由岩浆和水相互作用发生爆炸而形成。在地表下形成了深切到围岩的圆形火山口，并被一个低矮的碎屑环包围。常常会积水而形成火山湖。
琼北第四纪火山区分布着为数众多、大小不一的射气岩浆喷发成因的低平火山口。
Maars of various scales produced by phreatomagmatic eruption are widely distributed in the Quaternary volcanic region of the Northern Hainan Island.
工作区内琼北新生代火山共计5 9个，火山结构类型可分为火山锥、熔岩穹、熔岩湖与低平火山口等。
There are 59 Cenozoic cones, the volcanic texture of which may be classified into cone, dome, lava lake and maar.
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