A volcanic hazard is the probability a volcanic eruption or related geophysical event will occur in a given geographic area and within a specified window of time. The risk that can be associated with a volcanic hazard depends on the proximity and vulnerability of an asset or a population of people near to where a volcanic event might occur.
Lava flows Different forms of effusive lava can provide different hazards. Pahoehoe lava is smooth and ropy while Aa lava is blocky and hard. Lava flows normally follow the topography, sinking into depressions and valleys and flowing down the volcano. Lava flows will bury roads, farmlands and other forms of personal property. This lava could destroy homes, cars, and lives standing in the way. Lava flows are dangerous, however, they are slow moving and this gives people time to respond and evacuate out of immediate areas. People can mitigate this hazard by not moving to valleys or depressed areas around a volcano.
Pyroclastic materials (tephra) and flow Tephra is a generalized word for the various bits of debris launched out of a volcano during an eruption, regardless of their size. Pyroclastic materials are generally categorized according to size: dust measures at <1/8 mm, ash is 1/8–2 mm, cinders are 2–64 mm, and bombs and blocks are both >64 mm. Different hazards are associated with the different kinds of pyroclastic materials. Dust and ash could coat cars and homes, rendering a car unable to drive with dust accumulation in the engine. They could also layer on homes and add weight to roofs causing a house to collapse. Also, ash and dust inhaled could cause long-term respiratory issues in people inhaling the particles. Cinders are flaming pieces of ejected volcanic material which could set fire to homes and wooded areas. Bombs and blocks run the risk of hitting various objects and people within range of the volcano. Projectiles can be thrown thousands of feet in the air and can be found several miles away from the initial eruption point. A pyroclastic flow is a fast-moving (up to 700 km/h) extremely hot (~1000 °C) mass of air and tephra that charges down the sides of a volcano during an explosive eruption.
Air travel hazards Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft where the particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Dangerous encounters in 1982 after the eruption of Galunggung in Indonesia, and 1989 after the eruption of Mount Redoubt in Alaska raised awareness of this phenomenon. Nine Volcanic Ash Advisory Centers were established by the International Civil Aviation Organization to monitor ash clouds and advise pilots accordingly. The 2010 eruptions of Eyjafjallajökull caused major disruptions to air travel in Europe.
Mudflows, floods, debris flows and avalanches When pyroclastic materials mix with water from a nearby stream or river, they can turn the watercourse into a fast moving mudflows. These are called lahars; when the lahar contains large material such as blocks of rock and trees, it is a volcanic debris flow. Lahars can form directly from a pyroclastic material flow flowing into a river, or could possibly form after the main eruption. The latter are referred to as secondary lahars and form when rain wets the ash and debris already on a landscape and stick together, rolling along the topography. It's estimated it can only take 30% water to initiate ash into a lahar. The thicker and/or more fast-moving a lahar, the more potential to destroy things in its path, thus making it more dangerous than a slower and/or more diluted lahar. Lahars and mudflows can damage buildings, wildlife and cars and can prove difficult to escape once caught in them. The lahars can coat objects, wash objects away and can knock objects down by their force. Lahars, debris flows and mudflows that travel into a river or stream run the potential for crowding the waterway, forcing the water to flow outward and causing a flood. The volcanic matter could also pollute the water, making it unsafe to drink. The debris ejected from the volcano adds to the sides of the slope with each eruption, making the sides steeper each time. Eventually the slope gets so steep it fails and an avalanche ensues. These avalanches carry material and debris for very long distances at very short intervals. This makes a warning system nearly impossible because the slope failure could occur at any time. The avalanche will destroy anything in its path including personal property, houses, buildings, vehicles and possibly even wildlife. If the impact of the materials in the avalanche doesn't destroy the person or object at first contact, damage could result from the weight of prolonged material on the objects.
Volcanic gases Large, explosive volcanic eruptions inject water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 16–32 kilometres (9.9–19.9 mi) above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The SO2 emissions alone of two different eruptions are sufficient to compare their potential climatic impact. The aerosols increase the Earth's albedo—its reflection of radiation from the Sun back into space—and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years; sulfur dioxide from the eruption of Huaynaputina probably caused the Russian famine of 1601–1603.
Acid rain
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