Jupiter Then And Now

Internal structure

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.^[37] Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths
-copied from Wiki.

In Poul Anderson's Three World's To Conquer (London, 1966), Chapter 15, p. 105, Jupiter has:

a metallic core;
thousands of miles of solid hydrogen;
a shell of ice.

When an equilibrium is broken so that the pressure in one volume drops below a critical value, a mass of ice changes to a less dense crystalline phase in a large explosion. Liquefied water bursts through the planetary surface, cooling and congealing into the mountainous cone of a volcano. Thus, Jovian volcanoes are molten water whereas seas and clouds are ammonia.