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AMCS Bulletin 19 / SMES Boletín 7 — 2006

171

Lava Tubes of the Naolinco Lava Flow, El Volcancillo, Veracruz, México
Guillermo Gassós1 and Ramón Espinasa-Pereña2
2 1 Club de Exploraciones de México, Sección Veracruz, A.C. Sociedad Mexicana de Exploraciones Subterráneas A.C., ramone@cablevision.net.mx

Six caves up to nearly a kilometer long have been discovered on the Naolinco lava flow,which was emmited by El Volcancillo 870 years ago and reached a length of about 50 kilometers. All of the caves seem to be remains of a master tube which probably fed most of the lavas that form the flow. Of particular interest is the fact that at least two of the caves capture and carry surface streams of considerable size. The water

Abstract

does not return to the surface until the springknown as El Descabezadero, the birthplace of the Actopan River.

North of Cofre de Perote a series of small eruptive vents are called the Las Lajas Cinder Cones. Over a dozen volcanic vents have been recognized and some of them have been dated (Siebert and Carrasco-Núñez, 2002). La Joya cinder cone complex is one of the oldest, and

Las Lajas Cinder Cones and lava flows

produced about 20km3 of basaltic flows that extend about 14 kilometers SE to underlie the city of Xalapa, capital of the state of Veracruz, about 42,000 years B.P. Many younger volcanic vents and lava flows exist in the area (Fig. 1). The youngest lava flows dated by Siebert and Carrasco-Núñez (2002) originated from El Volcancillo (2,700 m.a.s.l.), a twin crater located 4 kilometers southeast of the town of LasEl Volcancillo

Figure 1. Las Cumbres volcanic complex and the Volcancillo lava flows.

172
Vigas which erupted 870±30 y.B.P. The cone complex straddles a sharp crested ridge between two valleys carved into the slope of Las Lajas volcano, a subsidiary cone of Cofre de Perote. It fed two lava flows that traveled down different drainages. The Toxtlacoaya A’a flow, originating from thesoutheastern crater, has a length of approximately 12 kilometers, while the Río Naolinco pahoehoe flow, which originated in the northwestern crater, traveled over 50 kilometers. The eastern crater occupies the summit of a steep sided scoria cone that is breached in two places on its southern side. Large lava benches surround the inner crater and mark the highest stand of a former lava lake whichoverflowed the breach, generating a short lava flow which shortly stopped at the end of the first steep slope. We believe that most

AMCS Bulletin 19 / SMES Boletín 7 — 2006 of the Toxtlacoaya lava flow issued not from the breached upper cone but from a pair of vents at the northeastern base of the cone, based on lava flow morphology. The lava flow crusted over forming a large lava tube with a bigskylight, 20 meters in diameter, which overflowed frequently forming a small shield. Quarrying of a lower entrance and the building of an Oleoduct collapsed most of the cave, leaving a semi-natural rock arch giving the cave its name, Cueva del Arco (Figure 2) Siebert and Carrasco-Núñez (2002) claim that the 35 meter thick lava pile visible on the walls of Cueva del Arco (Figure 3), actually 45 meters,according to our survey, represent the minimum thickness of the lava
Figure 3. Cueva del Arco. Notice the two cavers, one on rope and the other at the bottom.

Toxtlacoaya lava flow

flow, and do not consider that the tube could have been originally much smaller, and the present height was caused by thermal erosion, as suggested by the passage cross section. The western or main crater is 200meters wide and 90 meters deep. It partially truncates the eastern scoria cone and was produced by collapse of a small lava lake that overflowed the western scoria cone. In both craters we find the same sequence of events: building of a scoria cone by lava fountaining, followed by the emission of lava which formed a lava lake. In the western crater, the scoria cone was overtopped over an arc of...
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