Lava flows are almost always too slow to run over people, but they can certainly run over houses, roads, and any other structures.
Pyroclastic flows are mixtures of hot gas and ash, and they travel very quickly down the slopes of volcanoes. They are so hot and choking that if you are caught in one it will kill you. They are also so fast (100-200 km/hour) that you cannot out-run them. If a volcano that is known for producing pyroclastic flows is looking like it may erupt soon, the best thing is for you to leave before it does.
Some of the good ways that volcanoes affect people include producing spectacular scenery, and producing very rich soils for farming.
Water vapor, the most common gas released by volcanoes, causes few problems. Sulfur dioxide, carbon dioxide and hydrogen are released in smaller amounts. Carbon monoxide, hydrogen sulfide, and hydrogen fluoride are also released but typically less than 1 percent by volume.Gases pose the greatest hazard close to the vent where concentrations are greatest. Away from the vent the gases quickly become diluted by air. For most people even a brief visit to a vent is not a health hazard. However, it can be dangerous for people with respiratory problems.
The continuous eruption at Kilauea presents some new problems. Long term exposure to volcanic fumes may aggravate existing respiratory problems. It may also cause headaches and fatigue in regularly healthy people. The gases also limit visibility, especially on the leeward side of the island where they become trapped by atmospheric conditions.
Source of Information: Volcanic and Seismic Hazards on the Island of Hawaii by Christina Heliker, 1991, U.S. Geological Survey General Interest Publication.
A deadly eruption
The 1815 explosive eruption of Tambora volcano in Indonesia and the subsequent caldera collapse produced 9.5 cubic miles (40 cubic kilometers) of ash. The eruption killed 10,000 people. An additional 80,000 people died from crop loss and famine.
To put it mildly, ash is bad for jet aircraft engines. Apparently the problem is much more severe for modern jet engines which burn hotter than the older ones. Parts of these engines operate at temperatures that are high enough to melt ash that is ingested. Essentially you end up with tiny blobs of lava inside the engine. This is then forced back into other parts where the temperatures are lower and the stuff solidifies. As you can imagine this is pretty bad. One problem that I heard about is that pilots start losing power and apply the throttle, causing the engine to be even hotter and melt more ash.Added to this is the fact that ash is actually tiny particles of glass plus small mineral shards–pretty abrasive stuff. You can imagine that dumping a whole bunch of abrasive powder into a jet engine is not good for the engine. This has been a pretty non-scientific explanation of the problem. I just found an article that describes the problem a little more technically.
“The ash erodes sharp blades in the compressor, reducing its efficiency. The ash melts in the combustion chamber to form molten glass. The ash then solidifies on turbine blades, blocking air flow and causing the engine to stall.” This comes from the FAA Aviation Safety Journal, Vol. 2, No. 3.
The distance you have to evacuate depends entirely on what kind of eruption is going on. For example, Pinatubo, one of the largest recent eruptions sent pyroclastic flows at least 18 km down its flanks, and pumice falls were hot and heavy even beyond that. For example, pumice 7 cm across fell at Clark Air base which is 25 km from the volcano! A 7 cm pumice won’t necessarily kill you but it does mean that there is a lot of pumice falling, and if you don’t get out and continuously sweep off your roof it may fall in and you’ll get squashed.On the other hand, the current eruption at Ruapehu is relatively small. In fact, there were skiers up on the slopes when the eruptions commenced, and even though they were only 1-2 km from the vent they managed to escape. The volcanologists routinely go up on the higher slopes of Ruapehu during these ongoing eruptions to collect ash and take photographs.
So you see, you need to know something about what you think the volcano is going to do before you decide how far to run away. I guess if you have no idea of what the volcano is planning, and have no idea of what it has done in the past, you might want to be at least 25-30 km away, make sure you have a good escape route to get even farther away if necessary, and by all means stay out of low-lying areas!
Cities and Towns
The effect an eruption will have on a nearby city could vary from none at all to catastrophic. For example, atmospheric conditions might carry ash away from the city or topography might direct lahars and pyroclastic flows to unpopulated areas. In contrast, under certain atmospheric, eruption and/or topographic conditions, lahars, pyroclastic flows, and/or ash fall could enter the city causing death and destruction.
This scenario brings up several interesting problems. How do you evacuate a large population if there is little warning before the eruption? Where do these people go? If an eruption is highly likely yet hasn’t happened yet how long can people be kept away from their homes and businesses?
I should point out that in most volcanic crises geologists advise local civil defense authorities. The civil defense authorities decide what to do concerning evacuations, etc.
The IAVCEI has a program to promote research on “Decade” Volcanoes. Decade volcanoes are likely to erupt in the near future and are near large population centers. Mount Rainier in Washington and Mauna Loa in Hawaii are two Decade volcanoes in the U.S. Other Decade volcanoes include Santa Maria, Stromboli, Pinatubo, and Unzen.
What happens to the towns around a volcano when it erupts depends on many things. It depends of the size and type of eruption and the size and location of the town. A few examples might help. The 1984 eruption of Mauna Loa in Hawaii sent lava towards Hilo but the eruption stopped before the flows reached the town. The 1973 eruption of Heimaey in Iceland buried much of the nearby town of Heimaey under lava and cinder. The 1960 eruption of Kilauea in Hawaii buried all of the nearby town of Kapoho under lava and cinder. In 1980, ash from Mount St. Helens fell on many towns in Washington and Oregon. The 1902 eruption of Mount Pelee on the island of Martinique destroyed the town of Saint Pierre with pyroclastic flows. In 1985, the town of Armero was partially buried by lahars generated on Ruiz. For more examples see Decker and Decker (1989).