**5. Discussion and interpretation**

The installation of the crater of Lechmine n'Aït el Haj is the result of explosive phenomena associated with concentric collapses at the crater. These explosions imply the meeting of basaltic magma with water, here probably underground and/ or superficial water. The crater is probably open on the path of lake or streams that feed the depressions that emerge near the limestone chain in the north of the limestone plateau. The involvement of groundwater of karstic origin in the phreatomagmatic activity is justified by the position of the maar on the path of faults of cryptokarstic origin (**Figure 3**). The second phreatomagmatic phase highlighted at LNH is probably due to an input of underground water that interacts with the magma and causes the deposition of the last pyroclastic breccia (**Figure 15**).

Tectonic analysis of the fractures in the quaternary pyroclastic deposits in the LNH crater allows reconstructing the stress systems. That helped to highlight vertical markers coupled with horizontal shifts. These localized distensions are confirmed by the geometry of the fractures, the presence of tectonic markers, the mixed eruption style of the maar (strombolian-phreatomagmatic), and the mechanisms of syn-eruptive tectonics.

The eruptive activity of LNH is controlled by an NW-SE to WNW-ESE subsidence, with a changing depending on the eruptive style evolution. Thus, the activity of the LNH maar, whose first phreatomagmatic then shifted to strombolian; there is a transition from an extending system to a strike-slip system. This permutation of the stress regime σ<sup>2</sup> - σ3 can be linked to a short-term instability (at the scale of an eruption) that can be enhanced by several factors, notably: 1. the accumulation of volcanic products that vary the position of the center of gravity and deform the edifice [49], 2/a sudden change in the composition of the magmas, notably the increase in the SiO2 content [50] or in water [51], and thus of the eruptive behavior.

During the explosions, collapse phenomena along curved fractures contribute to the widening of the crater [51]. The direction of collapse, particularly sectoral, is generally normal to the direction of active faults (normal or inverse), to alignments of parasitic cones and preferential directions of intrusion [52]. In a strike-slip context similar to that of LNH emplacement, collapse tends to have a parallel direction to the fault direction [53]. However, these relationships between tectonics and the direction of collapse are not always obvious [53]. The direction of the substrate slope, reflecting local tectonics, erosion, or volcanic activity, is generally parallel to the direction of sectoral collapse [54, 55].

The shallow diatremes suggest an eruption where the water-magma interaction remained at shallow levels. It is also an indication of a water-saturated or weakly lithified to unconsolidated host rock [47], which is consistent with the LNH volcanic structure. The multiple vents observed in this eruptive center constitute another indication of an eruption hosted in a substrate weakened by karstic corrosion or poorly consolidated sedimentation [56]. The substrate is unable to support the sloping walls of the diatreme and eventually collapses and obstructs the vent, provoking its migration and explosion in another place [56]. This explains the structure of the LNH maar, distinguished by the non-centered position of the first phreatomagmatic explosion, comparing to the center of the Strombolian explosion. The transition between magmatic and phreatomagmatic eruptive styles is explained by the variation in the groundwater supply and/or variable magma flow [57]. The preservation of the dykes in contact with the diatreme means that they took place during the later phases of the eruption, otherwise they would have been destroyed by the progressive mixing of the diatreme when deeper explosions transported material upwards [42]. This suggests that in the final stages of the eruption, the explosive fragmentation is reduced to become predominantly magmatic.

**165**

**6. Conclusion**

**Figure 15.**

mixed edifice set up during the quaternary.

*(in yellow: Phreatomagmatic activity; in brown: Strombolian activity.*

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

The application of the methods of tephrostratigraphy and geophysic to the pyroclastic deposits of the volcano Lechmine n'Aït el Haj in the Causse of the Middle Atlas allowed understanding and interpretation of the volcanic dynamics of this

*Reconstruction of the volcanic evolution of the maar of Lechmine n'Aït el Haj in the Causse of Middle Atlas.* 

The LNH maar is a large-diameter explosion crater, settled in the Liasic limestone substrate and the overhanging Plio-Quaternary basaltic flows that cover the plateau. The fragments of lithics from the substrate constitute an important part of the projected products (pyroclastics), associated with the juvenile magma. The data provided by the pyroclastic deposits allow us to estimate the importance, frequency, and chronology of the eruptions of the LNH volcano. It is structured by two phases of eruptive activity, phreatomagmatic and strombolian. The pyroclastic projections of the first phreatomagmatic phase offer numerous variants with a clear dominance of Liasic limestone fragments which constitutes 60 to 70% of the deposits. Their very clear stratification is due to the rhythmicity of the explosions. The juvenile fragments only

present 30 to 40% of projections in the form of scoria, bombs, and blocks.

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*

**Figure 15.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

The installation of the crater of Lechmine n'Aït el Haj is the result of explosive phenomena associated with concentric collapses at the crater. These explosions imply the meeting of basaltic magma with water, here probably underground and/ or superficial water. The crater is probably open on the path of lake or streams that feed the depressions that emerge near the limestone chain in the north of the limestone plateau. The involvement of groundwater of karstic origin in the phreatomagmatic activity is justified by the position of the maar on the path of faults of cryptokarstic origin (**Figure 3**). The second phreatomagmatic phase highlighted at LNH is probably due to an input of underground water that interacts with the magma and causes the deposition of the last pyroclastic breccia (**Figure 15**).

