Engineering topics

Marsh, Bruce D.

Publications and source records attributed to Marsh, Bruce D..

7 records

NASA NTRS1990

Transient magmatic convection prolonged by solidification

Fluid dynamic experiments have been conducted on the solidification of a paraffin layer, in order to elucidate the transient stage of convection created in cooling magma by the fact that strong changes in viscosity with crystallization lock up within an inwardly propagating crust much buoyancy that would otherwise be available to drive convection. The interior of the magma remains isothermal, and the temperature decreases uniformly until it is locked at the convective liquidus; the crystals are fine hairlike dendrites without major compositional differentiations. Measurements over time are presented of crust thickness, convective velocity, and heat transfer.

Brandeis, Genevieve↗

NASA NTRS1989

The convective liquidus in a solidifying magma chamber - A fluid dynamic investigation

An experimental study of the solidification of a layer of paraffin cooled from above is reported. When the fluid is superheated, convection sets in at the beginning of cooling but rapidly decreases in intensity. Once the layer has lost its superheat, convection ceases and further cooling is by conduction. The interior temperature then remains constant until encountered by the upper crust. The convective liquidus is defined as the temperature threshold below which convection is very weak or nonexistent. In natural basaltic systems the convective liquidus, although strictly unknown, may be very close to the true liquidus. Rapid convection may therefore not be a dominant process during the crystallization of many magmna chambers; instead, convection is part of an overall intimate balance between phase equilibria, crystal growth, and heat transfer.

Brandeis, Genevieve↗

NASA NTRS1989

The delineation and interpretation of the earth's gravity field

In an attempt to understand the mechanical interaction of a growing lithosphere containing fracture zones with small and large scale mantle convection, which gives rise to geoid anomalies in oceanic regions, a series of fluid dynamical experiments is in progress to investigate: (1) the influence of lithosphere structure, fluid depth and viscosity field on the onset, scale, and evolution of sublithospheric convection; (2) the role of this convection in determining the rate of growth of lithosphere, especially in light of the flattening of the lithosphere bathymetry and heat flow at late times; and (3) combining the results of both numerical and laboratory experiments to decide the dominate factors in producing geoid anomalies in oceanic regions through the thermo-mechanical interaction of the lithosphere and subjacent mantle. The clear existence of small scale convection associated with a downward propagating solidification front (i.e., the lithosphere) and a larger scale flow associated with a discontinuous upward heat flux (i.e., a fracture zone) has been shown. The flows exist simultaneously and each may have a significant role in deciding the thermal evolution of the lithosphere and in understanding the relation of shallow mantle convection to deep mantle convection. This overall process is reflected in the geoid, gravity, and topographic anomalies in the north-central Pacific. These highly correlated fields of intermediate wavelength (approx. 200 to 2000 km) show isostatic compensation by a thin lithosphere for shorter (less than or equal to approx. 500 km), but not the longer, wavelengths. The ultimate, dynamic origin of this class of anomalies is being investigated.

Marsh, Bruce D.↗

NASA NTRS1989

Magma chambers

Recent observational and theoretical investigations of terrestrial magma chambers (MCs) are reviewed. Consideration is given to the evidence for MCs with active convection and crystal sorting, problems of direct MC detection, theoretical models of MC cooling, the rheology and dynamics of solidification fronts, crystal capture and differentiation, convection with solidification, MC wall flows, and MC roof melting. Diagrams, graphs, and a list of problems requiring further research are provided.

Marsh, Bruce D.↗

NASA NTRS1988

The delineation and interpretation of the earth's gravity field

A series of fluid dynamical experiments in variable viscosity fluid have been made and are in progress to study: (1) the onset of small scale convection relative to lithosphere growth rate; (2) the influence of paired fracture zones in modulating the horizontal scale of small scale convection; (3) the influence of the mantle vertical viscosity structure on determing the mode of small scale convection; and (4) the 3-D and temporal evolution of flows beneath a high viscosity lid. These experiments extend and amplify the present experimental work that has produced small scale convection beneath a downward-moving solidification front. Rapid growth of a high viscosity lid stifles the early onset of convection such that convection only begins once the lithosphere is older than a certain minimum age. The interplay of this convection with both the structure of the lithosphere and mantle provide a fertile field of investigation into the origin of geoid, gravity, and topographic anomalies in the central Pacific. These highly correlated fields of intermediate wavelength (approximately 200 to 2000 km), but not the larger wavelengths. It is the ultimate, dynamic origin of this class of anomalies that is sought in this investigation.

Marsh, Bruce D.↗

NASA NTRS1988

Dynamic compensation in the central Pacific Ocean

The intermediate-wavelength geoid (lambda similar to 2000 km) and sea-floor topography fields in the central Pacific Ocean were studied in terms of static and dynamic compensation models. Topographic features on the sea-floor with lambda less than 1000 km were found to be compensated both regionally, by the elastic strength of the lithosphere, and locally, by displacing mantle material to reach isostatic adjustment. The larger-scale sea-floor topography and the corresponding geoid anomalies with lambda similar to 2000 km cannot be explained by either local or regional compensation. The topography and the resulting geoid anomaly at this wavelength were modeled by considering the dynamic effects arising from viscous stresses in a layer of fluid with a highly temperature-dependent viscosity for the cases of: (1) surface cooling, and (2) basal heating. In this model, the mechanical properties of the elastic part of the lithosphere were taken into account by considering an activation energy of about 520 kJ/mol in the Arrhenius law for the viscosity. Numerical predictions of the topography, total geoid anomaly, and admittance were obtained, and the results show that the thermal perturbation in the layer, which accounts for the mass deficit, must be located close to the surface to compensate the gravitational effect of the surface deformation. For the case of basal heating, the temperature dependence of viscosity results in a separation of the upper, quasi-rigid lid from the lower mobile fluid, hence inhibiting the development of a compensating thermal perturbation at shallow depths. The results clearly rule out small-scale, upper-mantle convection as the source of these anomalies. Instead, the geophysical observables can be well explained by a shallow, transient thermal perturbation.

Hinojosa, Juan Homero↗

NASA NTRS1987

The delineation and interpretation of the Earth's gravity field

The geoid and topographic fields of the central Pacific were delineated and shown to correlate closely at intermediate wavelengths (500 to 2500 km). The associated admittance shows that anomalies having wavelengths less than about 1000 km are probably supported by the elastic strength of the lithosphere. Larger wavelength anomalies are due to dynamic effects in the sublithosphere. Direct modeling of small scale convection in the asthenosphere shows that the amplitudes of observed geoid and topographic anomalies can be independently matched, but that the observed admittance cannot. Only by imposing an initial regional variation in the thermal regime is it possible to match the admittance. It is proposed that this variation may be due to differences in the onset time of convection beneath the lithosphere of different ages. That is, convection beneath thickening lithosphere is strongly dependent on the rate of thickening (V) relative to the rise time for convection. The critical Rayleigh number contains the length scale K/V, where K is thermal diffusivity. Young, fast growing lithosphere stabilizes the underlying asthenosphere unless it has an unusually low viscosity. Lithosphere of different age, separated by fracture zones, will go unstable at different times, producing regional horizontal temperature gradient that may strongly influence convection. Laboratory and numerical experiments are proposed to study this form of convection and its influence on the geoid.

Marsh, Bruce D.↗