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Debris flows at Lassen Volcanic Center are often caused by hydrothermal alteration of rocks in the Sulphur Works area. But why does hydrothermal activity make volcanic rock collapse? It's all in the mechanics. When they're erupted, volcanic rocks are dense and strong, because their primary (original) minerals are mostly silicates like feldspar, olivine, pyroxene, and quartz. But in volcanic areas, gases and water can combine to form acids that change the original volcanic minerals into other, weaker minerals. These include kaolinite (a clay used in cosmetics), smectite clay (which can absorb water), and illite. 

One of the main characteristics of clay minerals is that their molecules are arranged in sheets, which are able to slide past each other just like sheets of paper. This means they are very week and easily broken in one direction. When a hydrothermally altered rock has had lots of its original silicate minerals replaced, it can be much weaker and breaks or crumbles easily. They can also be more colorful, which is why geothermal areas are often filled with bright white, red, yellow, and orange rocks!