The photo above is of Mt. St. Helens today. This once beautiful mountain was changed dramatically on May 18, 1980. The eruption that occurred was a Plinian eruption, which is the most violent eruption classification.

As you learned in the last lesson, different magmas have varying amounts of silica and gas that cause the lava to either be thick and pasty or thin and runny. The thickness and thinness of the magma will determine how a volcano will erupt and what kind of a cone will form.
Volcanoes will erupt for two reasons

The magma deep under the crust is less dense than the surrounding rock causing it to rise.
As the magma approaches the surface of the Earth the gas that is in the magma will come bubbling out because the pressure surrounding the magma will decrease nearer the surface.

Have you ever had a can of soda pop explode all over the room? This "eruption" of pop is caused by the same scientific principle that causes a volcano to erupt violently. When you open the pop can the pressure is released so quickly that the gas that is dissolved in the pop comes rushing out along with some of the pop.

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