Short answers, pulled from the story.
Metamorphic rock forms when an existing rock, called the protolith, is subjected to temperatures greater than about 200 degrees Celsius and pressures of 100 megapascals or more, causing it to recrystallize into a new texture or mineral composition without fully melting. The protolith can be igneous, sedimentary, or an earlier metamorphic rock.
Metamorphic rocks form 12% of the Earth's land surface. They also make up a large part of the Earth's crust, with the lower continental crust consisting mostly of metamorphic rock that has reached the granulite facies.
James Hutton, the Scottish naturalist often described as the father of modern geology, first noted in 1795 that certain rock beds in the Scottish Highlands had originally been sedimentary rock transformed by great heat. His friend James Hall later confirmed the role of pressure using a cannon barrel as a pressure vessel.
Index minerals are minerals whose presence in a metamorphic rock indicates the approximate temperatures and pressures at which that rock was metamorphosed. Examples include sillimanite, kyanite, staurolite, andalusite, and some garnet; each is stable only within specific pressure-temperature conditions.
On the 17th of August 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake, Montana, composed of schist. The pronounced planes of weakness characteristic of schist caused a massive landslide that killed 26 people who were camping in the area.
Foliation is a distinctive layered or banded texture that develops when a rock is compressed along one axis during recrystallization, causing platy minerals like mica and chlorite to align in parallel. Slate, phyllite, schist, and gneiss are all foliated metamorphic rocks, while marble is generally not foliated because it lacks platy minerals.