Well, it isn't too hard to figure out, now is it. I would imagine a volcanic hazard would be some sort of hazard that is related to volcanoes. Now that would be a pretty short article if I left off with that, though, wouldn't it? Let's have a brief discussion about some of the specific hazards that are associated with volcanoes.
First of all, there's the most obvious one: lava. As much as lava-like you see at hawaii- is associated with volcanoes, it isn't really that common. The flowing runny lava is called basalt and is mostly associated with rifts and hotspots-places where mantle rock can reach the surface without too much trouble. The good thing about lava is that it looks really nice and it's easy to get out of the way; very few people die as the direct result of it. Depending on the viscosity of the lava (resistance to flow), it can move at a variety of speeds ranging from snail-paced to faster than you or I can run. Sluggish lava (usually called a'a, although in some cases it may not even be basalt) is easy to walk away from, but it causes a great deal of property damage. The slow lava flows, because they are so viscous, can be enormous and bull-doze over houses and property. Fast moving flows form rivers of lava with banks on either side. Don't jump in and you'll be ok. When lava leaves the channel, it quickly slows down as it spreads out and cools.
Next comes stupidity. That's right, when tourists visit a volcano without the common-sense that God gave most centipedes, there's bound to be trouble. For the love of God, if you're visiting a volcano and you don't work there; read and obey all the signs! People die every year because they venture out onto lava deltas, or get too close to skylights, or some other generally bad idea. Lava deltas are particularly deadly because they're a great place to watch the lava flow into the ocean, but some people don't realize that the lava is flowing just a few feet under them or don't seem to notice that there are large cracks in the flow behind them. The deltas are not structurally stable at all and they collapse into the ocean on a fairly regular basis. If you are caught on one of these deltas when it collapses, you don't have to worry about drowning because you'll be crushed to death, burned by the acidic steam, or immersed in molten lava in just a few moments. Obeying the posted signs and using common sense will keep you safe in almost all situations. Even simply staying on marked trails is important; people have been scalded to death at Yellowstone because of this.
Next comes gasses. Most of what volcanoes emmit is just vaprous water, but they also leave us a number of other abundant gifts such as carbon dioxide (CO2), sulfur dioxide (SO2), carbon monoxide (CO), sulfuric acid (H2S), hydrochloric acid (HCl), and hydrofluoric acid (HF). Not as common are fluorine, helium, nitrogen, and argon. Volcanic gasses don't usually kill people directly, but they lead to a number of other different hazards. The gasses can be emmitted during the eruption from the main vent(s) or from fumaroles along the volcano. The most notable loss of life as the direct result of gasses is at Lake Nyos, where a 1986 limnic eruption killed 1700 people by asphyxiation.
A major and spectacular hazard is the pyroclastic flow. The dangerous flows usually occur when the flank of a volcano collapses and erupts explosively out of the newly created hole. The incandescent cloud flows out of it, following the contours of the land, burning and crushing everything in it's path. A huge number of volcanically related deaths are associated with these which can now be reasonably predicted ahead of time. The most famous disaster in history associated with this is probably Pompeii. When you are overcome by a pyroclastic flow, all of the moisture in your body may be vapourized, instantly mummifying you...that is, if there's anything left to mummify.
Floods; yes, volcanoes can cause some massive floods. Many volcanoes are capped with large glaciers and so when they erupt, the heat instantly melts them and the water comes rushing off the side of the mountain. Armero, in Colombia, was completely wiped out even though it was miles away. 23,000 people died as a result. Also a flooding danger are jokulhlaups which are glacial bursts. These occur when a volcano underneath a large glacier melts a lake into the underside, which is dammed by ice. When the ice dam breaks, it creates an enormous rush of water that washes over everything in its path. These are a major problem in Iceland.
