Short answers, pulled from the story.
Schist is a medium-grained metamorphic rock that forms during mountain building, typically from fine-grained sedimentary rocks like mudstone or shale under regional metamorphism. Heat and pressure first convert mudstone to slate, then to phyllite, and finally to mica schist as minerals recrystallize into larger, platy grains.
Schist splits easily because it contains abundant platy minerals such as mica, talc, chlorite, or graphite, whose grains align in a preferred direction under nonhydrostatic stress. This alignment, called schistosity, creates parallel planes of weakness along which the rock readily breaks into thin flakes or slabs.
The word schist is derived from the ancient Greek verb meaning "to split," referring directly to the rock's most distinctive physical property.
Schist is medium-grained with well-developed schistosity, making it one of three textural divisions of metamorphic rock. Gneiss has poorly developed schistosity and thicker layering, while granofels has no discernible schistosity. Slate is a fine-grained precursor to schist in the metamorphic sequence, and before the mid-nineteenth century the two were not clearly distinguished by miners.
On the 17th of August, 1959, a magnitude 7.2 earthquake near Hebgen Lake in Montana destabilized a mountain slope composed of schist, triggering a massive landslide that killed 26 people. The schist's pronounced planes of weakness made the slope vulnerable to failure when the earthquake struck.
Schist bedrock contains schistosity planes that form structural discontinuities, reducing the strength of rock masses and increasing the risk of failure in tunnels, foundations, and slopes. The hazard can exist even in undisturbed terrain because the aligned planes of weakness are an inherent feature of the rock.