This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated from the release of initially fluidized or nonfluidized columns of fine particles (diameter d = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d 0 = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness (d 0 ) to a maximum of about twice that of flows on a smooth substrate when d 0 = 1.5-3 mm, before decreasing moderately at higher roughness values of d 0 = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was strongly reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward air flux escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially nonfluidized flows and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles (d = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on subhorizontal topographies.
The Serdán-Oriental lacustrine basin in the eastern part of the Trans-Mexican Volcanic Belt holds a volcanic field of >30 monogenetic vents. Among them, the ~1000-m-high, ~11 km3 Las Derrumbadas rhyolite twin domes dominate the interior of the basin and are surrounded by smaller scoria cones, lava flows, shields, tuff rings, and maars. Of interest in this area are rare endemic species encountered in some of the maar lakes, as well as the large number of pre-Hispanic archeological sites indicating that the lacustrine environment became attractive as a dwelling hub during the late Holocene. We conducted a stratigraphic and radiocarbon dating study to reconstruct the volcanic history, assess the impact of past eruptions on the environment and pre-Hispanic populations, and evaluate future volcanic hazards. Accordingly, at least 10 volcanoes were identified to be < 25,000 BC of which eight are Holocene in age (Alchichica, Tecuitlapa, Atexcac, Cerro El Brujo, Tepexitl, Aljojuca, Derrumbadas, Piedras Negras). Hence, the central part of the Serdán-Oriental basin should be considered potentially active and new eruptions are likely to occur in the future. Furthermore, we show that the ~AD 20 Las Derrumbadas eruption is one of the most voluminous silicic effusive eruptions during the Holocene worldwide. This eruption possibly triggered a migration of human populations from dispersed rural hamlets in the central part of the basin toward fewer larger urban sites (e.g. Cantona) at its margins. Finally, the young ages of the maars imply that the unique biodiversity of their crater lakes must have developed over remarkably short timescales.
The eruption of the ∼10 km3 rhyolitic Las Derrumbadas twin domes about 2000 yrs ago has generated a wide range of volcano-sedimentary deposits in the Serdán-Oriental lacustrine basin, Trans-Mexican Volcanic Belt. Some of these deposits have been quarried, creating excellent exposures. In this paper we describe the domes and related products and interpret their mode of formation, reconstructing the main phases of the eruption as well as syn-and-post eruptive erosional processes. After an initial phreatomagmatic phase that built a tuff ring, the domes grew as an upheaved plug lifting a thick sedimentary pile from the basin floor. During uplift, the domes collapsed repeatedly to form a first-generation of hetero-lithologic hummocky debris avalanche deposits. Subsequent dome growth produced a thick talus and pyroclastic density currents. Later, the hydrothermally-altered over-steepened dome peaks fell to generate 2nd generation, mono-lithologic avalanches. Subsequently, small domes grew in the collapse scars. From the end of the main eruptive episode onwards, heavy rains remobilized parts of the dome carapaces and talus, depositing lahar aprons. Las Derrumbadas domes are still an important source of sediments in the basin, and ongoing mass-wasting processes are associated with hazards that should be assessed, given their potential impact on nearby populations.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5752296
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