The hikers spotted the wall of ash and attempted to run from it:
However, it the cloud of volcanic ash quickly filled the sky:
As they ran, it continued to gain on them, thundering down the side of the volcano:
Until it they were finally engulfed by it:
Preliminary reports on the 30 people killed by the eruption indicate that the majority of them died from ash inhalation.
Many of the hikers who were killed by -- or who are still trapped on -- the volcano were on religious pilgrimages, as the volcano, Japan's second tallest, is considered sacred.
Watch the entire video YouTube below.
Here is a video of the same eruption, only from the air, also via YouTube.