An alluvium is a very tiny, extinct volcano.
When an alluvium is created, a tiny hole opens up in the ground and a small amount of magma spurts up and solidifies quickly, sealing the hole and creating the very small rock projection that is the alluvium. An alluvium then never erupts again, otherwise it would likely be classified as just a plain, old volcano.
The spurting of an alluvium is so brief and the amount of magma that is ejaculated is so small that you might not even notice that it erupting unless you are standing right over top of it when it happens. In that case, you are likely to get a really, really, really nasty burn on your foot and possibly as high as somewhat above the ankle, but that’s probably about all.
In describing alluviums to laypeople, geologists are wont to describe them as usually being not much taller and, at most, only slightly wider than the average erect penis. Don’t blame the writers and editors here at The Words Project for the reportage. We strive for accuracy. That’s just the way geologists minds work.