![]() They often occur as avalanches when lava domes collapse or volcanic slopes shake. This volatile-rich material can travel over great distances at fast speed and these flows also called nuée ardente(French for fiery cloud). Many of the world's large accumulations of rhyolitic and dacitic debris are ash flows. The 1883 eruption of Krakatoa threw ash so high into the atmosphere that the finer dust eventually spread all around the Earth in just 14 days. The 1783 Laki fissure eruption in Iceland covered the entire island in ash, completely devastating the vegetation, and sent a dust cloud over Europe and North America for several months. As the ash cloud collapsed, the ash suffocated most of the victims. In the 79 AD eruption of Vesuvius, which partly destroyed the Roman town of Pompeii, the eruption plume rose to a great height and covered the surrounding area with pumice and hot ash to an extent of 450 km2. Because ash falls consist of relatively loose material, many deposits rapidly erode and are washed elsewhere. When the ash from these eruptions covers a large area, they form sheet deposits. These are relatively rare and occur only a few times every 100 years. Very powerful eruptions may hurl ash over 10 km into the air. Most ash clouds are only ejected 1 km into the air and fall back onto the volcanic cone. The extent of the ash fall depends upon the height that the ash is ejected along with the speed and direction of the prevailing wind at the time. The finer materials of explosive volcanic eruptions are ejected into the air, eventually falling to the ground as a layer of ash. Some of the largest and most spectacular lava tubes on Earth are found at Undara in northern Queensland. ![]() Then, when the eruption wanes, most of the lava drains out forming a hollow lava tube. These form when the outer surface of a lava flow hardens but the inside remains hot and lava continues to flow through. Eruptions that eject large amounts of bombs or lapilli (little stones) build up scoria cones or cinder cones. Large blocks of lava (bombs) cannot be ejected far from a volcanic vent because of their weight. This fragmental material is classified on the basis of grain size. The pyroclastic material erupted from an explosive volcanic eruption may be ejected as fragments, resulting in scoria cones or ash fall deposits, or it may spread outwards in ash flow deposits. This is an example f how volcanic gases released during eruptions can create both short-term and long-term environmental hazards. This led to the death of several thousand sheep from fluorosis during the 1995-1996 eruption of Ruapehu. However, this fluorine-bearing ash is soluble in the acidic digestion system of animals. Much of this fluorine forms slowly from soluble inorganic compounds which release fluorine to the soil over a long time. The volcanic ash from these eruptions is six times more concentrated in fluorine than the original magma. The greatest concentration of harmful gases occurs during early water-charged magmatic eruptions. Ruapehu volcano on the North Island of New Zealand has a vent-related hydrothermal system and is covered by a 10 000 000 m3 acidic crater lake where volcanic gases accumulate. They include water vapour, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide and hydrofluoric acid. This degassing magma is driven up into high eruption clouds and also hugs the ground as pyroclastic flows, clouds of magma droplets, ash clouds and gases. The reduction in pressure as the magma approaches the surface drives out bubbles of dissolved gases in bubbles and expands the volume of the magma. Magmas with explosive volatile contents can vary in styles of eruption. ![]()
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