THE INTERACTION OF A PLUME WITH A RHEOLOGICAL BOUNDARY - A COMPARISONBETWEEN 2-DIMENSIONAL AND 3-DIMENSIONAL MODELS

The discrete nature of volcanic hot spot chains, whose origin is belie ved to be related to mantle plumes, is suggestive that the plumes feed ing them may be pulsating. A possible origin of the pulsations is the interaction of mantle plumes with a theological interface in the trans ition zone. We have studied the time-dependent, three-dimensional inte raction of an upwelling mantle plume with a theological interface that separates a Newtonian lower mantle from a Newtonian upper mantle with rt,lower viscosity. Previous two-dimensional (2-D) work demonstrates that when a hot plume enters a non-Newtonian layer with lower viscosit y, the hot material is carried from the interface in pulsating, diapir ic events. A purely Newtonian analog to this model mimics this behavio r in a qualitative manner if the viscosity contrast across the interfa ce is larger than 2 orders of magnitude. In this work, we have used th is Newtonian model to make three-dimensional (3-D) modeling feasible. A combined spectral/finite difference code has been employed to solve the 2-D and 3-D convection equations in Cartesian coordinates, We have varied the Rayleigh number (scaled to the constant viscosity of the l ower mantle) between 10(4) and 10(5) and the viscosity contrast of the layers between 10 and 1000. In 2-D situations with low viscosity cont rasts, a plume thins in a steady state fashion as it enters the upper layer. At moderate and higher viscosity contrasts (above 50), the plum e shows a pulsating behavior. This transition fi om ''thinning'' to '' pulsating'' is found to be strongly dependent on viscosity contrast an d only moderately dependent on Rayleigh number. The frequency of the p ulsations is linearly dependent on Rayleigh number and viscosity contr ast. In three dimensions, the plume thins less and is therefore less s usceptible to disruption by shear and the plume moves through the uppe r layer in a steady state fashion. This disappearance of pulsating beh avior in three dimensions indicates a case where conclusions about plu me dynamics in two dimensions do not apply to 3-D situations.