Geophysical research suggests that volcanic magma forms near the base of the Earth's crust and moves upward to a shallow magma chamber before erupting at the surface. Magmas rise because they are less dense than the rocks at lower depths, and their heat probably weakens surrounding rocks. The upward movement of magma may also be due to expanding gases within the molten rock or to chemical reactions that dissolve rocks above the magma. Volcanic material moves toward the surface through channelways, or volcanic conduits, and is extruded through vents at the Earth's surface. Eruptions take different forms depending on the composition of the magma when it reaches the surface.
Consolidated ash is called tuff.
These eruptions cover large areas and often produce ropy, or pahoehoe, lava flows. Thicker basalt lava breaks into chunks or blocks, forming blocky lava flows, called aa. Lava is the fluid phase of volcanic activity. Whether lava or pyroclastics are being ejected, the eruption is normally accompanied by the expulsion of water and gases, many of which are poisonous. Lava usually forms long, narrow rivers of molten rock that flow down the slopes of a volcano.