Garrotxa Volcanism:Propagating Crack or Plumeless Model
The Garrotxa alkaline volcanoes occur along a series of
pre-existing fault lines and there is a increasing body of theory that suggests
this is not as a result of 'hot spots' or deep-seated magma plumes. Volcanism
requires both a source of melt (apparently without exceptionally high
temperatures) and weaknesses in the Earth’s surface, to allow it to escape.
The plume hypothesis requires that plate movements are driven by two independent
modes of convection – plate tectonics and plumes. The former is driven by
forces at plate boundaries – ridge push and slab pull, and the other is driven
by heat from the Earth’s core. The new theory suggests that only one of these
is needed – plate tectonics - with plate movements creating the fault lines
that act as 'escape points' for magma from the upper mantle.
Bruce Railsback (University of Georgia) has developed these ideas in a
series of six diagrams below, with the youngest (and largest) volcanoes found in
the direction of a propagating crack. Plate motion may happen, but it is not
essential to this model.
The Garrotxa volcanoes do not show a clear age-direction
relationship required by Professor Railback's Propagating Crack model. However,
stretching and thinning of the crust in the western Mediterranean resulting from
plate movements has lead to a series of cracks developing within the
Amer-Brugent fault system. These lines of weakness would provide the conditions
for alkaline magma to force its way from the upper mantle to the surface.