Hermit crabs "eating" vesicular basalt

[Lordhermitcrab]

In my crabitat, I have one huge and thick cholla log going up onto the cork bark platform.

I was put off by the fact that crabs oftentimes get wedged in the tubes in the past, so I plugged it with a rock in my room. One of many rocks, in fact.

Turns out, from my recent geology class, that this rock is vesicular basalt. Neato. But what's strange is sometimes I see Tessa, one of my mid-sized crabs, "eating" it. I put quotes on that, because it looks like she is. You know how crabs eat, right? Left or right claw pinches something off, brings it to the mouth and little appendages come down and grab it. Tessa's doing that.

Basalt's a hard rock so I don't know if she's actually getting anything off of it.

Anyone seen something like this before? Is there a problem here?

[Crabinski]

Not to worry! When my crabs were back in the old 10g, I added a lovely piece of red vesicular basalt to the crabitat. It was like a doughnut with a flat bottom and they all loved climbing it, sitting in it, pushing one another out of it to get a better seat and, of course, "eating" it. Basalt is essentially volcanic ash so no harm if any is actually consumed.

[Lordhermitcrab]

What do you mean it's volcanic ash? I thought it was a very common, solid, igneous rock.

Also it often contains a lot of magnesium. Could they be after that?

[Crabinski]

What do you mean it's volcanic ash? I thought it was a very common, solid, igneous rock.

Also it often contains a lot of magnesium. Could they be after that?

Basalt, both extrusive vesicular and almost non-porous in appearance intrusive varieties, is formed from molten rock (magma when it stays within the volcano, lava when it erupts or trickles out). Vesicular basalt is created when lava erupts at about 2000 degrees and, due to the huge difference in temperature between the lava and the surrounding air, begins to dry in mid-air, falling to the ground as black, brown or red lava bombs with all those visible air pockets and what looks like a crumbly texture. As vesicular basalt erodes, it essentially weathers into volcanic ash, very healthy for plants. Magma/lava is comprised of a variety of elements including magnesium, calcium, sodium and potassium so there are many tasty aspects for hermit crabs.