The morphology and formation of flow-lobe tumuli on Icelandic shield volcanoes

Abstract

Three types of tumuli are recognized on monogenetic shield volcanoes in Iceland, namely (1) lava-coated tumuli, (2) upper-slope tumuli and (3) flow-lobe tumuli. Flow-lobe tumuli grade into lava rises, which are larger inflation structures, but both structures have a similar mode of emplacement. The areal distribution of flow-lobe tumuli in the flow field differs from that of lava-coated tumuli and upper-slope tumuli. Flow-lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields, whereas the other two tumulus types are more frequent in the proximal parts of the flow fields. The flow-lobe tumuli also differ from the two other tumulus types as regards their morphology: they are significantly larger, have shallower flanks, and do not have extensive outflows from the cracks. The vesicle distribution within the flow-lobe tumuli is similar to that of P-type flow lobes: a fairly dense zone is situated between the upper and lower vesicular zones. Each flow-lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust. Major lava tubes are mostly absent from the areas where flow-lobe tumuli form and, therefore, the emplacement of flow-lobe tumuli must be related to secondary, short-lived pathways of lava. The flow-lobe tumuli inside the flow fields of Icelandic shield volcanoes are morphologically identical to those flow-lobe tumuli that are found in the margins of flow fields of other Icelandic volcanoes. These observations allow us to model the flow-lobe tumuli as morphological variants of P-type flow lobes forming along terminating secondary lava paths. Flow-lobe tumuli form at low lava-supply rates of the order of 10 −2 − 10 −5 m 3/s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core. We model the tumulus as a circular elastic plate that becomes uplifted, inflated and fractured by a uniform magmatic overpressure at its bottom. We conclude that (1) the effective thickness of the lava crust that is subject to bending is 0.2–0.4 m and remains essentially constant during the formation of the tumulus, and (2) for flow-lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano, the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m. This elevation difference can give rise to a static magmatic overpressure of 2.5 MPa in the tumulus, whereas the overpressure needed to form a typical flow-lobe tumulus is only 0.2–1 MPa.