The Lechmine N'kettane is a Quaternary volcano, located within the Middle Atlas Volcanic
Field (MAVF) in central Morocco. It is built on the faulted contact between Liassic
limestone and Plio-Quaternary fluvio-lacustrine deposits. In map-view it consists
of an elliptical maar crater, surrounded by a tephra ring, within which a scoria cone
is nested in its northern crater zone. The Lechmine N'Kettane volcano is monogenetic
in the sense of its small eruptive product volume and lack of evidence of significant
time breakthrough it grows. The volcano formed from an eruptive locus that migrated
laterally and vertically within the short duration of the eruption in a zigzagging
pattern, along a complex set of generally NE-SW and NW-SE-trending faults. It represents
a perfect example of how a volcano form and evolve under the influence of a combination
of specific factors such as the lithological characteristics of the substrate, its
hydrogeological parameters, magma flux and the local structural framework of the country
rocks. The petrographic, granulometric and morphological (including terrain modelling)
analyses of the Lechmine N'kettane pyroclastic deposits show that it was constructed
in four eruptive phases with variable eruptive styles. The first, relatively dry,
phreatomagmatic phase, that took place on a NE-SW fault in the northeastern part of
the crater, was generated by the interaction between the ascending basaltic magma
with meteoric water in the karst aquifer hosted by the Liassic limestone. The second
phase is represented by a magmatic scoria fallout deposit whose explosion locus moved
westward, along the same NE-SW fault. During the third phase, the explosion center
migrated southward, along a NNW-SSE fault, and produced the last phreatomagmatic event
by interaction of magma and water-saturated Plio-Quaternary sediment. The fourth eruptive
phase is a purely scoria event, corresponding to the construction of the nephelinitic
scoria cone in the northwestern part of the tephra ring. Between eruptive products
formed in respective eruptive phases no evidence was recognized to establish significant
time gaps between their formation.
