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Pressure Buildup
The eruption process starts with pressure building up within a magma chamber below Earth's surface. Magma contains several dissolved gases. When the pressure on the magma is greater than the vapor pressure of the gas, it remains dissolved. If this pressure is disrupted, it can cause the gases to form small gas bubbles, known as vesicles, within the magma. The gas bubbles push to the surface because they are less dense than the magma.
Eruption
Some volcanoes erupt violently, causing massive destruction within minutes, whereas the magma of other volcanoes seeps out so slowly there is no harm done. Once magma leaves a volcano, it is known as lava. How a volcano erupts depends on the composition of the magma. Magma's thickness, or viscosity, and its gas content are the main factors in this determination. More viscous magma or magma with high gaseous content tends to erupt more violently than those with low viscosity and gas content.
Tephra
During explosive eruptions, magma blasts into the air, where it breaks into pieces known as tephra, which can range in size from minuscule fragments to boulders as large as houses. The hot tephra that is shot out of a volcano races down the sides of the volcano and can destroy everything in its path. Particles of tephra smaller than two millimeters are called ash. Ash blankets the ground like snow although it doesn't melt. Thick blankets of ash can suffocate plants, animals and people.
Mudflow
When ash combines with water from melting snow or ice, or streams, mudflows are formed. Mudflows can be hot or cold and can be just as destructive as the initial eruption. They can move as fast as 60 miles per hour and reach distances of about 190 miles away. It is the speed and distance of these mudflows that causes so much destruction.