Pyroclastic flows are nothing like the slow predictable movement of lava. Molten rock creeps along the surface but very rarely takes lives as it is easily outrun. Lava flows have a devastating effect on houses and other structures as it can destroy these very easily.
Heavy ash fall spat out by the volcano can make breathing hard and flatten houses. The ash is especially more dangerous if wet. Volcanic ash is material thrown out of a volcano as particles, and less than 2mm across. Ash can damage crops or force an area to be abandoned.
Earthquakes can be associated with volcanic activity but they are usually small and shallow. Volcanic earthquakes are almost always due to the movement of magma. When a volcano starts to show some unrest the earthquakes tend to be deep. As the magma batch moves to the surface the earthquakes likewise migrate upward. Most of these earthquakes are probably too small to make it into the news other than in the local area. The complication, of course, is that most of the world's volcanoes occur in areas that are very tectonically active (subduction zones).
Mud flows travel very quickly and cannot be out-run. They can be triggered by seismic activity prior to an eruption or the eruption itself. Because they carry so much sediment and debris they leave thick deposits of mud, sand, boulders, and other debris behind, often many meters thick.
A volcanic eruption can effect the atmosphere for years if it is powerful enough. Sulphur gasses erupted into the stratosphere chemically change into sulphuric acid droplets too small to see with the naked eye. “[Sulpheric acid is] Formed in the upper atmosphere when the sulphurus gasses emited by a volcano are converted by a chemical process in the upper atmosphere to sulphuric acid which then condenses into a fine mist” (SITE).
Following the erruptio of Mount Pinatubo in 1991 their was a world wide cooling effect. Solar energy reaching the earths surface was reduced 5-10%. Spectacluar sunsets were witnessed world wide for years to some. This is because the particles that the volcano shoots into the stratosphere have a light scattering effect. This effect can vary depending on the size of the particles and how long they remain in the atmosphere.
When these gasses are realised into the troposphere, such as in non explosive eruptions, the gasses have a resinence of about seven days, but when realised into the stratosphere they can remain their for years. (SITE)
Mud slides
Ash clouds
Fountains of fire
Flows of molten rock
Flood basalts
The most important part of prepairing for a volcanic eruption is to predict when the next eruption will occur and how powerfulthe eruption will be. Prediction of a volcanic eruption is not a simple task. As magma rises up into and through the volcanic cone, there are warning signs. Small earthquakes occur, water is heated giving off more steam and there is greater release of the gases commonly found in magma such as hydrogen sulphite. It takes several of these changes to take place before volcanists can be sure of volcanic eruption. Volcanisits all over the world closely monitor volcanoes for any sign of activity so that an evacuation plan can be arranged.
Rising magma before an eruption causes temperatures of rocks and gases to increase. These temperatures are closely watched. Satellites can be used to monitor heat changes but this is expensive and has a degree of inaccuracy. In many countries, volcanists observing the volcano will take temperature readings from the steam vents. This is far more accurate than a satellite and not as expensive.
When magma starts to rise towards the surface, the gasses the volcano realises change. Volcanoes have been known to realise more sulpher dioxide as the magma rises in its chamber.
The conductivity of the earth is also very helpful in measuring the magma chamber. Electrical currents can be sent through the volcano to a receiving station. The station can tell how long it took the electrical current to reach the station and what electrical resistance it encountered. Soundings similar to this have been used on mount Vesuvious have revealed that there is a chamber of magma 5 miles below the crater, and that a large amount of rock debris is blocking the funnel (SITE). This research has helped those monitoring the volcanoe to know that the next eruption of Vesuvious will be explosive. The blockage on the funnel means that the volcano is collecting gasses and pressure so when it does explode it will be catastrophic.
Education is a key to surviving a volcanic eruption. People need to be aware that they are living in a volcano danger area and what they should do if an eruption should occur. In the surrounding areas of Vesuvious aproxametly 750,000 people are currently living on the remains of dry lava from the eruption in 1944. when the volcano erupts again, the chances of a high death tole are high.