IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P1_3B-O17

Room A5

Date/Time: July 21 15:30-15:45

Magma withdrawal below ground-water table as a trigger to form large maars over highmagma discharge rate fissure-fed lava spatter/scoria cones: the Al Wahbah Crater(Harrat Kishb, Saudi Arabia)
Karoly Nemeth1 , Mohammed Rashad Moufti2 , Nabil El-Masry2 , Atef Qaddah2
1Institute of Agriculture and Environment, Massey University, Private Bag 11222, Palmerston North,New Zealand, 2 Faculty of Earth Sciences, King Abdulaziz University, P.O. Box 80200, Jeddah, 21589,
Saudi Arabia
E-mail: k.nemeth@massey.ac.nz

Al Wahbah Crater is one of the largest and deepest Quaternary maar crater (NW-SE-elongated, appx.2.3-km
wide and 250-m deep with an irregular near-perpendicular crater wall) not only in the Arabian Peninsula but
also among other craters of volcanic fields on Earth. It is located in the SW edge of the bimodal (alkali olivinebasalt/phonolite) Harrat Kishb (2Ma-Recent) that formed lava flows over 6000-km2. Al Wahbah Crater cuts deeply
into the pre-eruptive surface exposing the Proterozoic diorite basement that is covered by at least two basanitic
lava sheets and a lava spatter/scoria cone defining the crater as a maar. The half-sectioned pre-maar cone in theNW demonstrate that the maar collapse post-date the lava schield and cone formation. The lack of evidences
to support significant time break between the pre-maar cone-building pyroclastic beds and the dune-bedded
accidental lithic-rich pyroclastic beds that form the tuff ring suggests that the pre-maar cone (and its lava flow)
and the tuff ring formation is in a time-continuum and they formed in the same eruptive episode. In this respect
Al Wahbah’s eruption is unique and differs from maar formation that implies initial magma-water interaction-driven
explosions near surface followed by gradual explosion locus down-migration as a result of gradual exhaustionof ground-water sources (Lorenz’s model). Al Wahbah seems to have been followed an opposite eruption pathstarted with an initial lava shield and cone-building phase that have been intervened by phreatomagmatic explosive
eruptions that have been culminated in a maar collapse and tuff ring formation. This scenario can be best explainedby the drop of the magma discharge rate causing magma withdrawal below regional ground-water table. The drop in
the magmatic pressure in the conduit due to the decrease in magma rise speed enhanced ground-water to enter tothe top of the withdrawn magma column through cracks along the unsupported solidified mnargin of the conduit wall.
As a result, effective thermohydraulic explosion took place initially in the time when the same volcanic system still
emitted lava flows that rafted cone material and damping tuff breaccia on top of the still moving lava flows allowing
protrusions of lava into the freshly deposited wet pyroclastic blanket. In the main phase of the maar-forming eventthe reduced magma discharge rate shifted the efficiency of the explosive phreatomagmatic magma fragmentation
to its maximum to produce base surges that were travelled radially at least 2-km from their source leaving behind aquickly thinning blanket of dune-bedded pyroclastic deposit that covered, plastered and engulfed everything aroundthe newly formed maar. It seems Al Wahbah represents a rare but potentially one of the most hazardous volcanic
phenomena documented in the Harrat Kishb that is essential to understand to develop accurate volcanic hazard
scenarios at the Harrat Kishb and elsewhere.

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