Bombas y lapillis esferoidales compuestos en los depósitos freatomagmáticos de la caldera de Los Marteles (Gran Ganaria)

Zusammenfassung

In this paper are presented the results of a study carried out in the composed lapilli and bombs located in the tephra ring of Los Marteles caldera (Gran Canaria island), with the aim to analyze their textural and structural characteristics (optical and electronic microscopy), and to establish their formation mechanism. According to their textural and structural features, composed pyroclasts can be divided up into five different types: 1) nucleated, 2) apparently coherent, 3) scoriaceous, 4) loaded, and 5) concentrically banded. All of them exhibit spheroidal shapes and an inner clastic structure, sometimes apparently coherent, and are composed by an amalgam of crystals and crystal fragments, achnelithic glass (achneliths; droplets originally generated by a lava spray), scoriae, and volcanic lithic fragments. The spheroidal composed lapilli and bombs are generated by the agglutination and coalescence of such components inside the eruptive column generated in a hydrovolcanic eruption.