A quote from this very informative article: "how many people hike at places like Mount St. Helens, Lassen Volcano National Park or Mount Rainier every day without specific knowledge that they are hiking on a potentially active and explosive volcano?"

Even a tiny bit of reading by visitors to these areas would reveal their nature (to say nothing of the broad clue in the full title for Lassen) and the recent volcanic history for each (active fumaroles near the summit of Rainier have sheltered bivouacking climbers on more than one occasion). In addition to the threat of volcanic eruption, the hazards of a sudden lahar - a quick slide of wet volcanic ash triggered by several possible mechanisms, is as great as the problems of a volcanic eruption.

And yet Fujisan is the most frequently climbed mountain in the world, and indeed it is beautiful and worthwhile....

I wonder if even a certified climbing helmet would be of much use when rocks start falling from the sky.