Tectonic analysis of the fractures in the quaternary pyroclastic deposits in the LNH crater allows reconstructing the stress systems. That helped to highlight vertical markers coupled with horizontal shifts. These localized distensions are confirmed by the geometry of the fractures, the presence of tectonic markers, the mixed eruption style of the maar (strombolian-phreatomagmatic), and the mecha-

The eruptive activity of LNH is controlled by an NW-SE to WNW-ESE subsidence, with a changing depending on the eruptive style evolution. Thus, the activity of the LNH maar, whose first phreatomagmatic then shifted to strombolian; there is a transition from an extending system to a strike-slip system. This permutation of the stress regime σ<sup>2</sup> - σ3 can be linked to a short-term instability (at the scale of an eruption) that can be enhanced by several factors, notably: 1. the accumulation of volcanic products that vary the position of the center of gravity and deform the edifice [49], 2/a sudden change in the composition of the magmas, notably the increase in the SiO2 content [50] or in water [51], and thus of the eruptive behavior. During the explosions, collapse phenomena along curved fractures contribute to the widening of the crater [51]. The direction of collapse, particularly sectoral, is generally normal to the direction of active faults (normal or inverse), to alignments of parasitic cones and preferential directions of intrusion [52]. In a strike-slip context similar to that of LNH emplacement, collapse tends to have a parallel direction to the fault direction [53]. However, these relationships between tectonics and the direction of collapse are not always obvious [53]. The direction of the substrate slope, reflecting local tectonics, erosion, or volcanic activity, is generally parallel to

The shallow diatremes suggest an eruption where the water-magma interaction remained at shallow levels. It is also an indication of a water-saturated or weakly lithified to unconsolidated host rock [47], which is consistent with the LNH volcanic structure. The multiple vents observed in this eruptive center constitute another indication of an eruption hosted in a substrate weakened by karstic corrosion or poorly consolidated sedimentation [56]. The substrate is unable to support the sloping walls of the diatreme and eventually collapses and obstructs the vent, provoking its migration and explosion in another place [56]. This explains the structure of the LNH maar, distinguished by the non-centered position of the first phreatomagmatic explosion, comparing to the center of the Strombolian explosion. The transition between magmatic and phreatomagmatic eruptive styles is explained by the variation in the groundwater supply and/or variable magma flow [57]. The preservation of the dykes in contact with the diatreme means that they took place during the later phases of the eruption, otherwise they would have been destroyed by the progressive mixing of the diatreme when deeper explosions transported material upwards [42]. This suggests that in the final stages of the eruption, the explosive fragmentation is reduced to become predominantly magmatic.

**5. Discussion and interpretation**

nisms of syn-eruptive tectonics.

the direction of sectoral collapse [54, 55].

**164**

*Reconstruction of the volcanic evolution of the maar of Lechmine n'Aït el Haj in the Causse of Middle Atlas. (in yellow: Phreatomagmatic activity; in brown: Strombolian activity.*

## **6. Conclusion**

The application of the methods of tephrostratigraphy and geophysic to the pyroclastic deposits of the volcano Lechmine n'Aït el Haj in the Causse of the Middle Atlas allowed understanding and interpretation of the volcanic dynamics of this mixed edifice set up during the quaternary.

The LNH maar is a large-diameter explosion crater, settled in the Liasic limestone substrate and the overhanging Plio-Quaternary basaltic flows that cover the plateau. The fragments of lithics from the substrate constitute an important part of the projected products (pyroclastics), associated with the juvenile magma. The data provided by the pyroclastic deposits allow us to estimate the importance, frequency, and chronology of the eruptions of the LNH volcano. It is structured by two phases of eruptive activity, phreatomagmatic and strombolian. The pyroclastic projections of the first phreatomagmatic phase offer numerous variants with a clear dominance of Liasic limestone fragments which constitutes 60 to 70% of the deposits. Their very clear stratification is due to the rhythmicity of the explosions. The juvenile fragments only present 30 to 40% of projections in the form of scoria, bombs, and blocks.

Two important eruptive sequences marked the first phreatomagmatic phase composed of stratified deposits with an abundant lithic fraction of limestone from the Liasic basement and juvenile pyroclasts. The accidental lithics decrease during the emissive process. This phreatomagmatic activity is initiated by lake water attested by lacustrine deposits.

A second explosive phase of the strombolian eruption style follows the first phreatomagmatic phase. It begins with the effusion of a thick basalt flow due to a collapse inclined slightly to the west, and then it is a pyroclastic plume that will be launched with the fallout of different sizes and shapes depending on the proximity of the eruptive center.

The last pyroclastic breccia surrounds the crater of LNH. It occurs in discontinuous pyroclastic deposits with well-sorted bedding where the fraction of lithics is less abundant than that of the early phreatomagmatic stages.

The tectonic analysis allowed the reconstruction of the stress systems and the highlighting of the mechanisms of syn-eruptive tectonics which had an important impact on the transition of the eruptive style (phreatomagmatic-strombolian).

LNH is only one example of the 105 monogenic volcanoes of the Causse of the Middle Atlas, this study represents a first step towards the discovery of this province at the scale of volcanoes, in order to build a model of volcanic dynamics in this region, starting from the approaches mentioned above. Scientific knowledge can be exploited in addition to the natural potentialities of the region to build a model of development that fits with the particularities of this territory. Finally, the richness of the Causse of the Middle Atlas in recent monogenic volcanoes with well-preserved forms in a karstic geological context, as well as the great variety of morphology, both in terms of flows and volcanic devices, make this territory a privileged area to establish a natural geopark accessible to all and easily mediatized.
