Precautionary measures that can be taken before and after a volcanic eruption

The most effective precautionary measure is to move far away from any active or dormant volcano, barring that, other precautionary measures for before a volcanic eruption is to constantly monitor the volcano for activity and to have an evacuation plan in place in case the volcano does erupt.
During an eruption it all depends on what type of eruption it is. If it is a cinder cone, just staying away is good enough. If it is a lava floe, you can just walk away, most lava moves pretty slowly, do not ever walk on the lava, it may look solid but often has molten lava running beneath it. Stay away from where lava meets water, there can be explosions as the lava cools quickly. If the Eruption is a pyroclastic flow, the only thing you can do is pray that the pyroclastic flow runs out of steam before it reaches you. Pyroclastic flows can move at several hundreds mph and at temperatures of hundreds of degrees Fareignhiet.
Also in any type of eruption stay away from any streams that flow from the volcano, you are at risk from a type of flood called a lahar. Lahars are fast flowing rapid overflowing floods filled with all sorts of debris, and if the volcano has a glacier on it, the entire glacier can melt adding to the floodwaters.

Afterwards it is best to stay away from the area affected by the volcano. Poisonous gases can rise out of the ash and any lava may not be completely solidified, you could fall through. Sometimes there are pockets of gas buried in the volcanic ash, so don't dig through it, the gas will usually work its way out, but that could take a long time.

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Advantages/Disadvantages of living next to a volcano

Disadvantages
Sea - during a volcanic eruption earthquakes happen, and tsunamis may be created. Also, if it is a volcanic island, the island may be destroyed, and there is no escape.
Lava flow - these are very slow moving, but destructive as they cannot be stopped and they set fire to everything in their path.
Pyroclastic flow - these are impossible to outrun, travelling at about 300 km/h, and are extremely destructive
Mudflow/lahar - these are mud rivers that have the consistency of cement, and destroy everything in their path, including buildings
Advantages
Sea - there tends to be abundant sea life near volcanic islands, so there will be lots of seafood
Fertile soils - The volcanic ash from eruptions is very fertile and good for farming
Tourism - Volcanos are good for tourism, bringing towns lots of money
Quarry - There are lots of stone deposits from volcanic eruptions, which can be converted into quarries.
Geothermal energy - There will be natural vents near the volcano, in the ground which can be used to provide geothermal energy.

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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.

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Formation of Volcanoes

Volcanoes form when hot material from below rises and leaks into the crust. This hot material, called magma, comes either from a melt of subducted crustal material, and which is light and buoyant after melting, or it may come from deeper in the interior of a planet and is light and buoyant because it is very hot.

Magma, rising from lower reaches, gathers in a reservoir, in a weak portion of the overlying rock called the magma chamber. Eventually, but not always, the magma erupts onto the surface. Strong earthquakes accompany rising magma, and the volcanic cone may swell in appearance, just before an eruption, as illustrated in this picture. White arrows in the picture show the volcano getting bigger as magma rises inside. Scientist often monitor the changing shape of a volcano, especially prior to an eruption. The different reasons why a volcano forms are via plumes or hot spots in the lithosphere or as a result of subduction of the nearby lithosphere 

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