volcano, lahar flows
"Because lahars are confimed to valley bottoms, people can avoid them by seeking high ground."
Lahars can travel at up to 60mph (100kmh), so you have very little time to react;. Camp Muir is 1.5 miles from the summit, so a lahar could reach Muir in as little as 90 seconds after eruption.
If you need to make a snap decision, look for the highest hill nearby, and run in that direction. (run laterally, not towards the peak)
check the recent seismic data