Eruption dynamics of pleistocene maars and tuff rings from the Azrou-Timahdite district (Middle Atlas, northern Morocco) and its relevance to environmental changes and ground water table characteristics
Introduction
In this paper, we focus on a set of hydrovolcanic edifices from a small basaltic area (≈500 km2) situated between the towns of Azrou and Timahdite (Northern Morocco) (Fig. 1b) and which belongs to the Magmatic Middle Atlas (MA) Province (Fig. 1a). These edifices, scattered among small scoria cones and basaltic lava flows (Fig. 1, Fig. 2), are peculiarly interesting because they display an exceptional diversity with respect to morphologies and ejecta types, sometimes even within a single edifice.
Hydrovolcanic eruptions have been described for more than fifty years (i.e. Thorarinsson et al., 1964) in many places in the world. Although their dynamics and ejecta deposit mechanisms are now well documented (e.g., Heiken, 1974; Fisher, 1979; Wohletz and McQueen, 1984; Fisher and Schmincke, 1984; Lorenz, 1986; Lorenz, 1987; Sohn and Chough, 1989; Németh et al., 2001; Brand and White, 2007, 2009; Nemeth et al., 2012; Agustin-Flores et al., 2014; Agustin-Flores et al., 2015, Németh and Szabolcs, 2020), they are still not fully understood. We know that, in continental areas such as the MA, hydrovolcanic eruptions result from explosive sub-aerial activity related to the interaction between a rising magma and water, either trapped at depth (aquifer) or stored at the surface (pool, marsh, river, etc.). In addition to their specific structures, ejecta are characterized by their low juvenile pyroclast content in contrast with the purely magmatic ejecta occurring in scoria cones. Purely phreatic eruptions do not even contain any juvenile magma (Cas and Wright, 1988). Two types of continental hydrovolcanic edifices are distinguished: “maars” and “tuff rings”. Maars form when the rising magma interacts with water trapped at depth but in moderate quantities. In this case, the water is entirely vaporized. Typically, maars are comprised of craters with diameters of a few hundreds meters but not larger than 3 km (White and Ross, 2011) and relatively great depths (but generally less than 100 m), with sub-vertical edges and flat floors that are always below the level of the surrounding terrains. These craters are either completely or partially surrounded by relatively thin (ten to several tens of metres) crescent-shaped embankments of bedded ejecta with sub-vertical inner slopes and gentle outer slopes. In contrast, tuff rings are formed when rising magma interacts with abundant water close to or at the ground surface. In this second case, the water is not entirely vaporized and therefore part of it remains in a liquid state. The associated craters are larger and shallower than maars and often even their floors are at the level of the surrounding terrains. Ejecta are generally richer in juvenile pyroclasts than maars and the inner and outer sides of embankments display a low and approximately equal slope.
The volcanoes of the Azrou-Timahdite Area (ATA) are all of Pleistocene age. The Pleistocene was a period of strong climatic instability in North Africa: dry and wet stages followed each another while glacial and interglacial stages alternated in Europe (Lisiecki and Raymo, 2007). The exact age and duration of the dry and wet stages are still topics of debate (Jimenez, 2010; Grant et al., 2017). Although still poorly constrained from an age point of view, the Pleistocene volcanic activity in the Azrou-Timahdite area (ATA) has been long enough [between 2.5 and 0.5 Ma according to Harmand and Cantagrel (1984) and El Azzouzi et al. (2010)] to encompass several dry and wet phases.
The ATA hydrovolcanic edifices are also interesting because they lye on a karst plateau of medium altitude (between 1,750 and 1,950 m). The karst regions are very sensitive to fluctuations in rainfall (Jacob et al., 2008), and therefore to climate variations. Such variations result in changes in both the water table depth of the perched aquifer located in the epikarst (Williams, 2008) and in the water table depth of underneath karst aquifer. If a volcanic activity happens in the same time as these climate variations, the hydrodynamic changes in the karst must have an impact on the volcanic dynamics. This has already been emphasized in the Miocene to Pliocene Bakony-Balaton Highland volcanic field in Hungary (Németh et al., 2001) and in a Plio-Pleistocene district of Central Mexico (Aranda Gomez and Luhr, 1996).
The aim of this article is twofold: to highlight the peculiarities of the ATA hydrovolcanic eruptions and try to relate them with the hydrodynamic changes into the regional karst subjected to the Pleistocene climatic instability.
