**3. The Middle Atlas Volcanic Province (MAVP)**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

maar and its eruptive dynamic.

other planets such as Mars [19].

order of a few days to a decade [23].

broad area over millions of years [30].

years) [21, 22].

**2. General features of monogenic volcanism**

(1,2%). The three first types are exclusively Plio-Quaternary (3.77–0.60 Ma), the K-Ar age of the last one is Middle Miocene (16.25–5.87 Ma), and Plio-Quaternary (3.92–0.67 Ma) [9]. This magmatism results from a partial melting that occurred at around 2 GPa, i.e. near the lithosphere–asthenosphere boundary beneath the Middle Atlas (60–80 km) [10]. The analysis of the Middle Atlas aeromagnetic data allowed the characterization of the regional magnetic anomalies, their location and delimitation corresponding to the major accidents of the Middle Atlas [11, 12]. 30% of MAVP volcanoes are represented by maar-diatreme-type volcanic systems [6]. They have a negative shape forming a crater that intersects the preeruptive surface. Lechmine n'Aït el Haj is the first maar ever studied in this region. After a previous work on the analysis of the eruptive sequence of the maar [13], the aim of this chapter is to understand the structural context of the formation of this

Magma system volumes can occur in an intraplate context devoid of a mantle plume [14]. This type of volcanism is characterized by "small" volcanoes with magmatic system often basaltic [4–6], derived mostly from a mantle that stays in the crust just the time to allow a minor fractional crystallization [15]. These monogenic volcanic fields occur in any tectonic setting [2, 16–18], not only on Earth but also on

Much research has been done on monogenic volcanoes, focusing on their nature from source to surface [15, 20]. The rapid ascent of magma and the short eruptive history of volcanoes allow to understand, for example, the magmatic evolution of the systems that fed several small volcanoes over long periods of time (millions of

In many cases, the eruptive style is controlled not only by the internal properties

of the magma but also by the external environmental conditions to which it has been exposed. The resulting morphology is often related to the mechanism of the dominant eruptive style, making it important criteria in the study and especially in the classification. The estimated time for the formation of these volcanoes is in the

Monogenic volcanoes are referred to as the product of a single eruption [24], however, a magma ascent is not always related to a single magma influx, which usually involves several episodes producing a geochemical evolution even over a single eruption [15, 20, 25]. This magma may come directly from the mantle or from a volume of magma trapped in a zone of contrasting density, such as the upper mantle/ crustal boundary [25]. The magma ascension begins with an inter-connectivity between the small volumes of magma in the mantle and their vertical migration forming dykes [26, 27]. These dykes generally follow pre-existing structures such as

The magma of monogenic volcanoes is often primitive. It rarely expresses itself

Once a batch (or batches) of magma begins its ascent to the surface, it faces continuous outgassing and interactions with the host environment. Once at the surface, this magma can produce a volcanic eruption that can be explosive or effusive. This is controlled by the characteristics of the eruption which are determined at a superficial depth (≤1–2 km) by the balance between internal and external factors [4].

individually; it tends to form several monogenic volcanoes in a volcanic field, where there may be tens or even thousands of individual volcanoes [29]. However, a monogenic volcanic field could experience repeated monogenic eruptions across a

faults of the basement rocks as they move towards the surface [28].

**150**

The Atlas mountain chain is the result of the Oligocene compression induced by the Europe-Africa convergence and continent-continent collision. Its basement is structured during the Hercynian orogeny [307,5 ± 6,8–364,0 ± 8,2] [31]. It is continental and outcrops only at a few inlier [32]. The cover consists mainly of Jurassic deposits. It is deformed in the folded Middle Atlas, whereas it is sub-tabular in the Causse of the Middle Atlas [32]. This latter zone is distinguished with a simple structural style of inclined blocks, manifesting itself in the topography by a succession of subhorizontal layers (Tabular Middle Atlas). It is the area where the Plio-Quaternary volcanism occurred with a hundred of eruptive centers aligned in a sub-meridian direction (**Figure 1**). It is crossed by the Tizi n'Tritten Fault (TTF) and a network of faults between the North Middle Atlas Fault NMAF and TTF. All these accidents, which are at least Hercynian, from direction N45 to N70, affect the recent Quaternary deposits [8].

The emission points located between Azrou and Timahdite have an NNW–SSE orientation along 50 km. The basalt flow covers the ancient Quaternary formations up to the Saïs plain in the north (**Figure 1**). They are channeled to the East by the

### **Figure 1.**

*Structural map of the Middle Atlas with the location of the volcanic field [33]. SMAF: South Middle Atlas fault, AOF: Ait Oufella fault, NMAF: North Middle Atlas fault, TTF: Tizi n'Tretten fault. Jurassic, cretaceous, and Paleogene synclines: 1: Bekrite, 2: Timahdite, 3: Bou-Anguer, 4: Aïn-Nokra, 5: Oudiskou, 6: Tirhboula, 7: Ait Oufella, 8: El-Mers, 9: Guigou.*

**Figure 2.**

*Geographical location of the study area. Most of the volcanoes and lava flow units are settled between Azrou and Timahdite where the cryptokarstic cavities are dominating.*

depression of the Guigou Valley and to the West by the Wadis of the Beht and Oum Rbia. Most of the volcanoes are strombolian (70%) disseminated in the entire volcanic area. on the other hand, maars are focused in the eastern part of the volcanic province [5] (**Figure 2**). The karstic-carbonate nature of the region combined with the fluctuations in rainfall contributes to the formation of the water table in the epikarstic level [34, 35]. The variation of the karstic water at this level has an impact on the volcanic dynamic and the changing of the eruptive style. In The MAVP, 8% of volcanic vents have witnessed a shifting between wet and dry dynamics during the same monogenic eruption [5].
