Magma is composed of molten rock and is stored in the Earth’s crust. Lava is magma that reaches the surface of our planet through a volcano vent. Lava flow, Hawai. Courtesy of the U.S. Geological Society.agma is composed of molten rock and is stored in the Earth’s crust. Lava is magma that reaches the surface of our planet through a volcano vent. Lava flow, Hawai. Courtesy of the U.S. Geological Society.