HR: 1330h
AN: V22A-0562    [PDF]
TI: Rubbly Pahoehoe Lavas: An Important Component of Icelandic Basaltic Lava Flows
AU: * Guilbaud, M
EM: m.guilbaud@open.ac.uk
AF: Earth Sciences Dpt, Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Self, S
EM: stephen.self@open.ac.uk
AF: Earth Sciences Dpt, Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Blake, S
EM: s.blake@open.ac.uk
AF: Earth Sciences Dpt, Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Thordarson, T
EM: torvth@hi.is
AF: University of Iceland, Dunhaga 3, Reykjavik, IS-107 Iceland
AU: Keyzthelyi, L
EM: laz@usgs.gov
AF: USGS Astrogeology, 2255 N. Gemini Dr., Flagstaff, AZ86001 United States
AB: Eruptions at rift zones often produce basaltic lava flows. Structural and textural study of historic and prehistoric flows gives important clues about emplacement processes. The 1783-4 eruption of Laki produced 14.7 km$^{3}$ of lava from a fissure that opened along the Eastern Volcanic Zone of Iceland. The resulting 600 km$^{2}$ lava flow-field presents a wide range of surface morphologies that we explored to study emplacement mechanisms. We present results from preliminary field, macroscopic, and microscopic analysis. Field observations show that along a single flow surface morphologies change from (1) flat (with a coherent continuous, pahoehoe(phh)-like crust), to (2) slabby (with a disrupted surface made of phh-like slabs), to (3) rubbly (with a rough surface covered by loose vesicular blocks), and then to (4) folded rubbly (with ridges of rubble several meters high). Features characteristic of flow growth by inflation are abundant. The changes are unidirectional in the above order but the sequence can be repeated. This occurred if the fluid lava stored within the core of the flow broke through the front. Field relationships indicate that the majority of the flows were initially emplaced as small lobes of phh-type lava that gradually changed into slabby phh and then rubbly phh through progressive disruption, shearing and compression of the surface. During its entire advance, the lava flow never reached the point of incessant surface renewal with formation of aa-type clinker, neither did it extensively develop the smooth filamentous surface common in Hawaiian phh flows. Thus, Laki lavas are intermediate flows. Microtexture analysis of samples collected along single flows will tell us what caused this evolution, such as crystallinity due to degassing or viscosity change. This flow type has been recognized on Reykjanes Peninsula in young fissure-fed lava flow-fields and in the Columbia River Basalts and Kerguelen Plateau flows. It is therefore an important component of many lava flow-fields, being in some cases a more frequently occurring flow type than the phh and aa end-members.
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting