Shallow
-
level sill emplacement can uplift Earth’s surface via forced folding,
providing insight into the location and size of potential volcanic eruptions. Linking the
structure and dynamics of ground deformation to sill intrusion is
thus critical in volcanic
hazard assessment. This is challenging, however, because: (1) active intrusions cannot be
directly observed, meaning that we rely on transient host rock deformation patterns to
model their structure; and (2) where ancient sill
-
fo
ld structure can be observed,
magmatism and deformation has long
-
since ceased. To address this problem, we combine
structural and dynamic analyses o
f the Alu dome, Ethiopia; a 3.5
-
km
-
long, 346
-
m
-
high,
elliptical dome of outward
-
dipping, tilted lava flows c
ross
-
cut by a series of normal
faults. Vents distributed around Alu feed lava flows of different ages that radiate out from
or deflect around its periphery. These observations, coupled with the absence of bounding
faults or a central vent, implies that Alu
is not a horst or a volcano, as previously thought,
but is instead a forced fold.
Interferometric synthetic aperture radar
data captured a
dynamic growth phase of Alu during a nearby eruption in
A.D.
2008, with periods of
uplift and subsidence previously
attributed to intrusion of a tabular sill at 1 km depth. To
localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be
recognized to be developing above an incrementally emplaced saucer
-
shaped sill, as
opposed to a tabular
sill or laccolith.