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How did Mount St. Helens go from this to this? Many people know that Mount St. Helens was volcano and it exploded in 1980. However, seconds before the explosion, Mount St. Helens had massive landslide in which almost half the mountain slid downward due to gravity. Welcome to MooMooMath and Science, an erosion caused by gravity. Although you may not see gravity, it is force of erosion. Gravity not only influences the movement of water, it also causes rocks and soil to move downward. This downward movement caused by gravity can be called mass movement and plays major role in shaping the Earth. Sand, rocks, and soil will move downhill until the slope becomes stable. The angle of repose is the steepest angle at which loose material will not slide downward. The angle of repose may change based on the size, weight, and shape, and also the moisture level of the objects, which may cause rockfalls and landslides. The most destructive mass movement happens suddenly and rapidly. rockfall happens when loose rock falls down steep slope. This steep slope may be man-made when creating road or created by nature. Rocks that range in size from small fragments to large boulders fall down the slope due to gravity. landslide is sudden and rapid movement of large amount of material downward. slump is the most common landslide and happens when block of material moves downward over curved surface. Heavy rains, deforestation, and earthquakes increase the chances of landslide. An earthquake triggered the Mount St. Helens landslide. rapid movement of large mass of mud is mudflow. When heavy rain mixes with soil and rock, slippery mass of mud may be created that can flow slowly rapidly downhill. Mudflows are also result of volcano and they're called lahars. Volcanic eruptions or heavy rains on volcanic ash may cause lahar that can travel up to 80 km an hour. If volcano occurs on snowy peak, the snow melts quickly and creates dangerous hot mudflow. Sometimes the mass movement is very slow. This is called creep. Water loosens the soil on steep slope. Plant roots and animals loosen the soil, which slowly moves downhill or creeps. Bent tree trunks may be evidence that creeping is occurring. If you'd like to know more about erosion, this playlist will help. And as always, thanks for watching. MooMooMath uploads new math and science video every day. Please subscribe and share.