カントリーロック
A maar forms when rising magma encounters sediment beds saturated with groundwater. The magma heats the groundwater to the point where the vapor pressure overcomes the weight of the overlying beds (the overburden pressure) and the beds are catastrophically blown out. Country rock is fragmented and expelled into the atmosphere together with fragments of the magma, creating a deep crater, the bottom of which sits below the pre-eruptive ground surface.