Geography

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.

The advantages seem to outweigh the disadvantages, because wherever there is a volcano, people live around it because of the rich fertile soil, despite the threat of an eruption. Examples of this are Naples (Mount Vesuvius), and Fuji City (Mount Fuji)

Volcanoes dominate the landscape in Nicaragua and Costa Rica – big, massive volcanoes rising up out of nowhere. Occasionally, steam or glowing red lava seeps out of tops. How do volcanoes form, anyway?
Most volcanoes are formed by the movement of tectonic plates on the surface of the earth. These plates are basically huge pieces of rock that ‘float’ on the mantle (a layer of the earth that is sort-of liquid rock). The tectonic plates are in constant motion, albeit very slow motion. They sometimes move toward each other, other times they’ll move apart, and still other times one will sink while the other rises above it.
PLATE TECTONICSr!
When a tectonic plate sinks, it sinks down into the mantle and becomes very hot. So hot, in fact, that the rock melts. This molten rock will gradually make its way up to the surface of the earth through a series of cracks. When it reaches the surface of the earth, we refer to it as lava. As layer upon layer of lava builds up, a volcano is formed.

There are many factors that determine what kind of lava flow will occur and what type of volcano it will be. The amount of gas trapped in the lava, the kinds of minerals making up the lava, and how much pressure can be trapped in the area all affect the eruption and formation of the volcano. You can read about the types of lava and lava flows.

http://en.wikipedia.org/wiki/File:Volcano_scheme.svg
Cross-section through a stratovolcano (vertical scale is exaggerated):
1. Large magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
What are the three layers the Earth is made of?

Crust
The crust is the outer layer of Earth. It is about 18 miles thick. It is the part we live on.
Mantle
The second layer is called the mantle. It is about 1,800 miles thick.
Core
The inner layer is called the core. What causes volcanoes to erupt?

The Earth's crust is made up of huge slabs called plates, which fit together like a jigsaw puzzle. These plates sometimes move.

Between the Earth's crust and the mantle is a substance called magma which is made of rock and gases.

When two plates collide, one section slides on top of the other, the one beneath is pushed down. Magma is squeezed up between two plates.

Did you know?

Volcanoes are like giant safety valves that release the pressure that builds up inside the Earth.

12. Lava flow
13. Vent
14. Crater
15. Ash cloud
A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant or inactive, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular
Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Historically, so-called volcanic winters have caused catastrophic famines.

When it reaches the surface and comes out of a volcano, magma becomes lava.

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Volcanic Eruptions
As well as the danger from the hot lava, an erupting volcano can trigger are life threatening things.

tsunamis flash floods earthquakes mud flows rock falls.

Effect of Volcanoes on people and the environment

Volcanoes can have a very serious effect on the lands and people around them when they erupt.

Buildings are destroyed and people are made homeless.

People are killed.

Clouds of ash cover plants making them inedible.

Poisonous gases kill people and animals.

Dust causes pneumonia and illnesses to the survivors.

Dark skies, severe winds and heavy rains may follow an eruption for months afterwards.

Active - eruptions can be anytime and often.
Dormant - has been a while since it has erupted, but could at anytime.
Extinct, meaning it hasn't erupted in a very long, long time so it probably won't ever again.