Why do some volcanic eruptions produce ash clouds that rise into the stratosphere?

Explosive volcanoes release magma rich in dissolved gases, and when pressure drops, those gases expand violently. The expanding gas fragments the magma into tiny ash particles that can be thrown many kilometers into the sky. Large eruptions such as Mount Pinatubo in 1991 injected material up to about 35 km high, reaching the stratosphere. Because the stratosphere is relatively stable and dry, ash and sulfur aerosols can travel far and remain aloft much longer than in the lower atmosphere.