Peperite

What happens when fire meets water? Various things, at various scales. If you toss a hot rock into cold water you'll get cracked-marble texture. If you pour a lava flow into a swamp you'll get palagonite. If you start an eruption under a glacier you'll get a tuya.

Peperite results when lava invades mud. This is a traumatic experience for both parties. The lava shatters, and the mud is baked and stirred. The ensemble is like a melee caught in stop-action, and that's what we have in this new addition to the gallery of volcanic rock types.
_{Peperite — photo courtesy Eddie Lynch; all rights reserved}