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Volcanic eruptions provide routes for material deep beneath Earth's surface to escape to the surface. They also allow a world to its heat. Active volcanoes on Earth and Venus are fed by subsurface molten rock. On Earth, the lava comes up from the mantle (which is the layer under the surface). Once there's enough molten rock — called magma — and enough pressure on it, a volcanic eruption . In many volcanoes, the magma rises up through a central tube or "throat," and out the top of the mountain. Volcanic activity can be fairly quiet, or it can be quite . In a very active flow, clouds of gas may come rolling out of the volcanic caldera. These are quite deadly because they're hot and moving fast, and the heat and gas and kill someone very quickly.