Maxwell L. Rudolph

Viscosity jump in Earth’s mid-mantle

A mysterious mid-mantle slowdown The viscosity of Earths deep interior plays a key role in mediating plate tectonics. Rudolph et al. combined several geophysical data sets to model the viscosity of the mantle. Mantle viscosity abruptly increases below 1000 km. The increase could explain the stalling of subducting slabs and the deflections of hot upwelling plumes around this depth. Although the viscosity increase explains some recent unexpected observations, the origin of the jump itself remains a mystery. Science, this issue p. 1349 Geodynamic modeling reveals a large viscosity increase in Earth’s mid-mantle. The viscosity structure of Earth’s deep mantle affects the thermal evolution of Earth, the ascent of mantle plumes, settling of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Based on a reanalysis of the long-wavelength nonhydrostatic geoid, we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth. We detect an increase in viscosity at 800- to 1200-kilometers depth, far greater than the depth of the mineral phase transformations that define the mantle transition zone. The viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation, plume deflection, and changes in large-scale structure and offers a simple explanation of these phenomena.

Large‐scale rigid‐body rotation in the mantle wedge and its implications for seismic tomography

Using a combined finite difference and marker-in-cell technique, we performed two-dimensional coupled petrological-thermomechanical numerical simulations of intraoceanic subduction. The simulations indicate that parts of the mantle wedge can become trapped between rheologically weak, hydrated, and partially molten upwellings (cold plumes) and the subducting slab. The structures form at various depths and develop circular, elliptic, or irregular shapes. The combined effect of the tractions caused by upwelling and subduction causes these regions to rotate. Our simulations investigate the parameters controlling the occurrence and long-term stability of such rigid, rotating structures. Circular rotating structures like “subduction wheels” are characteristic of models with relatively young (20–30 Myr) slabs and intermediate (2–5 cm/yr) subduction rates. We propose that the development of such circular features may explain some of the isolated seismic velocity anomalies in the mantle wedge.

Deep and shallow sources for the Lusi mud eruption revealed by surface deformation

The Lusi mud eruption, in East Java, Indonesia, began in May 2006 and continues to the present. Previous analyses of surface deformation data suggested an exponential decay of the pressure in the mud source but did not constrain the location, geometry, and evolution of the possible source(s) of the erupting mud and fluids. To map the surface deformation, we employ multitemporal interferometric synthetic aperture radar and analyze a well-populated L-band data set acquired by the Advanced Land Observing Satellite (ALOS) between May 2006 and April 2011. We then apply a time-dependent inverse modeling scheme. Volume changes occur in two regions beneath Lusi, at 0.3–2.0 km and 3.5–4.75 km depth. The cumulative volume change within the shallow source is ~2–3 times larger than that of the deep source. The observation and model suggest that a shallow source plays a key role by supplying the erupting mud, but that additional fluids do ascend from depths >4 km on eruptive timescales.

History and dynamics of net rotation of the mantle and lithosphere

The net rotation of Earths lithosphere with respect to the underlying mantle is the longest-wavelength component of toroidal flow in the mantle and is sensitive to both mantle buoyancy structure and lateral viscosity variations. The lithospheric net rotation in the geologic past implied by plate reconstructions using a hotspot reference frame for the past 100 Myr is up to five times greater than the present-day rate of lithospheric net rotation. We explore the role of lateral viscosity variations associated with subcontinental keels in producing the lithospheric net rotation for the geologic past and find that the introduction of subcontinental keels improves the agreement between modeled net rotation and the net rotation present in the plate reconstructions for the past 25 Myr. However, our models with continental keels produce at most 0.16°/Myr of differential rotation between the lithosphere and lower mantle for present-day, and explaining the most rapid rates of lithospheric net rotation during the Cretaceous and Paleogene remains challenging. This suggests the need for either an additional mechanism for generating lithospheric net rotation, or an adjustment to the absolute mantle reference frame relative to which plate motions are specified.

