A volcano is a place on the Earth's surface (or any other planet's or moon's surface) where molten rock, gases and pyroclastic debris erupt through the earth's crust. Volcanoes vary quite a bit in their structure - some are cracks in the earth's crust where lava erupts, and some are domes, shields, or mountain-like structures with a crater at the summit.
Magma is molten rock in the Earth's crust. When magma erupts through the earth's surface it is called lava.
The largest volcano on Earth is Hawaii's Mauna Loa. Mauna Loa is about 6 miles (10 km) tall from the sea floor to its summit (it rises about 4 km above sea level). It also has the greatest volume of any volcano, 10,200 cubic miles (42,500 cubic kilometers).
The largest volcano in our Solar System is perhaps Olympus Mons on the planet Mars. This enormous volcano is 17 miles (27 km) tall and over 320 miles (520 km) across.
