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Our transcription: Why is the eruption of a composite volcano so violent and explosive? The answer lies in the andesitic lavas of which these volcanoes are composed. Andesite contains more silica than basalt does, so it crystallizes at a lower temperature creating a more viscous lava. Crystals will often form in an andesite magma before an eruption thickening it even further. High viscosity traps the gases dissolved within the magma and prevents them from escaping easily. During the initial stages of the eruption, gas bubbles cannot break through the thick lava until pressures within them reach explosive proportions. We know what pent up gas pressures can do. The last of the eruption sprays fine droplets of lava into the air forming clouds of volcanic ash, and the escaping gas whips some of the lava into a foam called "pumice".
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