Disruption of tephra fall deposits caused by lava flows during basaltic eruptions

Abstract

Further information on publisher's website:http://dx.doi.org/10.1007/s00445-015-0974-3Publisher's copyright statement:The nal publication is available at Springer via http://dx.doi.org/10.1007/s00445-015-0974-3Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Observations in the USA, Iceland, and Tenerife, Canary Islands, reveal how processes occurring 13 during basaltic eruptions can result in complex physical and stratigraphic relationships between lava 14 and proximal tephra fall deposits around vents. Observations illustrate how basaltic lavas can 15 disrupt, dissect (spatially and temporally) and alter sheet-form fall deposits. Complexity arises 16 through synchronous and alternating effusive and explosive activity that results in intercalated lavas 17 and tephra deposits. Tephra deposits can become disrupted into mounds and ridges by lateral and 18 vertical displacement caused by movement (including inflation) of underlying pāhoehoe lavas and 19 clastogenic lavas. Mounds of tephra can be rafted away over distances of 100s to 1000s m from 20 proximal pyroclastic constructs on top of lava flows. Draping of irregular topography by fall 21 deposits and subsequent partial burial of topographic depressions by later lavas can result in 22 apparent complexity of tephra layers. These processes, deduced from field relationships, have 23 resulted in considerable stratigraphic complexity in the studied proximal regions where fallout was 24 synchronous or alternated with inflation of subjacent lava sheets. These mechanisms may lead to 25 diachronous contact relationships between fall deposits and lava flows. Such complexities may 26 remain cryptic due to textural and geochemical quasi-homogeneity within sequences of interbedded 27 basaltic fall deposits and lavas. The net effect of these processes may be to reduce the usefulness of 28 data collected from proximal fall deposits for reconstructing basaltic eruption dynamics. 29 30

Well-preserved fallout basaltic tuff in central Paraná-Etendeka Large Igneous Province: pyroclastic evidence of high fire-fountain eruptions

“…Dynamic and evolving environments prevail around volcanic vents, especially during long-lasting ood basalt eruptions. Massive lava ows can eventually cover extensive areas, potentially burying signi cant pyroclastic material, leading to complex interactions and overlaps between products from various vents, resulting in intricate stratigraphy (Brown et al 2015).…”

Stratigraphy and Geochemistry of a Fond St. Jean Cinder Cone, Dominica

“…The brecciated basalt unit SJB-1 (3d) shows matrix olivine with reaction rims altered to pyroxene. ____________________________________________________ to determine the temporal relationship of eruptive styles, as lava flows can disrupt pyroclastic deposits after emplacement through a number of complex mechanisms (Brown et al 2015).…”

Melt stripping and agglutination of pyroclasts during the explosive eruption of low viscosity magmas

“…However, standard field-based techniques used to quantify GSDs at ultramafic and mafic volcanoes are notoriously problematic. The deposits generally have poor preservation and can be rheomorphic in nature 11 , 20 , are often redistributed by coeval lava flows 39 , fractures can heal during eruption 40 , pyroclasts are susceptible to secondary fragmentaton 2 , 16 , 41 and scoria cones feature a complex stratigraphy, commonly resulting from numerous indistinguishable explosive events or episodes. Currently we rely on, and are limited to, real-time monitoring of active volcanoes via thermal videography to document the GSD produced by explosive mafic eruptions 16 , 38 , 42 .…”