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…showed last 75 words of 308 total…are going to have a more difficult time flowing. This can block a vent in the volcano, which causes a buildup of gases. At high temperatures and low surface pressures, the gases will expand and build up pressure against the ground. The pressure of the buildup of gases is what causes the great explosions. It is the viscosity of the magma, combined with the quantity of dissolved gases, which determines the nature of volcanic eruptions.