general statement vulcano eruption

The term Vulcanian was first used by Giuseppe Mercalli, witnessing the 1888-1890 eruptions on the island of Vulcano. His description of the eruption style is now used all over the world for eruptions characterised by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. Mercalli described vulcanian eruptions as "...Explosions like cannon fire at irregular intervals..." Their explosive nature is due to increased silica content of the magma. Almost all types of magma can be involved, but magma with about 55% or more silica (basalt–andesite) is most common. Increasing silica levels increase the viscosity of the magma which means increased explosiveness. They usually commence with phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground. This is usually followed by the explosive clearing of the vent and the eruption column is dirty grey to black as old weathered rocks are blasted out of the vent. As the vent clears, further ash clouds become grey-white and creamy in colour, with convolutions of the ash similar to those of plinian eruptions.

Structure Of a Volcano

 Structure of a Volcano.

A volcano constitutes a vent, a pipe, a crater, and a cone.

The vent is an opening at the Earth's surface.

The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption.

The crater is a bowl-shaped depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released.

Solidified lava, ashes, and cinder form the cone. Layers of lava, alternate with layers of ash to build the steep sided cone higher and higher

Types Of Volcano

Volcanologists have classified volcanoes into groups based on the shape of the volcano, the materials they are built of, and the way the volcano erupts.

The groups are:

• Composite Volcanoes (also called strato volcanoes),
• Shield Volcanoes (also called shields),
• Cinder Cones,
• Spatter Cones, and
• Complex Volcanoes (also called compound volcanoes).

Volcanic Eruption

(Fig. 2.8) Eruption.

An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcano's vents.  When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.  The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.

1. low water, low silica - runny lava flows (not viscous)

(Fig. 2.9)

(Fig. 2.10)

2. low water, high silica (very viscous) - pasty lava - often building domes

3. high water, low silica(not viscous) - fountain of runny lava

(Fig. 2.11)

(Fig. 2.12)

4. high water, high silica (very viscous) - explosion

Obstacles also influence the type of eruption.  When the pipe is blocked by a stopple or an accumulation of pumice, the pressure in the pipe will build up very high resulting in an explosion.

When magma reaches earth's surface it is called lava.  It may pour out in gentle streams called lava flows or erupt violently into the air.  Rocks ripped loose from the inside of the volcano or torn apart by the gas may be shot into the air with the lava.  These rocks blown out of a volcano are called pyroclastic rocks.  The rock fragments fall back to earth in many different shapes and sizes:

  Dust - particles less than 1/100 inch in diameter
Dust particles may be carried great distances. In a powerful eruption they may be carried around the earth several times.

  Ash - fragments less than 1/5 inch in diameter
Most volcanic ash falls to the surface and cemented together by water to form a rock called volcanic tuff.

  Bomb - A rounded piece of newly hardened lava which takes shape while flying through the air.

  Block - A piece of lava that has sharp corners.

  Cinder - Bubbly rock formed by liquid lava cooling in the air.

  Pumice - Cinder so bubbly that it floats in water.

Volcanic activity is classified by how often a volcano erupts.  A volcano may be active, intermittent, dormant, or extinct.  Active volcanoes erupt constantly.  Intermittent volcanoes erupt fairly regularly.  Dormant volcanoes are inactive, but not long enough to determine whether they will erupt again or

How To Survive Volcano 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.
Steps

 

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 kinds of eruptions.
• Stock up on necessities. Store at least a three-day supply of food and portable 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 lahar (or mud 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.
1.  

1Listen 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

2

.  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.

3.  

3

 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.

4.  

4

 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.

5.  

 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.

6.  

 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.

                  8

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