Download Towards a better understanding of hot spot volcanism

Sheet n°285 - January 2008
Towards a better understanding
of hot spot volcanism
© Clouart Valérie
M
ore scientific
knowledge is still
needed about the origin of hot spots. When
this type of volcanism is present right
at the centre of oceanic tectonic plates, it
gives rise to volcanic
constructions such
as the Hawaii archipelago or Reunion
Island. Although it is
already known that
classical hot spots
such as these result
from upsurges of
magma generated at
the Earth’s core-mantle boundary, the origin of some of them is
still a matter of debate.
Researchers from the
IRD and the University
of Chile investigated
the phenomena that
led to the recent activity of seven hot spots
located in the central
Pacific. Numerical
mechanical models
showed that variations
in movement of the
Pacific plate, generating shearing stresses
within it, could facilitate the rise of magma
towards the surface.
This discovery, if corroborated, would indicate that the formation
of certain hot spots
depended
on
the
movement of the tectonic plates. This type
of volcanism would
therefore be less static than hitherto thought and could sometimes proceed at relatively rapid time-scales
of around ten million
years.
Map showing the boundaries of tectonic plates in the Pacific. The area framed in black corresponds to the
geographical zone on which the researchers applied different numerical models. The red spots show the hot
spots listed in this region, the black arrows show the movement of the Pacific plate.
Most of the Earth’s listed active
volcanoes are located at the borders
between two tectonic plates, where
upsurge of magma from the mantle
is facilitated. When these magmatic
uprisings occur at a subduction zone,
where one tectonic plate plunges under
another, they give rise to volcanic
massifs such as the Andes cordillera.
Other volcanic chains are formed along
oceanic ridges, submarine regions
of ocean-floor extension. However,
some volcanoes are governed by
a completely different mechanism :
intraplate volcanism. As their name
suggests, these volcanic constructions
appear in the very centre of tectonic
plates. Scientists now know that some
of them, such as the Hawaii-Emperor
archipelago or Reunion Island, result
from magmatic upsurges generated
at the boundary between the Earth’s
core and mantle situated 2 900 km
deep. Others, such as those of the
central Pacific, display different
characteristics. They are anomalous,
in their simultaneously high number,
unusually high concentration and short
life-span and prompt scientists to look
for hypotheses other than a deepmantle plume to explain the causes of
intraplate volcanism.
Researchers from the IRD and the
University of Chile focused on a
group of islands and archipelagos in
the central Pacific Ocean (Samoa,
Cook, Rurutu, Austral, Tahiti, Marquis,
Pitcairn), each listed as resulting from
hot-spot activity. These scientists
aimed to find out if movements of the
Pacific plate where these seven hot
spots are located could be involved in
their formation. They used numerical
mechanical simulation models of the
effect of the westward displacement
of the Pacific plate on internal
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Sheet n°285 - January 2008
For further information deformations during the past 10 million
CONTACTS :
MURIEL GERBAULT
Laboratoire des
mécanismes de
transfert en géologie
Address :
Departamento de
Geologia, Facultad
de Ciencias Físicas y
Matematicas
Universitad de Chili
Plaza Ercilla 803
Casilla 13518 correo 21
Santiago
Chile
Tel : (56-2) 236 3464
[email protected]
VALÉRIE CLOUART
[email protected]
PRESS OFFICE :
GAËLLE COURCOUX
+33 (0)1 48 03 75 19
[email protected]
INDIGO, IRD PHOTO
LIBRARY :
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+33 (0)1 48 03 78 99
[email protected]
www.ird.fr/indigo
REFERENCE :
VALÉRIE CLOUARD,
MURIEL GERBAULT,
Break-up spots:
Could the Pacific
open as a
consequence of plate
kinematics?, Earth
and Planetary Science
Letters, 2007
Doi : 10.1016/
j.epsl.2007.10.013
KEY WORDS :
Hot spot, plate
tectonics, magma,
shearing
years. This model incorporates a
differential tension regime which acts
on the Pacific plate, the northern part
moving with greater velocity than the
southern part, which undergoes a kind
of braking effect exerted by the bloc
of the Australian plate (see the 3D
diagram). The model shows the region
to be the site of an East-West shear
band which superimposes on the
geographical zone where the seven
hot spots investigated in the study are
grouped.
Another model was subsequently built
up that takes into account the cooling
of the tectonic plate with increased
distance from the oceanic ridge that
generated it. This second model also
brought evidence of a shearing band,
but this zone appeared more diffuse
towards the east than in the first
simulation. Moreover, this more diffuse
shearing zone was superimposed on
an anomaly of the Earth’s surface
classically attributed to an upswelling of
the oceanic lithosphere. This anomaly,
caused by the upward pressure of
the underlying mantle, appears along
with an unexplained variation in the
ocean floor. This second numerical
model therefore indirectly allows the
geographical location of the hot spots
on an East/West line of weakened
lithosphere to be matched with a
variation in its thickness.
The existence of a hot spot is
usually considered to be linked to
the occurrence of a mantle plume,
a sort of giant bubble of magma
generated by the thermal convection
currents circulating in the mantle. This
magmatic bubble exerts an upward
pressure pushing on the base of
the oceanic lithosphere. Then, once
broken through, the latter allows the
magma to erupt through the Earth’s
crust. Although this process effectively
explains the origin of deep-seated hot
spots, it does not provide satisfactory
explanations for other forms of
intraplate volcanism such as that which
occurs in the African rift or certain
more recent hot spots situated in the
central part of the Pacific Ocean.
The results of this study suggest
an alternative scenario which
envisages the involvement of
shearing strain within the tectonic
plates during the formation of a
certain type of hot spot volcanism. In
the central Pacific, such deformation
could therefore be a step towards
the break-up of the Earth’s largest
tectonic plate into two in a timescale of
around ten million years. Furthermore,
if the movements of a tectonic plate
were effectively to play a role in the
formation of a hot spot, that would
signify that such spots would not be
so static as hitherto believed. The
characteristic time-scale for heattransfer processes in the mantle is
in the order of more than 100 million
years whereas the movement of the
plates occurs over shorter geological
time-scales of around ten million years.
Certain hot spots could thus change
and develop in space relatively rapidly,
in line with displacements undergone
by the tectonic plates.
Grégory Fléchet - DIC
Translation : Nicholas Flay
©IRD/ Gerbault Muriel
The southern part of the Pacific plate abuts against the
Australian plate, making it move more slowly than the
northern part, and causing an East/West shearing zone to
develop (dotted area).
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - [email protected]