This constant bending puts a lot of energy into Io from friction. Note that there are no craters, favoring the theory that the geysers continually renew the surface. Geyser at the horizon. Better view of the geyser. Here is what a geyser is on Earth.
Water is heated by hot rock.
The water is under high pressure because of the water above it. It doesn't boil because of the high pressure. The temperture goes up a little more. The water begins to boil, pushing some out at the surface. This releases some pressure, causing more boiling.