Zeolites from La Crosa de Sant Dalmai volcano

La Crosa volcano, also called “La Crosa de Sant Dalmai”, shares the space between the Catalan municipality of Vilobí d’Onyar (in the lands of Sant Dalmai) and those of Bescanó and Aiguaviva. Located in the NE of Catalonia, in Girona.

It is a wide volcanic maar or crater (over 1.200 m, one of the largest in Europe) and low altitude (156 meters above sea level) formed by a phreatomagmatic eruption, produced by an explosion caused by groundwater in contact with ascending magma from the main chamber. The violent explosions have formed a ring of pyroclastic projections, formed both by juvenile materials (of basaltic composition, chalk, tuff and some volcanic bombs) and by fragments of subterranean rocks (igneous and metamorphic fragments). The phreatomagmatic origin of the Crosa maar differentiates it from the rest of the volcanoes in the area, in which strombolian eruptions predominate. It must be said, however, that within this phreatomagmatic volcano we find a smaller cone of the Strombolian type.

The eruptive period of La Crosa is between 7.9 and 1.7 million years (My), characteristic of the volcanic processes of the La Selva depression. The oldest in the Empordà region date from 12 to 8 My and the most recent, from La Garrotxa, are between 500,000 and 10,000 years old.

Figure 1. View of La Crosa de Sant Dalmai volcano, from the Can Guilloteres viewpoint.

Cereals, fodder and hazelnuts are currently cultivated there. The ring of the maar is populated by lush forests of holm oaks and oaks, with replanting of pine and eucalyptus trees.Mining has also been carried out related to the volcanic chalk, such as that of Can Guilloteres (where there is currently the car park, viewpoint and explanatory panels) or that of Can Costa.

Figure 2. Old chalk mining works of Can Guilloteres.

Figure 3. Old chalk mining works of Can Costa.

Near a field next to Can Costa we collected some fragments of lava with numerous vesicules. These porous black materials include lithic fragments such as albite, quartz, granite, shale, olivine and volcanic glasses. And some crystallized minerals.

Figure 4. Mineralized vesicles.

Figure 5

Figure 6

SEM-EDS and Raman analysis have shown that spherical aggregates (figure 6) and translucent to white prisms (figure 5 and 7) are phillipsite-K. The colourless transparent rosettes on figure 5 have been determined as chabazite-K.

Figure 7

REFERENCES

• Pedrazzi, D., Bolós, X., Martí, J. (2014): “Phreatomagmatic volcanism in complex hydrogeological environments: La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, NE Spain)”. Geosphere, vol. 10, núm. 1, pp. 170-184. [online, april 2021].

• Bolós, X., Barde-Cabusson, S., Pedrazzi, D., Martí, J., Casas, A., Himi, M., Lovera, R. (2012): “Investigation of the inner structure of La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, Spain)”. Journal of Volcanology and Geothermal Research, vol. 247-248, pp. 37-48.

• Ferrer, M.C., Riesco, M.: “Geòtop 356. La Crosa de Sant Dalmai”. Generalitat de Catalunya. Departament de Medi Ambient i Habitatge Direcció General del Medi Natural. [online, april 2021].

• Spanish version here