A Magma "Ocean" inside Io?

What is the most volcanically active body in our Solar System? Hint: It's not a planet! It's Io, the closest moon to Jupiter. Plumes of volcanic eruptions were spotted on its surface by the Galileo spacecraft mission in 1997.

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As soon as Io was known to be volcanically active, debates began about the nature of the volcanism. Io's close orbit to the gas giant Jupiter causes what's known as intense "tidal heating." That's what creates conditions conducive to its volcanism. But what's the nature of the material risi