Editing Pyroclastic surge (section)

== Ground surge ==
These deposits are often found at the base of pyroclastic flows. They are thinly bedded, laminated and often cross-bedded.<ref name=":1">{{Cite journal|last1=Douillet|first1=Guilhem Amin|last2=Pacheco|first2=Daniel Alejandro|last3=Kueppers|first3=Ulrich|last4=Letort|first4=Jean|last5=Tsang-Hin-Sun|first5=Ève|last6=Bustillos|first6=Jorge|last7=Hall|first7=Minard|last8=Ramón|first8=Patricio|last9=Dingwell|first9=Donald B.|date=2013-10-13|title=Dune bedforms produced by dilute pyroclastic density currents from the August 2006 eruption of Tungurahua volcano, Ecuador|url= |journal=Bulletin of Volcanology|language=en|volume=75|issue=11|pages=762|doi=10.1007/s00445-013-0762-x|issn=1432-0819|pmc=4456068|pmid=26069385}}</ref> Typically they are about 1&nbsp;m. thick and consist mostly of [[Lithic fragment (geology)|lithic]] and [[Crystal|crystal fragments]] (fine ash elutriated away). They appear to form from the flow itself, but the mechanism is not clear. One possibility is that the head of the flow expands through entrainment of air (which is then heated). This then results in the flow front surging forward, which is then over-run by the rest of the flow.<ref name=":0" />
[[File:Douillet al 2013b Fig. 6 (Bull. Volcanol.).jpg|thumb|Dune bedform formed by the pyroclastic currents related to the 2006 eruption of Tungurahua (Ecuador). A. Outer shape of a lunate dune bedform and B. internal lamination. Note preferential aggradation on upstream face (backset laminations).<ref name=":1" />]]