and dirt into nearby Spirit Lake and then downward to the valley of the North Fork of Toutle River. The ice and snow caps that melted caused mudflows. The mud traveled down the same path. It was incredibly destructive. The mudflows tore down houses, steel bridges and blocked the Columbia River with its debris. The next day showed a very different MSH. The mountain had lost more that three quarter of a cubic mile of rock and was now 1,200 feet shorter. What used to be a lush green slope was now a gray wasteland that looked like the surface of the moon. Mount St. Helens was built by many eruptions over thousands of years. With each eruption, hot rock from inside the earth forced its way to the surface. This type of rock is called magma. Once the magma reaches the surface of the earth it is called lava. With some eruptions, the magma was liquid so the lava flowed out of the volcano and hardened. With others, the magma was thick so it burst violently with sprays of molten rock. It rained down as tiny bits of rock, (ash) and as rocks puffed up by gases (pumice). The two types of lava have Hawaiian names. Aa is a sharp stone that cools down to a surface that is hard to walk on. Aa occurs from high lava fountains. The lava chunks cool in the air and cannot form into flows when they land. Pahoehoe is a much smoother stone and dries into tubes that are sometimes hollow. Pahoehoe happens when eruptions are at high temperatures and low viscosity. The low viscosity lets the lava flow easily and a skin is formed on top. Lava that hardens is called pumice. The speed of both types of flows are hard to tell apart, but Aa is faster than Pahoehoe. A Pahoehoe flow moves around one yard a minute but with a slope it can move up to 400 yards per minute or 14 miles per hour. Aa flows are usually 61/2 to 16 feet thick, and Pahoehoe flows are about one foot thick. The width of both types of flows is usually around 100 yards wide. Because lava moves so slowly ...
