magma ocean on Io

Students always come to intro geology courses with the misconception that beneath our thin, rocky crust, there exists an ocean of magma. Intro geology students imagine that volcanoes are simply breaches in this ocean, where pressure forces this liquid rock to spew out.

The reality, of course, is that the mantle is solid, not liquid. The mantle's ability to flow over time makes it seem liquid-like, but it is in fact quite crystalline. Beginning geology students have no end of trouble over this distinction; maybe it's something wrong with the way I teach it, but I'd say at least a quarter of students just never quite get this fundamental fact about the earth. And when I throw in decompression and flux melting, students really start to have difficulty.

Now, to add to the confusion, it seems that Io, one of the moons of Jupiter, does in fact have a "magma ocean." In fact, estimates are that about 10% of the volume of Io involves liquified material. This may account for Io's strikingly-intense volcanic activity. Compared to our relatively tame planet, Io has frequent volcanic activity.