Volcanoes at destructive and constructive margins

Volcanic impacts are more hazardous near destructive margins than constructive ones. Discuss.

I agree that volcanic impacts are more hazardous near destructive margins than constructive ones.

Destructive margins occur where continental and oceanic plates converge together or oceanic and oceanic plates converge together. For the former, the continental crust has a rock density much lower than the oceanic crust. As these two plates meet the oceanic crust is submerged underneath the continental crust, as it is denser. This causes a subduction zone to form where deep sea trenches result. As the oceanic plate is submerged, it begins to melt due to increased heat and friction. The melting of the plate results in magma building up, which eventually rises up to the surface as volcanoes. As for the later, the denser plate will be subducted and some of the magma will rise through fractures in oceanic crusts and pour onto ocean floor, forming undersea volcanoes. If the magma forces its way up on offshore, then ‘island arcs’ form. The West Indies and Japan are two examples of island arcs.

A constructive margin is where two tectonic plates are diverging away from each other, which causes new crust to be created at the boundary between these two plates. When two plates diverge, initially, a rift valley may occur. Magma rises from the mantle filling in the gaps between the two plates, which can cause submarine volcano to occur. These volcanoes may rise above sea level thus forming islands. Two examples of this are the island of Surtsey, and Easter Island.

In destructive margins of oceanic and continental plate, the oceanic crust is normally highly filled with rocks rich in iron and magnesium rock, but as it rises through granitic continental plates, it may cause partial melting of both mantle rocks and the plate rocks, producing magmas of varying silicity. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silicity (high-viscosity) composite volcanoes.

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As for construction margins, the plates pull apart and “de-pressurises” the mantle beneath the plates and causes it to partially melt. The Magma rises up the cracks between the two retreating plates, with little contact to the surrounding rocks, it picks up very little silica content. This causes it to reach the surface as more fluid basic lava. Then they are built up slowly by the accumulation of thousands of highly fluid lava flows that spread widely over great distances, and then cool as thin, gently dipping sheets, forming shield volcanoes.

The composite volcanoes are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, and cinders. As lava is acidic, the lava is thick and sticky and this viscosity of the lava does not allow trapping gas to escape. Additionally, the lava solidifies in the central pipe, blocking the passage and these built up the pressure in the volcano. Therefore when the volcano erupts, there will be violent explosion as ash and act as a medium of transportation for lava and allow the lava to flow over them quickly. Also, pyroclastic materials will also be poured out.

The shield volcano is a gently sloping volcano that is flat near the top and is built entirely by basic lava. By its basic nature, the lava is more fluid and it flows faster than the lava in the composite volcano before solidifying. However, unlike the composite volcano, there will be no explosion when the volcano erupts at a constructive margin, as there is no pressure and thus gas can escape easily through the less viscous lava, and very little pyroclastic material or none is found within the volcano.

Shield volcano’s lavas are more fluid and can travel over a greater distance. When shield volcano erupts, it may be less noticeable as there is no explosion and people who are unaware could not escape in time. However, it is not very hazardous because it is the pryoclastic materials that create the deadly phenomena of one volcano.

Therefore, composite volcanoes at destructive margins are more hazardous than shield volcanoes at constructive margins, as there are explosions of pyroclastic materials. Pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. They are fiercely hot, contain toxic gases, and can incinerate, burn, and asphyxiate people. Pyroclastic flow can move at phenomenal, hurricane-force speeds and travel long distances so their threat is far reaching. When these pyroclastic flows and ash deposits are mixed with water from glaciers, Caldera Lake and rain, they become lahars and are more hazardous as they can flow even faster down the steep composite volcano and the force of lahars is so great that buildings may be crushed or carried away.

Moreover, volcanic impacts will be more destructive and hazardous on land. Constructive margins are usually found in between oceanic and oceanic plate, and the volcanoes may appear above the sea. Since these eruptions are surrounded by water, they behave differently than their land-based relatives. The lava that pours out onto the sea floor cools rapidly in the water. Certainly, there will be volcanoes that have not grown and are underwater; their eruptions might be invisible on sea surface and will not affect any human beings. On the other hand, volcanoes at destructive margins are usually located on land, but there are some exceptions that are also located on sea. Those that are on land are very hazardous as many people stay around the volcano because the volcano lava can form fertile land for agriculture uses. Thus, when the volcano erupts, the volcanic impacts are disastrous and hazardous as many people and their homes are affected. One example of such cases is the Mount Pinatubo in The Philippines. The human impacts of the disaster are staggering. In addition to the up to 800 people who lost their lives, there was almost one half of a billion dollars in property and economic damage.

In conclusion, the volcanic impacts at destructive margins are more hazardous than those at constructive ones. Composite volcanoes, at destructive margins, are more explosive than shield volcanoes, at constructive margins, because composite volcanoes contain pyroclastic flows and are mostly located on land.

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