hypothesis is currently subject to unusual controversy and discussion. There is
a remarkable work done by Don Anderson
of the California Institute of Technology
and Gill Foulger of Durham and others who proposed that
there is inconsistency between the predictions of the plume model and what is
observed. Like few sites (locations) proposed to be the result of plumes, Foulger noticed that tomography of the
mantle only shows anomalies in the upper mantle.
In Iceland Bijward and Spakman (1999) claimed to have demonstrated a bent plume <500 km
rising form a root zone >1000 km at the core mantle boundary with plume top
of approx. 1200 km and temperatures of 200-300 K. However, Montelli
et al. (2003) claimed enhanced capability for detecting Earth’s interior
structure, unambiguous evidence for at least 5 hotspots originating deep in in
the lower mantle, and their claim that heat source below Iceland has a shallow
origin. The expected heat flow beneath Iceland in the presence of plume does not
exist (Foulger 2003). Current data shows cooled lithosphere beneath
Iceland than at the same depth below the East Pacific Rise.
Foulger et al.
also claimed that / ratio should be higher near the center of the
plume, while / ratios increase
away from the presumed center of the plume, towards the edge of the Icelandic
was one of the original, fundamental properties assigned to plumes, but the
discovery, that; this did not occur for many pairs of “hot spots” was not
considered to be an obstacle, but explained by deflection by convection
currents in the mantle, lateral flow or “plume capture” by ridges. Koppers et al. (2001) used / age
progressions for the last 140 Ma along the Pacific Seamount trail and compared
these with Pacific Plate velocities, which were predicted by them from their
Euler poles of rotation. They come up with conclusion that relative plate
motion between hotspots was required to fulfil the observed age progressions.
They predicted that the Pacific hot spots show motion of between 10-60 mm/y for
the past 100 Ma.
The plume model
suggests that the bend in the Emperor Seamount chain demonstrates a change in
direction of movement of the Pacific Plate. Sea floor spreading strike slip
faults do not demonstrate this change in direction of movement.
In Iceland, a
hotspot track is not observed, according to Foulger.
There is no evidence for high temperatures required for hotspots and the
crustal structure is inconsistent. No picrite glass can be found, which is
common in areas of high temperatures. The temperature of calculated primary
melts is similar to mid-ocean ridges and there is no heat flow anomaly. The
mantle seismic anomaly does not extend down to the lower mantle. Iceland is now
found to be a ‘thickspot’ with a crust of about 30 km in thickness with normal
temperatures and a cold crust.
Foulger et al. (2003)
points out that picrites are not found in Iceland as Plume induced melting is
due to high temperature magmas. Central Iceland lavas erupt at temperatures of
~1240 C. This temperature is close to melt temperatures in depleted N-MORB, so
do not show a particularly high thermal anomaly.
longevity of plumes varies from about 80 Ma (e.g., Hawaii) to only a few Ma,
e.g., the Caroline chain in the Pacific Ocean. The plume head-tail model, which
arose from laboratory convection experiments, was applied to some melting
anomalies e.g., the Deccan Traps – Laccadive-Chagos ridge – Reunion system, which appear superficially to fit the model well. The many
LIPs without chains, and chains without LIPs, have also attributed to plumes,
however, and these problems largely ignored, along with the problem that the
predicted precursory kilometer-scale uplift is observed at some localities but
not at others.