 A volcano is an opening, or rupture, in a planet's surface or crust,
which allows hot, molten rock, ash, and gases to escape from below the
surface. Volcanic activity involving the extrusion of rock tends to
form mountains or features like mountains over a period of time.
Volcanoes are generally found where tectonic plates are diverging or
converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge,
has examples of volcanoes caused by "divergent tectonic plates" pulling
apart; the Pacific Ring of Fire has examples of volcanoes caused by
"convergent tectonic plates" coming together. By contrast, volcanoes
are usually not created where two tectonic plates slide past one
another. Volcanoes can also form where there is stretching and thinning
of the Earth's crust (called "non-hotspot intraplate volcanism"), such
as in the African Rift Valley, the Wells Gray-Clearwater volcanic field
and the Rio Grande Rift in North America and the European Rhine Graben
with its Eifel volcanoes.
Volcanoes can be caused by "mantle plumes". These so-called "hotspots"
, for example at Hawaii, can occur far from plate boundaries. Hotspot
volcanoes are also found elsewhere in the solar system, especially on
rocky planets and moons.  Cross-section through a stratovolcano: 1. Large magma chamber 2. Bedrock 3. Conduit (pipe) 4. Base 5. Sill 6. Branch pipe 7. Layers of ash emitted by the volcano 8. Flank 9. Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud 
| How to Survive a Volcanic Eruption
Volcanic activity comes in many different forms, from trickles of
lava to violent explosions that shoot rocks, ash, and gas hundreds of
feet into the air. The latter type, often called "plinian" eruptions,
can be as powerful and destructive as any nuclear weapon, but all types
of volcanic eruptions are dangerous. Fortunately, most volcanoes are
carefully monitored, and scientists can usually provide some advance
warning before a serious event. That said, if you live near a volcano
or get an opportunity to visit one, you are always at risk. As is the
case with most natural hazards, however, a little knowledge and
preparation can help you escape an eruption alive.
- Prepare for the worst.
If you live in the vicinity of a volcano, make preparations well in advance. If you reside in the shadow of a volcano, you should always be ready for an eruption.
Learn what kind of eruption is likely. You will need different strategies to deal with different kind of eruptions.
Stock up on necessities. Store at least a three-day supply of food and potable water at your home. In the event of an eruption, water supplies may become contaminated, so you can’t count on your well or public water. Keep a first aid kit, blankets, and warm clothing handy, and have a battery-powered radio and fresh batteries on hand so that you will be able to listen to advisories if the power goes out. Keep necessary medications together. Ideally, you should keep all these things in one place—a large container that you can carry, for example—so that you can quickly bring them with you if you need to evacuate.
Make a plan and know escape routes. If you live near a well-researched and well-monitored volcano, you can probably obtain a hazard-zone map from your local emergency management agency or, in the U.S., from the U.S. Geological Survey. These maps show the probable paths of lava flows and lahars (debris flows) and give estimates for the minimum time it would take a flow to reach a given location. They also divide the area around the volcano into zones, from high-risk to low-risk. Your local emergency agency may also have evacuation routes mapped out. Using this information you can get some idea of how safe your house or workplace is, and you can plan the best route of escape. Because volcanic eruptions are complex and, to some extent, unpredictable, you should have several alternative routes to reach one or more “safe zones.”
If you will be visiting a volcano, knowledge is your most important protection. Before going to the volcano, consult with local authorities, and heed their recommendations or warnings. Learn about the hazards you may encounter in the area of the volcano, and get a reputable guide to accompany you, if possible. Bring plenty of water in case you become unexpectedly trapped by a lava flow, and don’t overexert yourself. You’ll be able to react more quickly—and run for your life, if necessary—if you’re not fatigued.
- Listen for radio or TV advisories when an eruption occurs.
When a volcano erupts, immediately tune in to determine if you are in
immediate danger where you are and also to find out what is happening
around you. These advisories will be your “eyes” to see the larger
picture and help you assess the situation and make the right decisions.
- Leave the area promptly if told to do so. You may be ordered
to evacuate wherever you happen to be or, in some cases, evacuation may
simply be recommended. Either way, get out. In recent eruptions, many
people have been killed because they did not heed an evacuation order.
If you are lucky enough to get advance warning, use it wisely.
Conversely, if you are not instructed to evacuate the area, stay where
you are unless you can see immediate danger. Taking to the roads may be
more hazardous than staying at home.
- Get to high ground. Lava flows, lahars, mudflows, and
flooding are common in a major eruption. All of these can be deadly,
and all of them tend to travel in valleys and low-lying areas. Climb to
higher ground, and stay there until you can confirm that the danger has
passed.
