Definition of viscosity in geology9/13/2023 ![]() Vulcanian eruptions involve violent gas explosions that blow out sticky plugs of lava. Lava flows from Strombolian eruptions are not common though they do produce small amounts of ash. These regular explosions can produce impressive booming sounds, however, the eruptions are relatively small. The lava has a fairly high viscosity (it’s quite thick due to its high silica content), so gas pressure builds up before the material can be ejected from the volcano. They eject short bursts of lava 15 to 90 meters in the air. The effects are impressive but not particularly dangerous. Strombolian eruptions are named after Stromboli in Italy. The general eruption style is a steady lava flow from a central vent, which can produce wide lava lakes, ponds of lava forming in craters or other depressions. Eruptions can form fire fountains, Lava thrust up to 50m in the air for many hours. Instead, they produce low-viscosity, low-gas-content lava that flows over large areas producing gently sloping shield volcanoes and lava plateaus. These eruptions are generally not explosive or destructive and do not throw huge amounts of Tephra or pyroclastic material in the air. In Hawaiian eruptions, the lava is more basic and basaltic, with low gas pressures and low silica content. Large volumes of basaltic lava form large horizontal plains or plateaux. Icelandic eruptions are characterised by a persistent eruption along a fissure. Mount Kilauea, Hawaii, is an example of an active volcano. An active volcano has erupted recently and is likely to erupt again. A dormant volcano has not erupted in 10000 years. An extinct volcano will never erupt again. Volcanoes are found in three states – extinct, dormant and active. El Tiede on Tenerife is a strato-volcano. Not all volcanoes that form at hot spots are shield volcanoes. As the plate moves away from the hot spot active volcanoes lose their source of magma and become extinct. As the Pacific Plate moves north-west over the plume huge amounts of basalt have accumulated on the ocean floor to produce the Hawaiian islands. This 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. A simple cross-section of a composite volcano
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