The volcano did the frying, and the result was trapped into bubbles of rock where it could not come into contact with CO2 from the air. Although most of it is

The volcano did the frying, and the result was trapped into bubbles of rock where it could not come into contact with CO2 from the air. Although most of it is not directly as oxides but some form of silicates. (This forces the use of lots of water in the process -- you can go from CaO -> CaCO3 without much water, but to dissolve silicates you need the CO2 dissolved in water.)