-
Protect yourself from pyroclastics. While you want to get to
higher ground, you should also try to shield yourself from pyroclastics
which are rocks and debris (sometimes red-hot) that are sent flying
during an eruption. The most important thing to do is watch out for
them and get out of their range. Sometimes they actually rain down, and
in some types of eruptions, such as that which occurred at Mount St.
Helens in 1980, these missiles can land miles from the volcano’s
crater. Protect yourself by staying below the ridgelines of hills and
on the side of the hill opposite the volcano. If you are caught in a
hail of smaller pyroclastics, crouch down on the ground, facing away
from the volcano, and protect your head with your arms, a backpack, or
anything else you can find. - Avoid breathing poisonous gases. Volcanoes emit a number of
deadly gases, and if you are close to one when it erupts, these gases
could kill you in less than a minute. Breathe through a respirator,
mask, or moist piece of cloth—this will also protect your lungs from
clouds of ash—and try to get away from the volcano as quickly as
possible. Do not stay low to the ground, as some of the most dangerous
gases are heavier than air and accumulate near the ground.
- Get and stay inside. Unless you need to evacuate, the safest
place you can be is inside a strong structure. Close all the windows
and doors to protect yourself from ash and burning cinders.
- Receive medical treatment promptly for burns, injuries, and gas/ash inhalation.
Once you are safe, waste no time to get treatment or an examination.
Keep in mind, however, that you may need to wait a while if there are
people with more serious injuries.
Tips
- Respect the power of a volcano. Some blasts can absolutely devastate an area many miles wide within hours or minutes.
- Driving through heavy ash is dangerous. Visibility is impaired, the
roads may become slippery, and the radiator may become clogged. Keep
your headlights on, proceed slowly, and watch your car for overheating.
- You can almost never outrun a lava flow or lahar, but you may be
able to dodge it by getting out of its way, especially by climbing to
higher ground.
- If you live near a volcano, don’t wait for an eruption to prep
 are
your emergency supplies and contingency plans. You should make a plan
as a family, so that everyone knows what to do and where to meet up. - Before traveling to a volcano, learn as much as you can about what
you’ll encounter there, even if you will have an experienced guide. Not
only will you gain a better understanding of these fascinating
phenomena, you will also be better prepared to avoid danger and respond
appropriately if an eruption occurs.
Warnings
- Watch out for signs of fire if you are indoors. A red-hot pyroclastic can ignite a roof fairly quickly.
- Beware the danger of roof collapse if heavy ash accumulates. Clear
the roof of ash periodically, as several feet of ash can fall in a few
hours.
- Never try to cross a lava flow or lahar. Even flows that appear to
be cooled may simply have formed a thin crust over a core of extremely
hot lava. If you do cross a lava flow, you run the risk of being
trapped between flows if another suddenly develops.
- Don’t try to cross geothermal areas. Hot spots, geysers, and
mudpots are common on volcanoes. The ground around these is typically
very thin, and a fall through could result in serious burns or death.
Never try to cross these during an eruption, and otherwise cross them
only on safe, marked paths.
- Mudflows and flooding following an eruption generally kill far more
people than pyroclastics or lava. You can be in danger even many miles
from the volcano.
- A pyroclastic flow/surge can travel over 300 miles per hour.Dangers
Rainbow and volcanic ash with sulfur dioxide emissions from Halema`uma`u vent.
The most devastating effect of volcanic ash comes from pyroclastic flows. These occur when a volcanic eruption creates an "avalanche" of hot ash, gases, and rocks that flow at high speed down the flanks of the volcano. These flows can be impossible to outrun.[12] As well as being impossible to outrun, they are almost as difficult to predict. In many cases prediction has been based on the topography of a region, only to see a valley fill and overflow[13]. In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.
Volcanic ash (by itself) is not poisonous, but inhaling it may cause problems for people whose respiratory system is already compromised by disorders such as asthma or emphysema. The abrasive texture can cause irritation and scratching of the surface of the eyes. People who wear contact lenses should wear glasses during an ashfall, to prevent eye damage. Furthermore, the combination of volcanic ash with moisture in the lungs can create a substance akin to liquid cement.
Therefore, people should take caution to filter the air they breathe with a damp cloth or a face mask when facing an ashfall. Ash is very dense, as only 100 millimetres (3.9 in) of ash leads to the collapse of weaker roofs. A fall of 300 millimetres (12 in) leads to the death of most vegetation, livestock, the wiping out of aquatic life in nearby lakes and rivers, and unusable roads.Accompanied by rain and lightning, ashfall leads to power outages, prevents communication, and disorients people.
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