Are plumes predicted by realistic Are plumes predicted by realistic convection experiments and convection experiments and

numerical situations?numerical situations?

John WatsonJohn Watson

Convection ExperimentsConvection Experiments

Griffiths, RW – Griffiths, RW – The differing effects of compositional and The differing effects of compositional and thermal buoyancies on the evolution of mantle diapirsthermal buoyancies on the evolution of mantle diapirs

Laboratory experiment involving oils of Laboratory experiment involving oils of different viscosities and temperaturedifferent viscosities and temperature

Driving Forces for buoyancyDriving Forces for buoyancyINTRINSIC Vs TEMPERATUREINTRINSIC Vs TEMPERATURE

EntrainmentEntrainment

Photograph taken from Campbell and Griffiths, . Photograph taken from Campbell and Griffiths, . Implications of mantleImplications of mantle plume structure for the evolution of flood basalts plume structure for the evolution of flood basalts . Earth and . Earth and Planetary Science letters, 99, 79-93Planetary Science letters, 99, 79-93

Convection Experiments Convection Experiments part 2part 2

Results of studyResults of study ““Diapir convection is a function of the relative Diapir convection is a function of the relative

contributions of the two properties (intrinsic and contributions of the two properties (intrinsic and temperature).”temperature).”

Entrainment of surrounding material contributes to the Entrainment of surrounding material contributes to the driving forces.driving forces.

Research leads onto the Griffiths and Campbell paper - Research leads onto the Griffiths and Campbell paper - Stirring and structure in mantle starting plumesStirring and structure in mantle starting plumes

Convection Experiments Convection Experiments part 3part 3

A plume rises from the Core-A plume rises from the Core-Mantle-Boundary (CMB), via Mantle-Boundary (CMB), via a “narrow conduit”, through a “narrow conduit”, through which “buoyant liquid” flows.which “buoyant liquid” flows.

Plume head spreads to Plume head spreads to 1000km in diameter until 1000km in diameter until lithosphere causes the plume lithosphere causes the plume to spread laterally to around to spread laterally to around 2000km.2000km.

Cause of Flood Volcanism?Cause of Flood Volcanism? Diagram taken from P. Van Keken, Earth and planetary Diagram taken from P. Van Keken, Earth and planetary

science letters, 148, 1997science letters, 148, 1997

Limitations of Convection Limitations of Convection experimentsexperiments

Both these experiments and several others ignore Both these experiments and several others ignore PRESSURE as a parameter.PRESSURE as a parameter.

Griffiths et al state that the model cannot explain all Griffiths et al state that the model cannot explain all hot-spot events.hot-spot events.

Therefore the research cannot fully account for the Therefore the research cannot fully account for the original plume argument (Morgan, 1971).original plume argument (Morgan, 1971).

Numerical and computer simulationsNumerical and computer simulations

““Numerical simulations Numerical simulations of plumes reproduce of plumes reproduce

many of the many of the geophysical geophysical

observations.” observations.” (Courtillot et al, 2003)(Courtillot et al, 2003)

Picture taken from Picture taken from http://geology.about.com/library/weekly/aahttp://geology.about.com/library/weekly/aa011401a.htm011401a.htm

A Hotspot Alternative. The rise of a surface-A Hotspot Alternative. The rise of a surface-related explanation is turning things related explanation is turning things upside down upside down

Numerical and computer simulations Numerical and computer simulations part 2part 2

Ribe, NM and Christensen UR – Ribe, NM and Christensen UR – The The

dynamical origin of Hawaiian volcanismdynamical origin of Hawaiian volcanism.. A model for eruption rate predictions and timing of A model for eruption rate predictions and timing of

volcanism.volcanism. Model Model ASSUMESASSUMES plume model based on “surface plume model based on “surface

signatures” e.g.;signatures” e.g.; The swell under Hawaii, Geoid anomalyThe swell under Hawaii, Geoid anomaly Flexural arch produced by loading of the elastic lithosphereFlexural arch produced by loading of the elastic lithosphere

Numerical and computer simulations Numerical and computer simulations part 3part 3

Tarduno et al – Tarduno et al – The Emperor Seamounts: The Emperor Seamounts: southward motion of the Hawaiian hot-spot plume in southward motion of the Hawaiian hot-spot plume in Earth’s mantle.Earth’s mantle.

Based around computer modelling of lithology Based around computer modelling of lithology inclination values from three seamounts from the chain.inclination values from three seamounts from the chain.

Results are inconsistent with a fixed plume model.Results are inconsistent with a fixed plume model. Hypothesis: plume has remained fixed, but Hypothesis: plume has remained fixed, but

asthenospheric channelling of the plume alters course asthenospheric channelling of the plume alters course of the conduit, leading to bend in the Emperor chain.of the conduit, leading to bend in the Emperor chain.

Numerical and computer simulations Numerical and computer simulations part 4part 4

Inclinations show a Inclinations show a South/South-East motion South/South-East motion of 5 - 10of 5 - 10° during the last ° during the last 100myrs.100myrs.

PROBLEMPROBLEM – no – no evidence in this paper evidence in this paper for alteration of the for alteration of the plume conduitplume conduit

ConclusionsConclusions

Neither Convection experiments or Numerical Neither Convection experiments or Numerical simulations can prove Morgan's theory. simulations can prove Morgan's theory.

Key parameters overlooked.Key parameters overlooked.

The studies assume that their models are The studies assume that their models are realistic when compared to actual mantle realistic when compared to actual mantle conditions.conditions.