths.

Freed water from the rock lowers the melting point of the mantle rock which is underneath making them to undergo partial melting which in turn makes them to rise due to their lighter density in relation to the bordering mantle rocks and the pools which exist beneath the lithosphere. When stratovolcano erupts if produces ash in to the atmosphere. These are explosions of volcanic clouds and they cause serious hazard to the aviation safety. These eruptions also lead to the formation of mudflows and pyroclastic flows which are also very hazardous.

Pyroclastic is composed of fast moving ground-hugging and avalanche-like mixture of ash, hot debris, and gases that can travel at a very high speed. This type of volcano is found in places such as Tambora which is found in Indonesia, Krakatoa which is an island in Sunda Straight. The main difference between stratovolcano and other types of volcanoes is that it takes a giant shape which is very tall. Another distinctive feature is the strata from which the name originates from. It is the strata that give rise to a stratovolcano.

They are also known as composite volcanoes which due to the fact they created from different kinds of structures with different kinds of eruptions. Stratovolcanoes are mainly made of ash, lava, and cinder, where by ash and cinders pile on top of each other as lava flows on top of the ash resulting in to cooling and hardening and the process repeats itself. A good example of stratovolcano is Mayon Volcano in the Philippines, Mt. Fuji in Japan, and Mount Vesuvius and Stromboli which are both found in Italy.

Another characteristic of stratovolcano is that they have steep sides with a distinguishable cone shape which is frequently composed of many vents that sometimes erupt lava in different ways. This type of eruption leads to the formation of lava domes which are usually thick mounds which are formed during flow of super thick magma that could not flow further. Stratovolcanoes have been the most powerful and destructive type of volcanic eruption in the human history. They are able to send ash and volcanic rocks cubic of miles away in to the atmosphere in a series of extremely violent eruptions termed as “plinian-type”.

This type of volcano comes up with great pressure such that it can leave the place with no mountain but only the root of the volcanic activity. For stratovolcano to take place there should be great pressure in the strata that will be able to send ash and other volcanic materials to be forced out in to the surface. The friction between the oceanic plate and the tectonic place should exist so that pressure is generated so as to initiate the process. Stratovolcano is typically formed at convergent plate margins where one plate is forced to descend beneath the other adjacent plate at the place of subduction zone.

These zones can be found in any place in the world but them mainly common along the Pacific Ocean rim. Metamorphism. Metamorphism can be defined as the solid state recrystalization of the pre-existing rocks which occur due to physical and chemical conditions in the environment. These conditions include pressure, heat, and availability of chemically active fluids. Chemical, mineralogical, and crystological changes takes place during this time of the process. Metamorphism exists in three types namely; dislocation, contact, and regional metamorphism.

Prograde metamorphism occur in conditions where there is great increase in temperature and pressure while retrograde metamorphism takes place when there decreasing temperature and pressure in the process. Regional Metamorphism. Also known as barrovian metamorphism, this kind of metamorphism covers a large area in the continental crust and it is typically associated with areas such as mountain ranges especially the roots of previously eroded mountains and subduction zones.

Conditions which produced widespread regional metamorphosed rocks takes place during an oregenic event. Regional metamorphism is caused by collision of two adjacent tectonic plates or island arcs where continental plates produce extremely high compressional forces which are required for metamorphic changes necessary for regional metamorphism. These oregenic mountains are later faced with erosion leading to the exposure of intensively deformed rocks which are typical of their core.

The condition of the subduction slab as it plunges towards the mantle in the zone of subduction also produces regional metamorphism. Regional metamorphism can classified and described as metamorphic zones or metamorphic facies. Regional metamorphism usually gives the rock more indurate and at the same time, schistose, foliated or gneissic texture which consist of a planar arrangement in the minerals such that prismatic or platy minerals like homblende and mica get their longest axes arranged in a parallel position to one another.

This makes many of these rocks break and split in to one direction along the zones with mica. In the case of gneiss, minerals tend to segregate in to bands leading to formation of seams of mica and quartz in a very thin mica scheist consisting mainly of one mineral. Contact (Thermal) Metamorphism. Contact or thermal metamorphism normally takes place in arrears with intrusive igneous rocks and is a resultant of temperature change due to the intrusion of magma in to cooler country rocks. Contact metamorphic rocks are commonly referred to as homfels.

Rocks resulting from contact metamorphism may not exhibit signs of deformation and are frequently soft grained as compared to other metamorphic rocks. Greater changes occur where magma comes in contact with country rocks since the temperatures become highest in these boundaries. The area around the igneous rock that forms due to the cooling of the magma in the metamorphoosized zone is called metamorphism aureole. Aureoles usually show all the degree of metamorphism from this contact area to the unchanged country rocks which are many distance away.

Contact metamorphism is thus greater in adjacent areas but becomes desperate as it moves away from the distance of contact (Mathew, 1999). The size of the aureole will highly depend on the distance, heat of the intrusion, temperature difference, and the size of the rocks. Dikes usually have small aureoles due to minimal metamorphism while large ultramafic intrusions can result into significantly well-developed and thick contact metamorphism. In some cases, the magmatic fluids that come from the intrusive rocks may also take part in these metamorphic reactions.

This extensive addition of magmatic fluids will eventually result in to modification of chemical composition of these rocks. In such case, the metamorphism is graded to metasomaism. In case the intruded rock is rich in carbonate, a scam is formed. Magmatic waters rich in fluorine leaving cooling granite may result in to formation of greisens. Hydrothermal Metamorphism. Hydrothermal metamorphism takes place when there is an interaction between the rock and fluids of variable composition and high temperatures meet.

The difference that exists in composition between the invading fluid and the rock triggers a series of metamorphic and metasomatic reactions resulting in to the changes in the structure of the parent rocks. The hydrothermal fluid may be circulating ground water, magmatic fluid, or oceanic water. Ocean waters undergo convective circulation of water in the ocean’s floor where basalts produces intense metamorphism in areas spreading adjacent to the submarine volcanic areas. This type of metamorphism is found in areas which are rich in oil wells and geologists use this technology to search for deposits of metal ores which are valuable.

Foliation. Foliation is a result of preferred orientation of the minerals within a rock. Foliation is the penetrating of planar fabric in to the rocks during metamorphism. Foliation is found mostly in rocks affected by regional metamorphism. Foliated rocks are often compressed and distorted in their composition. Foliated rocks rocks also have realigned clays and micas through the physical rotation that occur to the mineral within the rock. Reference: Mathew, F. (1999). Physical Geography. Oxford. Oxford University Press.

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