So what else is there? We've covered floods, lava, gas, explosions...is that it? Not quite. Ash is a MAJOR hazard. Ash, which is microscopic grains of jagged glass, can be breathed in by people when it falls to the earth (this is not the same as organic ash which is formed when material, such as wood, is burned). The jagged glass gets into the lungs and starts scraping and cutting up the tissue. What you get is a potentially deadly condition known as silicosis. That's why, when you see pictures of people in cities around volcanoes, they're wearing surgical masks-breathing without them can be very dangerous. In addition to this, ash in the sky is extraordinarily dangerous to passing aircraft. When the ash enters an engine on the plane, it melts to the turbines, disabling the aircraft. One of the main tasks of volcanologists, apart from helping people around the volcano, is predicting where plumes of ash will be carried by the wind in order to direct air traffic around it.
The eruption of volcanoes can also cause very strong thunderstorms. While the eruption is pushing millions of tons of ash into the sky, the air is convecting and forming large thunderstorms. Though lightning associated with the eruptions have killed people before, the display is spectacular to see from a safe location.
Perhaps the biggest volcanic killer is famine. This is usually the result of one of two hazards: gas or climate change. A great example of gasses causing famine is Lakagigar, Iceland, in 1783. The eruption released huge amounts of the extraordinarily toxic fluorine which was absorbed by plants and eaten by the cattle. The cattle then died, resulting in the starvation and death of one quarter of Iceland's population. Gasses can also wreak havoc as acid rain, when the gasses combine with water and eat away at vegetation after rainfall.
Finally there's climate. It may surprise you to find out that volcanoes actually play a huge role in climate because of all the gasses and ash they release. Sulfur dioxide and carbon dioxide, two of the biggest greenhouse gasses, are released in enormous amounts from volcanoes enhancing global warming. The immediate result of eruptions, however, is not warming but cooling. The ash and gasses circle the globe temporarily causing it to cool dramatically. The biggest volcanic disaster would be the eruption of Tambora, which caused global temperatures to drop as much as 3 degrees celsius. It caused the 1816 "year without a summer" when the US was plagued with subfreezing temperatures during the middle of June, even in Georgia and Florida. While the pyroclastic flows from the eruption killed 12,000 people, 80,000 more died of starvation world-wide due to it's atmospheric impact.
You may begin to wonder why volcanoes are so hazardous. I mean, yeah, they're really dangerous to be around...but why do they actually kill so many people? Can't they just live somewhere else? The simple fact is this: volcanoes are everywhere and the majority of our planet's population lives near them. The entire Pacific Ocean is surrounded by a ring of volcanoes (also known as the ring of fire). Ocean islands are often composed of a volcano rising from the seafloor, and a good portion of Europe is volcanic. The western Americas, Central America, and the Carribbean is chalk full of volcanoes. Africa's Great Rift is full of volcanoes creating a swath through the continent with other sites of activity popping up in groups all over the place. Even Asia's got it's fair share of volcanoes...right up there with Antarctica! The fact is that volcanoes are almost everywhere and so we can't really just not live around them. Moreover, volcanoes produce some of the most fertile land which creates an enticing bonanza for farming...especially when you're just barely able to scratch out a living in some under-developed country. The other problem is that living far away from a volcano doesn't really make you safe. You've just read about climate changes that have caused famine throughout far-away places in the world. Even Nebraska, in the United States, is covered with ash from large eruptions in Nevada and Oregon from long ago. The fact is that the effects of volcanoes are far reaching and that's why we have volcanologists; people whose job is to learn about and try to understand volcanoes in order to save peoples lives in the future.
So this is where we come to the end of this short article. People dedicate their lives to studying these hazards, so this is by no means all there is to the story; this is just a quick summary of the different hazards a volcano can pose. I encourage you to look into these hazards more thoroughly, especially if you live in southern Alaska, Washington, some regions of northern California and Oregon, Hawaii, the Pacific coast of South America, along the Meditterainean, the Caribbean, Iceland, central Africa, Indonesia, eastern Asia, or the Pacific Islands.