Volcanos are openings in the Earth's crust.  When the crust is open it lets the molten (melted) rock from the mantle (center of the earth) come out. When the molten rock cools it is called Lava. 

Volcanos can form underwater or on land.  

Lots of volcanos are not active.

 http://kids.discovery.com/games/build-play/volcano-explorer  

(http://p2.trrsf.com/image/fget/cf/619/464/images.terra.com/2013/04/28/centro-da-terra-temperatura-pesquisa-grafico-bbc.jpg) 

http://www.kidscosmos.org/img/tour/kids/volcano_diagram.gif 

http://www.sciencekids.co.nz/sciencefacts/earth/volcano.html 

http://www.enchantedlearning.com/subjects/volcano/ 

http://www.weatherwizkids.com/weather-volcano.htm 
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