Magma is molten (igneous) rock formed when Earth's radioactivity heats rocks to high temperatures – the deeper in the Earth, the greater the temperature.
When melted rock flows at the surface it is called lava – Hawaii's and Iceland's basalt lavas fountain (below center) or flow (above right) freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (nuee ardentes, such as that which killed victims at Pompeii) and towering clouds (below right, below left; pyroclastic flow, Mount St. Helens, 2, Vesuvius, Pinatubo, Soufrière Hills Volcano on Montserrat, Ischia deposits). .
[links: images: webpages: Vesuvius victims 'died instantly' ]
Video The Ring of Fire
images : USGS : top right, schematic of stratovolcano; top left, basaltic lava; bottom right, Plinian eruption of Mt. Klyuchsvskaya in Russia; lava fountain and lava flow, Kilauea, Hawaii; pyroclastic cloud above erupting andesitic stratovolcano.