Effects Of Volcanism In New Zealand

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The natural process of volcanism occurs in New Zealand and can produce a multitude of hazards to both the natural and cultural environment. Volcanism occurs in New Zealand due to its location on the boundary of the Pacific Plate and Indo-Australian Plate. Due to convection currents in the asthenosphere, the tectonic plates are pushed together and one plate is pushed below the other creating a subduction zone. In the North Island the Pacific Plate is subducting below the Indo-Australian but in the South the Indo-Australian subducts beneath the Pacific Plate. As the oceanic lithosphere subducts beneath the continental crust (or vice versa in the South), the increase in temperature and pressure causes volatiles such as water to be expelled from …show more content…
The magma rises towards the earth’s surface and either breaks through weaknesses in the crust, forming volcanoes or sits beneath the crust and heats it, causing geothermal activity. Once the magma escapes through the earth’s surface it is called lava and this can build up over time to form the ‘shape’ of the volcano. There are different types of magma, varying in their silica content. For example, Mafic magma has a low silica content while Felsic magma has a high silica content. The amount of silica present in the magma determines its viscosity with high silica content causing a high viscosity and a low silica content produces magma with low viscosity. Volcanoes are found throughout both the North and South Islands of New Zealand although the majority are located in the North. There is a mixture of both active and dormant volcanoes. All of the volcanoes found in the South Island were once active but are now dormant. This is due to the plates moving and the magma no longer being able to reach the volcano. In the South Island subduction is occurring at 35mm/a compared to 45mm/a in the North Island (Wallace et al., …show more content…
A lahar is a mudflow that is produced due to volcanism. They are composed of water, sand, silt, ash and rock debris- also known as tephra. There are different forms of lahars, differing in the sediment size of rocks. For example, a debris-flow lahar contains particles of various sizes ranging from small sediments to large boulders while a hyper concentrated-flow lahar contains much smaller particles. The composition of the lahar influences the manner in which it flows, with large sediment size causing it to be more viscous while the smaller sediment size causes a much thinner slurry. Due to the high speeds that the lahars flow and the density of the mudflows, the lahars can pick up and transport debris such as vegetation and large boulders that are in its path and deposit them across the landscape. Lahars occur due to volcanic material mixing with water but do not always occur during or directly after a volcanic eruption. They can be formed due to eruptions in crater lakes, collapse of crater lakes, heavy rainfall, melting of snow and ice, condensation of ejected steam, and mixing of pyroclastic flows and water deposits such as lakes. Lahars flow down the slopes of volcanoes and through channels and gorges due to their low viscosity. Their speed is dependent on the gradient of the slopes and the density of the mudflow as well as the volume

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