How are Volcanoes Formed
Volcanoes form where magma escapes through a vent, which is a crack in the earth's surface. This often happens at plate boundaries such as at destructive and constructive plate boundaries. They are also formed at Hot spots such as those found in Hawaii.

The best description to use when answering this type of question is the one concerning the destructive plate boundary as outlined below.

As two plates move towards each other, the heavier oceanic plate is forced under the lighter continental plate at the subduction zone. Here the oceanic plate melts due to the friction/pressure and forms lava which is significantly different to the mantle. This rises through cracks in the rocks where it eventually erupts at the surface, to form a composite volcano

Shield Volcanoes

• Gentle slopes and a wide base

• Frequent eruptions of basic lava

• Lava flows at high speeds and for long distances before cooling

• Usually non-violent eruptions

Examples - Mauna Loa & Kilauea

Composite Volcanoes

• Steep sided with a distinctive cone shape

• High with a narrow base

• Explosive eruptions of lava & ash

• Layers of alternating ash & lava

• Secondary or parasitic cones formed when the main vent gets blocked

Examples - Montserrat, Etna & Mt St Helens

• Pyroclastic Flows - A cloud of red hot gas and ash which flows down the side of a volcano at 200km/hr

• Lahars - A mixture of ash with rain or glacier melt water which forms a deadly river of mud

• Lava Flow  - A river of molten rock 1000 degrees centigrade that can travel at 40mph

• Ash Fall - Millions of tons of ash can bury buildings, roads etc.

• Volcanic bombs - Rocks and lava the size of houses and cars can be hurled into the sky