Section snippets
Geological framework
Northern Morocco has a very complex geology because of its peculiar setting at the triple junction between the Atlantic Ocean, the Western Mediterranean and the West African Craton (Frison de Lamotte et al., 2008 and references therein). It includes three sub-parallel Cenozoic Alpine ranges from north to south, respectively: the Rif, the Middle Atlas (MA) and the High Atlas (HA) (Fig. 1a). Magmatic activity took place between the Upper Cretaceous and the Pleistocene in the HA and MA. The
Inventory of the hydrovolcanic edifices in the Azrou-Timahdite area
Referring only to morphological criteria, the ATA hydrovolcanic edifices can be described as either maars or tuff rings. Whatever the type of edifice, few of them have evolved until a Strombolian activity and none have emitted a lava flow, probably because of the small amount of magma mobilized at each eruption. Although the majority have diameters between 250 and 800 m and crater depths less than 20 m, several of them display quite large sizes (e.g. 4 km × 2 km for the Bou Ibalghatene
New precision on the age of the Azrou-Timahdite hydrovolcanism
Our field observations clearly show that the ATA hydrovolcanic activity is, as a whole, more recent than the Strombolian activity. Hydrovolcanic craters cut the lava flows of the neighbouring scoria cones and their ejecta cover these flows at the same time (e.g. Bou Ibalghatene, Lachine Ait el Hadj, Lachmine Izyar; Fig. 4). The hydrovolcanic activity could even correspond to the latest eruptive event in the MA.
The age of the ATA Strombolian activity is quite well documented. A dozen flows have
Volcanic dynamism and paleoclimate
Strombolian activity developed in the ATA during the Lower Pleistocene (between 2.6 and 0.8 Ma). The lack or thinness of the pheatomagmatic deposits under the Strombolian tuffs suggests that the underlying karst was at that time slightly supplied by surface water: the climate of the MA was therefore rather dry. In subtropical Africa, three peaks of aridity would have occurred near 2.8 Ma, 1.7 Ma and 1.0 Ma, coinciding with the onset and intensification of high-latitude glacial cycles (
Conclusions
In conclusion, we can say that the shift from the predominantly Strombolian activity to the entirely hydrovolcanic activity observed in the ATA corresponds to a climatic change from dry to wet that would have taken place between 800 and 610 ka and that has continued until at least 500 ka ago.
In the other hand, this work shows that the ATA is a particularly suitable site for evaluating the interaction between water and magma in a karst setting. In particular, it highlights the variability of the
Acknowledgements
This study was performed as part of a collaborative multi-disciplinary research project on volcanism and Upper mantle petrology of Middle Atlas (Northern Morocco) involving the Faculty of Sciences of Meknes (University Mouldy Ismail, Morocco) and Geosciences Montpellier (CNRS & University of Montpellier, France). Funding for research was provided by CNRS (INSU and DERCI, France), CNRST (Morocco), the French Ministry of Foreign Affairs (MAE), and the European FP7 ‘Marie Curie’ program, through
References (57)
- et al.
Phreatomagmatic eruptions through unconsolidated coastal plain sequences, Maungataketake, Auckland Volcanic Field (New Zealand)
J. Geotherm. Res.
(2014) - et al.
Origin of the joya honda maar, san luis potosi, Mexico
J. Volcanol. Geoth. Res.
(1996) - et al.
Lithospheric origin for Neogene-Quaternary Middle Atlas lavas (Morocco): clues from trace elements and Sr-Nd-Pb-Hf isotopes
Lithos
(2014) - et al.
Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene sinker butte volcano, western snake river plain, Idaho
J. Volcanol. Geoth. Res.
(2007) African climate change and faunal evolution during the Pliocene-Pleistocene
Earth Planet Sci. Lett.
(2004)- et al.
Short wavelength lateral variability of lithospheric mantle beneath the Middle Atlas (Morocco) as recorded by mantle xenoliths
Tectonophysics
(2015) Rocks composed of volcanic fragments
Earth Sci. Rev.
(1966)Models for pyroclastic surges and pyroclastic flows
J. Volcanol. Geoth. Res.
(1979)- et al.
Modelling the hydrologic functions of a karst aquifer under active water management – the Lez spring
J. Hydrol.
(2009) - et al.
A 3 million year index for North African humidity/aridity and the implication of potential pan-African humid periods
Quat. Sci. Rev.
(2017)