Contents

Introduction

Fig. 1. Passive subduction of young oceanic lithosphere
Jeroen van Hunen
Fig. 2. Sea water percolation at mid-ocean ridges
Stan Schoofs
Fig. 3. Hot small-scale upwellings
Jeroen de Smet
Fig. 4. Salt water intrusion in a coastal hydrogeologic system
Gu Oude-Essink
Fig. 5. Thermal convection in 2-D spherical mantle model
Hana Cizkova
Fig. 6. Mantle convection under an immobile lid
Volker Steinbach
Fig. 7. Convection in the Earth's mantle including a phase transition
Arie van den Berg
Recent publications


Introduction

A central theme of the research of the Theoretical Geophysics group is the planetary evolution. The research consists for the main part of numerical modelling studies of thermo-mechanical models of flow and transport processes in the interior of the Earth and other terrestrial planets. The planetary interiors are subject to secular cooling and the conditions for the flow processes involved change with the decreasing internal temperature. One of the research goals related to this secular cooling is to clearify the nature of the formation of continental crust and mantle root during the Archaean period 4-2.5 Ga before present in the Earths early history (Fig. 3). The other terrestrial planets: Mercury, Venus and Mars with their apparantly very different dynamics are interesting testing grounds for models of planetary dynamics, investigated by the Theoretical Geophysics group (Fig. 6). Another area of interest is the investigating of upper mantle convective circulation, in particular the interaction of oceanic plates and overriding continents in subduction zones (Fig. 1). The impact of the mechanical boundary imposed by the solid state phase-transition near 670 km on mantle convective circulation is another topic of current research in the Theoretical Geophyscics group, associated with the question of whole mantle or layered mantle convection (Fig. 5), (Fig. 6). Transport processes in porous media are getting increasingly important in geology and geodynamics. Within the Theoretical Geophysics group research in this area is aimed at the segregation of partial melt in mantle flow systems, which forms a key processes in the formation of oceanic and continental crust and mantle. Also groundwater transport processes (Fig. 4) and in particular hydrothermal circulation (Fig. 2) are investigated by means of numerical modelling. The main common characteristic of the different research topics of the Theoretical Geophysics group is the important role of transport processes on different scales ranging from the planetary scale in whole or layered mantle convection (Fig. 7), to kilometer scale in models of hydrothermal circulation in oceanic crust (Fig. 2) and groundwater flow systems (Fig. 4).

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Figure 1: Passive subduction of young oceanic lithosphere by an old overriding continent. Two model calculations show the importance of friction heat: in the left hand column, subduction ceases due to lack of frictional heating, while in the right column, where friction heat is incorporated, it continues for at several millions of years.

Jeroen van Hunen

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Figure 2: During crust formation at mid-ocean ridges, percolating seawater efficiently cools the magmatic intrusion below the ridge and alters the newly formed basalt.Temperature and pressure conditions at the base of the permable crust may lead to phase separation of the circulation seawater into a diluted vapor and a very saline brine. These images show the temperature and chemical field at a typical stage during the depletion of a brine-saturated layer at the bottom, due to mixing with the overlying seawater. Rising hot plumes appear as high temperature vents (so-called 'black smokers') at the seafloor.

Stan Schoofs








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Figure 3: Hot small-scale upwellings impinge on the stable depleted continental root (red layer in the Figure b). The arrows point to the associated partial melting diapiric event through which the root grows. Figure a are lateral temperature variations in the upper mantle convection model. White and black lines indicate counter-clockwise and clockwise flows, respectively. The thick black and thin white layers on top of the depleted zone in Figure b are the lower and upper crust, respectively.

Jeroen de Smet

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Figure 4: The pictures on the right show salt water intrusion in a coastal hydrogeologic system in the northern part of Noord-Holland, The Netherlands, where a non-uniform density distribution occurs. The salinisation of the top layer as a function of time is severe. This process is generated by lake reclamations during the past centuries, creating low-lying polder areas.

Gu Oude-Essink

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Figure 5: The results of numerical simulations of thermal convection in the mantle. 2-D spherical model with depth dependent viscosity was used to study the coupling between the two flow systems separated by an impermeable boundary. In the model with a viscosity increase at the boundary the strong viscous coupling develops, while in the model with a low viscosity zone beneath the boundary thermal coupling prevails, giving rise to seemingly continuous up- or downwelling structures.

Hana Cizkova

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Figure 6: The colour images on the right show the temperature field and the degree of melting associated with mantle convection under an immobile lid of 200 km thickness. The convective structure as depicted by the temperature field is compatible with observations by the Magellan space craft. The high degree of melting in the upper mantle indicates a versy unstable situation that may trigger a global resurfacing event, which seems to have happened on Venus some 700 million years ago.

Volker Steinbach

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Figure 7: Convection in the Earth's mantle including an endothermic phase transition at 670 km depth. A composite temperature and pressure dependent rheology has been used, combining both linear and non-linear creep mechanisms. (a)+(b) temperature and streamlines. The phase-transition causes a sharp increase in temperature, resulting in a low viscosity layer (c), where non-linear creep dominates (d). The predominance of non-linear creep may show up in anisotropy of seismic wave propagation.

Arie van den Berg

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