On the temporal evolution of long‐wavelength mantle structure of the Earth since the early Paleozoic

The seismic structure of the Earths lower mantle is characterized by a dominantly degree-2 pattern with the African and Pacific large low shear velocity provinces (i.e., LLSVP) that are separated by circum-Pacific seismically fast anomalies. It is important to understand the origin of such a degree-2 mantle structure and its temporal evolution. In this study, we investigated the effects of plate motion history and mantle viscosity on the temporal evolution of the lower mantle structure since the early Paleozoic by formulating 3-D spherical shell models of thermochemical convection. For convection models with realistic mantle viscosity and no initial structure, it takes about ∼50 Myr to develop dominantly degree-2 lower mantle structure using the published plate motion models for the last either 120 Ma or 250 Ma. However, it takes longer time to develop the mantle structure for more viscous mantle. While the circum-Pangea subduction in plate motion history models promotes the formation of degree-2 mantle structure, the published pre-Pangea plate motions before 330 Ma produce relatively cold lower mantle in the African hemisphere and significant degree-1 structure in the early Pangea (∼300 Ma) or later times, even if the lower mantle has an initially degree-2 structure and a viscosity as high as 1023 Pas. This suggests that the African LLSVP may not be stationary since the early Paleozoic. With the published plate motion models and lower mantle viscosity of 1022 Pas, our mantle convection models suggest that the present-day degree-2 mantle structure may have largely been formed by ∼200 Ma.

Eruptions at Lone Star geyser, Yellowstone National Park, USA: 2. Constraints on subsurface dynamics

We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geysers eruption cycle. Previously, we described measurements and analyses associated with the geysers erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5–40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover, ~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of ~10 during eruptions.

Mechanics of Old Faithful Geyser, Calistoga, California

Received 26 September 2012; revised 14 November 2012; accepted 19 November 2012; published 21 December 2012. [1] In order to probe the subsurface dynamics associated with geyser eruptions, we measured ground deformation at Old Faithful Geyser of Calistoga, CA. We present a physical model in which recharge during the period preceding an eruption is driven by pressure differences relative to the aquifer supplying the geyser. The model predicts that pressure and ground deformation are characterized by an exponential function of time, consistent with our observations. The geyser’s conduit is connected to a reservoir at a depth of at least 42 m, and pressure changes in the reservoir can produce the observed ground deformations through either a poroelastic or elastic mechanical model. Citation: Rudolph, M. L., M. Manga, S. Hurwitz, M. Johnston, L. Karlstrom, and C.-Y. Wang (2012), Mechanics of Old Faithful Geyser, Calistoga, California, Geophys. Res. Lett., 39, L24308, doi:10.1029/2012GL054012.

Quantifying melt production and degassing rate at mid‐ocean ridges from global mantle convection models with plate motion history

Author(s): Li, M; Black, B; Zhong, S; Manga, M; Rudolph, ML; Olson, P | Abstract:

Influence of seismicity on the Lusi mud eruption

Earthquakes trigger the eruption of mud and magmatic volcanoes and influence ongoing eruptive activity. One mechanism that could trigger an eruption is clay liquefaction. Here we model the propagation of seismic waves beneath the Lusi mud eruption (East Java, Indonesia) using available seismic velocity and density models to assess the effect of subsurface structure on the amplification of incident seismic waves. We find that using an updated subsurface density and velocity structure, there is no significant amplification of incident seismic energy in the Upper Kalibeng Formation, the source of the erupting solids. Hence, the hypothesis that the Lusi eruption was triggered by clay liquefaction appears unlikely to be correct. Independent constraints from gas chemistry as well as analyses of drilling activities at the nearby Banjar-Panji 1 gas exploration well and an analysis of the effects of other earthquakes all favor a drilling trigger.

Does quadrupole stability imply LLSVP fixity

arising from C. P. Conrad, B. Steinberger & T. H. Torsvik 498, 479–482 (2013)10.1038/nature12203The African and Pacific large low-shear-velocity provinces (LLSVPs) at present dominate the structure of the Earth’s lowermost mantle, but there is considerable debate as to whether these structures have remained fixed throughout geologic time or whether they have shifted in response to the changing configurations of mantle downwellings associated with zones of surface tectonic plate convergence. In a recent Letter, Conrad et al. performed a multipole expansion of the Earth’s plate motions from 250 million years (Myr) ago to the present and used the relatively stationary positions of quadrupole divergence to argue that the two LLSVPs have remained stationary at least for the past 250 Myr and further speculated that the two LLSVPs formed “stable anchors” in the more distant geologic past. Here we show that the quadrupole divergence of plate motions is not a representative diagnostic for overall plate divergence patterns, owing to cancellation effects in the multipole expansion. Hence, the conclusion by Conrad et al. that the presence of stationary quadrupole divergence implies fixity of the LLSVPs is not well founded. There is a Reply to this Brief Communication Arising by Conrad, C. P., Steinberger, B. & Torsvik, T. H. Nature 503, doi:10.1038/nature12793 (2013).