Thus, Jupiter and Saturn are gas giants, and Uranus and Neptune are ice giants, even though the vast majority of the "gas" and "ice" in their interiors is a hot, highly dense fluid that gets denser as the center of the planet is approached, and in the case of Neptune, may reach temperatures of 5,100 °C.
[1] In igneous petrology the term more specifically refers to the volatile components of magma (mostly water vapor and carbon dioxide) that affect the appearance and explosivity of volcanoes.
Volatiles in a magma with a low viscosity, generally mafic with a lower silica content, tend to vent as effusive eruption and can give rise to a lava fountain.
Some volcanic eruptions are explosive because of the mixing between water and magma reaching the surface, which releases energy suddenly.
[2] Approaching the surface, pressure decreases and the volatiles come out of solution, creating bubbles that circulate in the liquid.
However, the small percentage of gas present represents a very large volume when it expands on reaching atmospheric pressure.
[2] Simplifying, the solubility of water in rhyolite and basalt is function of pressure and depth below the surface in absence of other volatiles.
Usually, insufficient water and carbon dioxide exist in the deep crust and mantle, so magma is often undersaturated in these conditions.
[2] Making a comparison with the solubility of carbon dioxide in magma, this is considerably less than water and it tends to exsolve at greater depth.
Low solubility of carbon dioxide means that it starts to release bubbles before reaching the magma chamber.
Actually, the bubbles are composed of molecules that tend to aggregate spontaneously in a process called homogeneous nucleation.
[2] The nucleation process is greater when the space to fit is irregular and the volatile molecules can ease the effect of surface tension.
The balance between supersaturation pressure and bubble's radii expressed by this equation: ∆P=2σ/r, where ∆P is 100 MPa and σ is the surface tension.