An unusual bed of giant pumice in Mexico

Abstract

We report here that a remarkable bed of giant-sized pumice blocks occurs within an old caldera of the major late Quaternary rhyolitic volcano of Sierra La Primavera, Mexico (Fig. 1a). The bed lies within a 100-m thick sedimentary sequence distributed over a near-circular area 10km across. The margin of this circular area closely parallels an incomplete ring of prophyritic comendite lava domes (Fig. 1b), some of which may be shallow intrusions emplaced into the sediments. The vents of these domes are believed to lie on a ring fault (Fig. 1b). The sedimentary sequence, now gently updomed (Fig. 1c), is believed to have accumulated in a caldera lake, the shoreline of which lay up to 1 km outside the ring fault.