Rootless Cone Archetypes and Their Relation to Lava Flow Emplacement Processes
C.W. Hamilton (University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology,
1680 East-West Road, POST 504, Honolulu, HI 96822, USA, ph: +1 808-956-9503, fax:
1-808-956-6322, e-mail: christopher@higp.hawaii.edu); [*S.A. Fagents*] (University of Hawaii at
Manoa, Hawaii Institute of Geophysics and Planetology, 1680 East-West Road, POST 504,
Honolulu, HI 96822, USA, ph: + 1 808-956-3163, fax: +1 808-956-6322, e-mail: fagents@hawaii.edu); T. Thordarson (School of Geosciences, University of Edinburgh, Grant
Institute, The Kings Buildings, West Mains Road, Edinburgh EH9 3JW; ph: +44 (0) 131-650-8526 fax: +44 (0) 131 668 3184; e-mail: thor.thordarson@ed.ac.uk)
Rootless cones are common volcanic structures within mafic lava flows in Iceland and on Mars.
Rootless eruptions are generated by explosive interactions between molten lava and water-bearing substrate and they represent one of the most hazardous aspects of lava flow emplacement. On Mars, low-latitude rootless cone groups are of particular interest because they imply the presence of groundwater (or ice) in the equatorial region, which is generally thought to be devoid of water.
Although volcanic rootless cones have long since been known as the products of explosive lava-water interactions, the specific mechanisms of their formation remain poorly understood. To constrain the controls on the initiation of rootless eruptions, we have conducted detailed surveys of rootless cone groups within the 1783-84 Laki lava using Differential Global Positioning System
(DGPS) measurements (decimeter-scale accuracy in kinematic mode) and ArcGIS. In this study we identify five distinct rootless cone morphologies: (1) simple symmetrical cones with a single central explosion crater; (2) half-cone pairs with two symmetrical halves separated by a V-shaped depression; (3) simple cone chains with explosion centers aligned along a single axis; (4) parallel cone chains with cones aligned in two rows that are separated by a V-shaped depression; and (5) complex rootless cones containing numerous explosion centers that are distributed over a broad area without quasi-linear alignments. Morphological and structural evidence suggests that: (1) Simple cones are formed by localized explosive lava-water interactions that initiate below a lava tube. (2)
Half-cone pairs form where explosions are localized below the center of a lava channel. (3) Simple cone chains form when a cylindrical lava pathway (usually a tube) subsides into a compactable substrate. The subsidence causes maximum strain accumulation along flow axis, which results in fracturing of the pathway floor (analogous to formation of an axial cleft in an elongate tumulus).
The fracture allows molten lava to physically mix with the underlying water-bearing substrate to produce a rootless eruption. The explosions also disrupt the lava pathway and prevent further transportation of lava down-flow. The blockage may then promote enhanced lava inflation in the up-flow region, increase subsidence rates due to localized mass loading, trigger additional fracturing episodes, and cause the active explosion sites to migrate up-flow to form a quasi-linear chain of rootless cones. (4) Parallel cone chains form along lava pathways that have a broad rectangular form rather than a cylindrical shape. In this instance, the subsiding pathway will concentrate strain within hinge zones that run along the margin of the slab (analogous to the monoclinal fractures that bound lava-rise plateaus). Failure along the hinge zones produces explosion sites that are aligned along the margins of the pathway. Once the rootless eruption disrupts the pathway at one locality, the restriction will promote up-flow lava accumulation, increased subsidence rates, additional pathway failures, and an up-flow shift in the location of active explosion sites. Lastly, (5) complex rootless cones are generally not associated with a discrete lava pathway such as a tube, but rather with broad flow of lava. Basal failure at one locality within the broad flow does little to restrict the overall down-flow motion of the lava and so systematic alignments of crater centers do not form due to pathway effects. These rootless cone archetypes provide useful insight into how explosive lava-water interactions initiate and to what extent lava emplacement conditions affect cone group

morphology--both of which are important considerations for interpreting the significance of rootless cones on Earth and Mars.
POSTER
CORRESPONDING AUTHOR: C.W. Hamilton (Student)