describe the features and effects of a pyroclastic flow?
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Pyroclastic flow is a superheated cloud of volcanic debris & gases.\n\nPyroclastic flow can involve any of the following volcanic materials...\n\nAsh & Dust - small material that usually ejected into the air during an eruption; can stay suspended for weeks in the atmosphere.\n\nCinders - pea-sized material ejected from volcano; not molten\n\nBlocks - about the size of your palm; ejected out of a volcano and into the air. They are not molten.\n\nBombs - partially molten material ejected through the air during an eruption.\n\nGases - gases released by volcanoes include water vapor, carbon dioxide, nitrogen, sulfer, etc.\n\nPyroclastic flow generally follows all the rules of gravity. It flows downhill. So during an eruption all this material is channeled down into the valleys below the erupting volcano.\n\nThe dangers accompanying pyroclastic flow are suffocation from gases released w/ the flow, being buried beneath the flow as it moves rather swiftly at about 300 mph, and the heat of the material itself which is roughly 1500 degrees Fahrenheit.\n\nAnything in the way of the flow will be annihilated. This may include whole forests, buildings, vehicles, and human beings.
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Several ways. This also depends on the type of eruption. Often earthquakes occur before, or during an eruption causing things to topple. Also, often the size of the volcano itself changes. Lava cools to rock after flowing over the landscape, and kills the organic material it comes into contact with. The gas released during an eruption is also deadly! If it is an eruption like Mt. St. Helens was, the land is covered in ash and decimated by mudslides.
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Here is today's (nov 5) status report on St. Helen's from USGS: [not copyrighted]. \nMOUNT ST. HELENS UPDATE\n\nCurrent Volcano Alert Level WATCH ; Aviation Color Code ORANGE : Growth of the new lava dome inside the crater of Mount St. Helens continues, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. During such eruptions, changes in the level of activity can occur over days to months. The eruption could intensify suddenly or with little warning and produce explosions that cause hazardous conditions within several miles of the crater and farther downwind. Small lahars could suddenly descend the Toutle River if triggered by heavy rain or by interaction of hot rocks with snow and ice. These lahars pose a negligible hazard below the Sediment Retention Structure (SRS) but could pose a hazard along the river channel upstream.\n\nPotential ash hazards: Wind forecasts from the National Oceanic and Atmospheric Administration (NOAA), coupled with eruption models, show that any ash clouds rising above the crater rim today would drift eastward.\n\nPotential ash hazards to aviation: Under current eruptive conditions, small, short-lived explosions may produce ash clouds that exceed 30,000 feet in altitude. Ash from such events can travel 100 miles or more downwind.\n\nRecent observations: The volcano is not visible today owing to clouds and heavy rain. Anecdotal reports and signals recorded on acoustic flow monitors on Saturday night suggest that the rains triggered debris flows that descended both the South Fork Toutle River and North Fork Toutle River, but these remain unconfirmed by field checking yet.
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Even though I like Oz's answer, it is actually the flow of magma in the Earth's mantle on average about 30 miles below the surface, where molten rock flows from high to low, nearing the earth's outer core where the temprature reaches to near 10000 degrees F.  Another heating element (or actually energy element, that has a side effect of heating) is radioactive decay.  Though most of the Earth's heat energy is released throught the much thinner parts of the crust that makes up the ocean floors.
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Volcano is the result of Lava and gases gushing out of the surface of the earth at great heat and pressure.  The molten lava is running over the land and destroy the plants and trees and after cooling becomes rock there. Fresh lava  comes out and run in the available slope.  The result is a mountain is growing with fire and lava.  Thus the mountains are the result of volcano and lava.  When the pressure within the volcano is reduced the volcano takes rest.It will erupt at any time  and send lava and gases, fire in all the directions.  The centre of the earth is believed to contain very hot lava and gases which is likely to come out at any time in any part; and most likely in the soft surfaces .
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For magmas, just remember that viscosity increases greatly with silica (SiO2) content, and decreases with higher temperature. Silica content affects viscosity because SiO2 in magma tends  to form long, stringy molecules, making the magma thicker, or "stickier." And, viscous magma erupts more violently. Low viscosity magma, like silica-poor basalt, is runny and escapes easily to the surface without much "excitement."
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go to\nhttp://en.wikipedia.org/wiki/Volcanoe
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you need to Build Volcano Model... go to this page\nhttp://volcano.und.nodak.edu/vwdocs/volc_models/models.html
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http://www.fs.fed.us/gpnf/volcanocams/msh/
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Lava or pumice which is formed in a volcanic eruptions has air pockets in it. If smothed on one or two sides it could be used to cool dinner.
