THE MORPHOLOGY AND FORMATION OF FLOW-LOBE TUMULI ON ICELANDIC SHIELD VOLCANOS

Three types of tumuli are recognized on monogenetic shield volcanoes i n 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 pahoe hoe flow fields, whereas the other two tumulus types are more frequent in the proximal parts of the flow fields. The flow-lobe tumuli also d iffer from the two other tumulus types as regards their morphology: th ey are significantly larger, have shallower flanks, and do not have ex tensive outflows from the cracks. The vesicle distribution within the flow-lobe tumuli is similar to that of P-type flow lobes: a fairly den se zone is situated between the upper and lower vesicular zones. Each flow-lobe tumulus is an individual lobe that inflates and forms tensio n cracks in the lava crust. Major lava tubes are mostly absent from th e areas where flow-lobe tumuli form and, therefore, the emplacement of flow-lobe tumuli must be related to secondary, short-lived pathways o f lava. The flow-lobe tumuli inside the flow fields of Icelandic shiel d volcanoes are morphologically identical to those flow-lobe tumuli th at are found in the margins of flow fields of other Icelandic volcanoe s. These observations allow us to model the flow-lobe tumuli as morpho logical variants of P-type flow lobes forming along terminating second ary 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 la va crust as a result of magmatic overpressure in the associated lava c ore. We model the tumulus as a circular elastic plate that becomes upl ifted, inflated and fractured by a uniform magmatic overpressure at it s bottom. We conclude that (1) the effective thickness of the lava cru st that is subject to bending is 0.2-0.4 m and remains essentially con stant during the formation of the tumulus, and (2) for flow-lobe tumul i that are supplied with magma from tubes that originate in overflow f rom the lava lake of a shield volcano, the elevation difference betwee n Be tumuli and the surface of the source lava lake is commonly around 100 m. This elevation difference can give rise to a static magmatic o verpressure of 2.5 MPa in the tumulus, whereas the overpressure needed to form a typical flow-lobe tumulus is only 0.2-1 MPa.