A volcano is created where hot molten rock, volcanic ash and gasses escape from an opening at the Earth’s crust. Through their power they cause destruction along with providing ecological benefits that are invaluable to the area around them.
Volcanoes are classified as active or dormant based on the activity of the volcano. A volcano is considered active if it has erupted at least once in the past 10,000 years. If the active volcano is currently having an eruption it is an erupting volcano, otherwise it is considered to be dormant. If a volcano has not erupted at least 10,000 years with no sign of erupting in the foreseeable future, it is then considered to be extinct.
Home Science Tools: An introduction to volcanoes
Oracle ThinkQuest’s Resource for Volcanoes: A volcanic project by students for students

How Volcanoes are Created
There are two ways in which volcanoes are created: they are either formed due to plate movements or over hotspots on the Earth’s surface. Convergent and Divergent boundaries are the two kinds of plate regions which cause volcanoes to develop.
In the case of convergent boundaries, a subduction zone is created where one plate is forced to move beneath the other due to the force placed on it. When the subducted plate is pushed downwards it reaches a point in the Earth’s mantle where the rock is partially melted. This melted rock or magma rises to form a chain of volcanoes, usually in the form of an arc.
With divergent boundaries, rather than moving towards each other the two plates move in opposite directions. The gap that is created is often filled with molten lava resulting in volcanic islands. Islands which are created through volcanic activity are constantly growing over long periods of time.

Volcanoes created by hotspots can be far away from the plate boundaries. These are created due to unusual spots of hot magma at the Earth’s lithosphere. The extreme heat of the magma causes the lithosphere to melt at these spots which leads to volcanic activity. If the hotspot occurs on the ocean floor the volcano may rise high enough to create volcanic islands such as Java and Hawaii. Hotspots can also occur beneath continental plates which cause layer upon layer of accumulated lava known as flood basalts.

A Peek into Our Planets Plumbing: How volcanoes work

Facts and Figures

  • There are approximately 1500 active volcanoes in the world today
  • At any given time, an average of 30 to 50 volcanoes are erupting simultaneously on land
  • Hawaii is a volcanic island created by a hotspot located 3,000 kilometers below the Earth
  • The largest volcano on Earth is Mauna Loa in Hawaii. It is a shield volcano located on Big Island and measures 17,000 metres from base to top.
  • The top two most active volcanoes are both located in Italy. Etna has had continuous eruptions for 3,500 years and Stromboli has had continuous eruptions for 2000 years.

Mount St. Helens: Facts and figures of Mount St. Helen in the USA
Extreme Science: All about volcanic islands

The Science Behind Them
There are four main components of a volcano. The first is the vent which is located at the bottom of the volcano and is the opening at the Earth’s surface. The second component, the pipe is the passage way through which the magma travels from the vent to the surface of the volcano. The third part of the volcano is the crater which is a round shaped hollow at the top of the volcano structure. The last part is the cone which is created by layers of lava, ashes and cinder.

When people imagine a volcano they usually picture a mountainous structure with a hollow at the center. This is actually only one type of volcano. Geologists classify volcanoes based on the materials they are made of and the way that the volcano erupts.
Composite Volcanoes: These volcanoes are created by alternating layers of rock fragments and lava. They are tall with a conical structure and have intermittent, explosive eruptions.

Shield Volcanoes: Unlike composite volcanoes, shield volcanoes have a low lying, wide spreading cone.  The lava that erupts from shield volcanoes is not as viscous as others and flows freely and far from the crater.
Cinder Cones: Cinder cones are very common volcanoes and have a steep, conical structure. The rock fragments which create the cinder cones contain gas bubbles created by fast cooling of the magma when it contacts air.
Spatter Cones: Volcanoes formed by spatter cones have steep sides. This is due to lava which contains just enough gas to prevent it from flowing freely but does not cause it to form into small fragments. The lava which is discharged from spatter cones comes out as hot blobs.

Complex Volcanoes: As the name suggests, complex volcanoes have features of multiple types of volcanoes. They occur due to the change in characteristics of explosions or from the presence of multiple vents in the area of the volcano.
Volcano World: What volcanoes are made of and how they grow
Window to The Universe: Different types of volcanoes
Types of Volcanoes: Descriptions of different types of volcanoes with images
Adventure in Science: The different types of volcanoes

Their Role in Nature
Though the lava from volcanoes leaves a long trail of destruction behind it the role of volcanoes in nature can simply not be downplayed or replaced. Lava initially destroys everything in its path, however the volcanic ash overtime drastically increases the fertility of soil. This is why people regularly live next to active volcanoes. With enough water, plant life can grow back in a lava struck zone within one year. Surprisingly one of the gasses released by volcanoes is hydrogen, which when mixed with the oxygen in air creates water vapour providing support to the water cycle.
A Look at How Volcanoes Effect Society: A look at the role volcanoes play in the lives of people
The Geography Site: Why do people live near volcanoes?

How to Make a Volcano: Guidelines for making a model volcano.
Steps for Making a Soda Volcano: How to make an exploding model volcano.
How to Build a Soda Volcano: Easy steps for making a soda volcano.
A Booklet on Volcanoes by the US Geological Survey: A comprehensive booklet on volcanoes
Information on Volcanoes by the Environment Literacy Council: A description of volcanoes including their hazards and a list of resources
Volcanoes: Can we predict volcanic eruptions?
Volcanoes: Well known volcanoes in the world
Nature’s Fury:(PDF Document) Earthquakes, Volcanoes, and Tsunamis