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High pressure builds up inside the magma chamber due to gases & the actual heat inside.\n\nComposite volcanoes usually are the worst for explosive eruptions b/c of the amount of trapped gases that build up. They erupt lava that is high in silica which also makes it thick. This combination usually leads to deadly pyroclastic flows.
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1. Sedimentation.\n2. Compaction.\n3. Dewatering/degassing\n4. Lithification by heat and pressure.
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front stagnation point\nhttp://www.fluidmech.net/tutorials/raindrops/drop_flow.htm\nhttp://www.fluidmech.net/tutorials/raindrops/raindrop.htm
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INTRODUCTION\nThis project culminates my undergraduate studies at Keene State College as a Geochemistry/Spanish major. I decided to do this research with the intention of learning about the potential hazards involved in living in a volcanically active area, mainly in Quito, Ecuador where I spent the summer of 1995 in with my grilfriends' family, whom without their support and helpfulness, this could not have even been thought about! I was so amazed to wake up daily and look out the window to the south and be awe-stricken by the immensity and beauty of the snow-capped volcano Cotopaxi. Naturally, the first part of the paper describes the geomorphology and volcanic hazards near Cotopaxi as relevant to the development of volcanic hazard maps (see figure 1 (315 K), Figure 2 (99K), for example shows the areas where volcanic ash will fall.\n\nIn my research, I've looked at three main issues that are important for the understanding the methodology used in trying to interpret and make predictions based upon the known systems of a volcano. I've looked into great detail for the many methods that exist for making predictions for possible future use in volcanic predictions. Included are some samples collected in August, 1995 at Mount Cotopaxi, located at 78¡26' west longitude and 00¡41' south latitude. With these rudimentary samples, I started to realize the importance that these andesites have. Shown in this paper are hazard zonation maps which were made prior to the explosions of Mount Saint Helen's in Washington state and complement the maps made for Mount Saint Helen's. These maps have predictions on them showing possible future volcanic events. They are based on the volcano's past history, as can be seen with the eruptive history of Cotopaxi in Table 1, an analysis of some of the deposits located around the volcano in figure 3 (148K). Further attention is spent on other methods of analyzing the hazards, but based on shorter term, such as analysis of gases expelled, earthquake activity, and changes in the relative shape of the volcano. \n\nThis analysis of some of the volcanic deposits used in determining the eruption history of certain volcanoes worldwide, with particular attention paid to Mount Cotopaxi for the purpose of making predictions about future eruption events and for future benchmark studies for hazard mitigation in volcanically active areas of the world. The study of volcanic mineral spring waters and a general comparison between other non-volcanic waters will aid in learning about the influence of the volcanic system. A comparison between the chemical variations of some volcanic mineral waters and some non-volcanic spring fed waters is also made \n\nCotopaxi is a volcano located about 50 km south of Quito, Ecuador. At 5,897 metres it is the second highest in the country—the highest being the Chimborazo. Cotopaxi is technically referred to as a stratovolcano. It has a height of more than 3,000 metres when measured from its base, which has a width about 23 km. It has one of the only equatorial glaciers.\n\nOccasional claims that Cotopaxi is the highest active volcano in the world are known to be incorrect. Llullaillaco volcano is considerably higher and is definitely in an active phase, having erupted as recently as 1877. Ojos del Salado is higher still, and there are indications near its summit of recent activity, although its remoteness precludes a definitive statement as to whether it is currently active.\n\nThere have been more than 50 eruptions of Cotopaxi since 1738. Numerous valleys formed by powerful lahars (mudflows) surround the volcano. This poses a high risk to the local population, their settlements and fields. The city Latacunga has been completely destroyed at least twice in its history. Cotopaxi's most violent eruptions in historical times occurred in the years 1744, 1768, and 1877. In the 1877 eruption pyroclastic flows descended all sides of the mountain, with lahar
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viscous felsic or high silica magmas
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convection, advection and radiation
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National Geografic website has a lot of information about it. \nIf you go to the linlk provided and then to the section of Natural Forces, you will find a lot of material to support your research. \n\nSecond link is about simulate and understand these events, explaning how it works at all their levels. You can find it to Earthquake, Hurricane, etc. I've putted the link to Vulcanoes\n\nGood luck