Which process could lead directly to the formation of pumice rock

Pumice rocks are formed due to the air that is trapped in between the rocks, a body of magma spurting out with dissolved gas under pressure. As the magma cracks through the Earth’s surface, the sudden pressure drop causes the gas to come out of the solution. The flash of gas form the vent slices the magma and blows it out as a molten lather. The lather hastily solidifies as it flies into the air and falls back to Earth as pieces of pumice.