J. C. Mora

Emplacement of pyroclastic flows during the 1998–1999 eruption of Volcán de Colima, México

After three years of quiescence, Volcan de Colima reawakened with increasing seismic and rock fall activity that reached its peak on November 20, 1998, when a new lava dome forced its way to the volcano’s summit. The new lava rapidly reached the S–SW edge of the summit area, beginning the generation of Merapi-type pyroclastic flows that traveled down La Lumbre, and the El Cordoban Western and Eastern ravines, reaching distances of 3, 4.5, and 3 km, respectively. On December 1, 1998, the lava flow split into three fronts that in early 1999 had reached 2.8, 3.1, and 2.5 km in length, advancing down the El Cordoban ravines. The lava flow fronts disaggregated into blocks forming pyroclastic flows. One of the best examples occurred on December 10, 1998. As the lava flow ceased moving in early 1999, activity became more explosive. Strong blasts were recorded on February 10, May 10, and July 17, 1999. The last event developed a 10-km-high eruptive column from which a pyroclastic flow developed from the base, traveling 3.3 km SW from the summit into the San Antonio–Montegrande ravines. Regardless of the mechanism of pyroclastic-flow generation, each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension. When the basal avalanche lost velocity and eventually stopped, the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic-flow deposit and an upper dune-bedded surge deposit. The dilute pyroclastic flow scorched and toppled maguey plants and trees, and sandblasted vegetation in the direction of the flow. At the end of the dilute pyroclastic-flow path, the suspended particles lifted off in a cloud from which a terminal ash fall was deposited. The basal avalanche emplaced block-and-ash flow deposits (up to 8 m thick) that filled the main ravines and consisted of several flow units. Each flow unit was massive, monolithologic, matrix-supported, and had a clast-supported steep front (ca. 1.5 to 2 m thick) composed of boulders up to 1.7 m in diameter. The juvenile lithic clasts had an average density of 1800 kg/m3. The dilute pyroclastic flow emplaced overbank deposits, found on valley margins or beyond the tip of block-and-ash flow deposits. They consist from bottom to top of a massive medium to coarse sand-size flow layer (2–4 cm thick), a dune-bedded surge layer (2–10 cm thick), and a massive silt-size layer (0.5 cm thick). The total estimated volume of the pyroclastic-flow deposits produced during the 1998–1999 eruption is 24×105 m3.

Petrology of the 1998–2000 products of Volcán de Colima, México

The 1998–2000 activity of Volcan de Colima generated a series of lava flows and block-and-ash flows of andesitic composition (59–61% SiO2). Juvenile clasts from both lava flows and pyroclastic deposits contain phenocrysts of plagioclase>two pyroxenes>Fe–Ti oxides and rare amphibole set in a groundmass of the same minerals and rhyolitic glass (74–77% SiO2). Prior to the eruption, the andesitic magma was stored at a temperature of ∼900°C based on titanomagnetite–ilmenite equilibrium. The magma had an oxygen fugacity of 10−11.1, corresponding to 0.8 log units above the NNO oxygen buffer, and water contents in the rhyolitic melt of ∼2 wt%, suggesting that the magma was water-undersaturated at depth. The presence of amphibole with clear signs of disequilibrium, quartz xenocrysts, and plagioclase (An 50) and microphenocrysts (An>50) indicate the injection of a new hotter and more mafic andesite into the magma chamber. This process was the main trigger of the 1998–2000 activity.

Eruptive History of the Tacaná Volcanic Complex

Tacana is the northernmost volcano of the Central American Volcanic Arc, and one of the four volcanic structures of the Tacana Volcanic Complex (TVC), from oldest to youngest: Chichuj, Tacana, and San Antonio volcanoes, and Las Ardillas dome. Geologic and radiometric data show that volcanic activity of the TVC began around 225 ka with the construction of Chichuj volcano within the 2 Ma old San Rafael Caldera. The edifice of Tacana began its construction west of Chichuj volcano around 50 ka. San Antonio volcano, and Las Ardillas Dome formed southwest of Tacana volcano during Late Pleistocene. Effusive and explosive eruptive activity has alternated from all eruptive centers of the complex. Flank collapses of Chichuj, Tacana, and San Antonio edifices have generated debris-avalanches. At least four plinian -subplinian events—two of which rank ~5 on the Volcanic Explosivity Index (VEI)—and nine other smaller explosive eruptions occurred at Tacana during the Holocene, the most recent one around 150 year BP. The 1949 and 1986 phreatic explosions from Tacana attracted scientific and public attention to the complex. At present, Tacana represents the second most dangerous volcano in Mexico after Popocatepetl.

Palaeomagnetic results from the Chiapanecan Volcanic Arc, Chiapas, Southern Mexico: geomagnetic and geodynamic significance

This article presents the first palaeomagnetic results from 13 independent cooling units in the Chiapanecan Volcanic Arc (ChVA). Six sites were directly dated by Ar–Ar or K–Ar methods: their dates range from 2.14 to 0.23 Ma. We isolated the characteristic palaeodirections for all 13 lavas. Eleven non-transitional directions yield a mean direction with inclination, I = 30.7°, declination, D = 4.1°, and precision parameters k = 63 and α95 = 5.8°. The corresponding mean palaeopole position is Plat = 83.3°, Plong = 203.8°, K = 227, A 95 = 5.1°. The mean inclination is in good agreement with the expected value for the last 5 million years, as derived from the synthetic North American polar wander path [Besse and Courtillot 2002, Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300], but a measured rotation of the palaeodeclination of about 8° with respect to the expected direction suggests the possibility of a clockwise rotation of the studied ChVA units. We have estimated the characteristics of palaeosecular variation through study of the scatter of virtual geomagnetic poles, obtaining a palaeosecular variation parameter S b = 14.5° with upper limit S U = 19.6° and lower limit S L = 11.7°, in reasonable agreement with the fit of model G [McFadden et al., 1988, Dipole/quadrupole family modeling of paleosecular variation: Journal of Geophysical Research, v. 93, no. B10, p. 11583–11588; 1991, Reversals of the Earths magnetic field and temporal variations of the dynamo families: Journal of Geophysical Research, v. 96, no. B3, p. 3923–3933] to the Johnson et al. [2008, Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows: Geochemistry, Geophysics, Geosystems, v. 9, no. 4, ID Q04032, doi:10.1029/2007GC001696] databases for the last 5 million years. In those cases in which age determinations are available, the polarity obtained for the studied flows is consistent with their stratigraphic positions, except for the Huitepec site, which probably reflects the transitional geomagnetic regime prior to the Matuyama–Brunhes geomagnetic reversal.