A volcano is a vent or chimney which transfers molten rock known as magma from depth to the Earth's surface.Magma erupting from a volcano is called lava and is the material which builds up the cone surrounding the vent. A volcano is active if it is erupting lava, releasing gas or generates seismic activity.hield volcano. When magma is very hot and runny, gases can escape and eruptions are gentle with considerable amounts of magma reaching the surface to form lava flows. Shield volcanoes have a broad, flattened dome-like shape created by layers of runny lava flowing over its surface and cooling.