Phreatomagmatic plioquaternary volcanism in the Middle Atlas: Analysis of the eruptive sequence of the Lechmine n’Aït El Haj maar

Lechmine n’Aït El Haj (LNH) is a monogenetic plioquaternary maar, lying in the volcanic province of the MiddleAtlas. It is a 110-m-deep crater located in the Liassic limestones. The tephra deposits surrounding the crater are mainly made up of depositional units (surges and projectas) interpreted as deposits of phreatomagmatic origin. They are topped by a small unit of massive breccia tuff reflecting magmatic deposits. The maar is a result of the interaction between the ascending magma and karstic water, in an intraplate volcanism context. Water, causing this eruption, is drained by an open system of fractures in the limestone. The explosion started by phreatomagmatic dynamism, producing a big stack of pyroclastic deposits and pyroclastic falls. During the eruption, the crater grows progressively from the eruptive center to the Northwest. The upper part of the phreatomagmatic deposits is characterized by a typical mud crack structure. A transition to a strombolian dynamism occurred throughout the end of volcanic activity. Meanwhile, a lava flow, coming from the volcanic plateau, discharged in the crater’s center. With the eruption resumption, the lava is strongly fragmented; therefore, a small cone is created especially in the northern flank of the maar. Towards the end of the volcanic activity, a supply of karstic water causes another transition of the eruptive style from strombolian to phreatomagmatic dynamism. A significant karst collapse in the southern flank of the LNH maar has occurred, leading to its current morphology.