A volcano is a mountain where lava (hot, liquid rock) comes from a magma chamber under the ground.
Most volcanoes have a volcanic crater at the top. When a volcano is active, materials come out of it. The materials include lava, steam, gaseous sulfur compounds, ash, and broken rock pieces.
When there is enough pressure, the volcano erupts. Some volcanic eruptions blow off the top of the volcano. The magma comes out, sometimes quickly and sometimes slowly. Some eruptions come out at a side instead of the top.
Volcanoes are found on planets other than Earth. An example is Olympus Mons on Mars.
Volcanologists are scientists who study volcanoes using methods from geology, chemistry, geography, mineralogy, physics, and sociology.
How volcanoes are formed
There are two main processes.
Volcanoes are made when two tectonic plates come together. When these two plates meet, one of them (usually the oceanic plate) goes under the continental plate. This is the process of subduction. Afterwards, it melts and makes magma (inside the magma chamber), and the pressure builds up until the magma bursts through the Earth’s crust.
The second way is when a tectonic plate moves over a hot spot in the Earth’s crust. The hot spot works its way through the crust until it breaks through. The caldera of Yellowstone Park was formed in that way, as were the Hawaiian Islands.
An active volcano is currently erupting, or it has erupted. An example of an active volcano is Mount St. Helens in the United States (US). An active volcano is either erupting or “sleeping.”
A dormant volcano is “sleeping,” but it could awaken in the future. Mount Rainier in the United States is considered dormant.
An extinct volcano is not active. It is not expected to erupt ever again. Edinburgh Castle in Scotland is located on top of an extinct volcano.
Edited from source