9/1/2023 0 Comments Define viscosity volcanoThis is a fixed area of volcanic activity where magma rises from a mantle plume. Hawaii was formed due to a volcanic hot spot. The chain of islands that form Hawaii lie at the centre of the Pacific Ocean, thousands of miles away from a plate margin. Iceland owes its existence to effusive eruptions at a divergent margin between the North American and Eurasian plates. Examples of shield volcanoes include Mount Kilauea and Maunaloa on Hawaii. Shield volcanoes are usually found at divergent (constructive) boundaries and sometimes at volcanic hotspots. Although these eruptions destroy property, death or injury to humans rarely occurs.Ī simple cross-section of a shield volcano Eruptions tend to be frequent but relatively gentle. Shield volcanoes produce fast flowing basic (fluid) lava that can flow for many miles. Shield volcanoes are low with gently sloping sides and are formed from layers of lava. Over time denudation creates a more varied relief. When they are formed they have a slope of 1°. One example is the Deccan Plateau in central India. When basic lava flows from many fissures large areas can be covered by free-flowing lava known as flood basalts. The products of effusive eruptionsĪn effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. Without magma providing support below a caldera is formed when the sides of the volcano collapse. They form when an explosive eruption destroys the cone and the magma chamber below is emptied. CalderasĪ caldera is a volcanic crater, usually more than 2km in diameter. ![]() There are 66 strato-volcanoes in the Andes that have recently erupted. Convergent plate boundaries and explosive eruptions give rise to chains of strato-volcanoes. Examples of composite volcanoes include Mount Fuji (Japan), Mount St Helens (USA) and Mount Pinatubo (Philippines). Composite volcanoes are usually found at destructive plate margins. The thick lava cannot travel far down the slope of the volcano before it cools. This viscous lava has a lot to do with why they are shaped the way they are. Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes. When composite volcanoes erupt they are explosive and pose a threat to nearby life and property. ![]() It is suggested that viscosity is roughly measured by how long it takes the treacle to flow from one end of a boiling tube to the other.A simple cross-section of a composite volcano The "Treacle Test" experiment is designed to enable students to investigate how temperature, volatiles and crystals may affect viscosity. A “crystal mush” will clearly have increased viscosity. Again, this applies particularly to the cooler, more viscous magmas typical of destructive plate margins. Some magmas have already begun to crystallise by the time they reach the surface. The release of gas during eruption is particularly likely to be explosive if the magma is both viscous (as gas is released, so viscosity is immediately increased) and volatile rich. A high volatile content decreases viscosity (like adding water to treacle), and is probably the main factor in enabling some highly viscous (but also volatile-rich) melts to reach the surface at all. Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. Magma invariably contains small amounts of dissolved gas (water, CO 2 etc) which is released as pressure is removed. Rhyolites may reach the surface at temperatures of less than 900 oC, and so have much higher viscosity. Not surprisingly, magmas formed by partial melting of mantle rocks are much hotter – well over 1200 oC for some Hawaiian basalts – than is the case for crustally derived melts. ![]() Magma temperatures reflect the melting points of their mineral components. These magmas erupt as basalts or intrude as gabbro, and are far less viscous. Low-silica magmas are typically formed by partial melting of mantle rocks beneath mid-ccean ridges or at “hot spots” like Hawaii. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive (e.g. Such magmas erupt as andesites and rhyolites or are intruded as granite masses. Silicate-rich magmas are typically formed at destructive plate boundaries, by partial melting and/or assimilation of crustal rocks (which are richer in silica than the rock of the mantle). ![]() The nature of volcanic eruptions is highly dependent on magma viscosity and also on dissolved gas content.
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