How do volcanoes create ash clouds?

When magma rises, pressure drops and dissolved gases expand rapidly. If the magma is viscous, the gases cannot escape easily, so pressure builds until the eruption becomes explosive. The magma tears apart into fine glassy fragments and crystals, which form an ash cloud that can drift hundreds or thousands of kilometers. Ash is not soft fireplace ash; it is abrasive volcanic rock dust that can damage engines, crops, and lungs.