Leif Karlstrom

Triggering of the largest Deccan eruptions by the Chicxulub impact

New constraints on the timing of the Cretaceous-Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental fl ood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1–1.0 J/m 3 throughout the upper ~200 km of Earth’s mantle, suffi cient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava fl ows, which together may account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous-Paleogene time, a huge pulse of mantle plume–derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava fl ows known on Earth. High-precision radioisotopic dating of the main-phase Deccan fl ood basalt formations may be able either to confi rm or reject this hypothesis, which in turn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed signifi cantly to the CretaceousPaleogene extinction.

The role of magmatically driven lithospheric thickening on arc front migration

Volcanic activity at convergent plate margins is localized along lineaments of active volcanoes that focus rising magma generated within the mantle below. In many arcs worldwide, particularly continental arcs, the volcanic front migrates away from the interface of subduction (the trench) over millions of years, reflecting coevolving surface forcing, tectonics, crustal magma transport, and mantle flow. Here we show that extraction of melt from arc mantle and subsequent magmatic thickening of overlying crust and lithosphere can drive volcanic front migration. These processes are consistent with geochemical trends, such as increasing La/Yb, which show that increasing depths of differentiation correlate with arc front migration in continental arcs. Such thickening truncates the underlying mantle flow field, squeezing hot mantle wedge and the melting focus away from the trench while progressively decreasing the volume of melt generated. However, if magmatic thickening is balanced by tectonic extension in the upper plate, a steady crustal thickness is achieved that results in a more stationary arc front with long-lived mantle melting. This appears to be the case for some island arcs. Thus, in combination with tectonic modulation of crustal thickness, magmatic thickening provides a self consistent model for volcanic arc front migration and the composition of arc magmas.

Mega-earthquakes rupture flat megathrusts

Mega-earthquakes go the flat way Megathrust faults in subduction zones cause large and damaging earthquakes. Bletery et al. argue that certain geometric features of the subduction zones relate to earthquake size. The key parameter is the curvature of the megathrust. Larger earthquakes occur where the subducting slab is flatter, providing a rough metric for estimating where mega-earthquakes may occur in the future. Science, this issue p. 1027 Large earthquakes in subduction zones are most likely to occur where the subducting slab is relatively flat. The 2004 Sumatra-Andaman and 2011 Tohoku-Oki earthquakes highlighted gaps in our understanding of mega-earthquake rupture processes and the factors controlling their global distribution: A fast convergence rate and young buoyant lithosphere are not required to produce mega-earthquakes. We calculated the curvature along the major subduction zones of the world, showing that mega-earthquakes preferentially rupture flat (low-curvature) interfaces. A simplified analytic model demonstrates that heterogeneity in shear strength increases with curvature. Shear strength on flat megathrusts is more homogeneous, and hence more likely to be exceeded simultaneously over large areas, than on highly curved faults.

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.

Fluvial supraglacial landscape evolution on the Greenland Ice Sheet

Supraglacial stream networks incise via thermal erosion of underlying ice, reflecting a balance between localized fluvial incision and dynamic topography from underlying ice flow. We analyze high-resolution digital elevation models of the ice surface and bedrock in the southwest Greenland Ice Sheet from 1000-1600 m elevation to quantify the importance of fluvial erosion. At wavelengths greater than ice thickness, bedrock dominates surface topography so supraglacial drainage basins are fixed spatially. At smaller wavelengths, fluvial erosion significantly affects topography. Stream longitudinal profiles exhibit positive mean curvature and consistent power law scaling between local channel slope and drainage area, suggestive of adjustment toward topographic steady state. We interpret these observations with a model for fluvial thermal erosion on top of a flowing ice substrate that predicts concave up steady state longitudinal profiles, where average concavity is most sensitive to melt rate and the relative magnitudes of ice flow and fluvial erosion.

Modeling Volcanic Processes: Magma chamber dynamics and thermodynamics

Overview Magma chambers are continuous bodies of magma in the crust where magma accumulates and differentiates. Both geophysical and geochemical techniques have illuminated many aspects of magma chambers since they were first proposed. In this chapter, we review these observations in the context of heat and mass transfer theory. This chapter reviews heat transfer calculations from magma chamber to the surrounding crust, and also considers the coupled stress fields that are generated and modified by the presence of magmatic systems. The fluid dynamics of magma chambers has received considerable attention over the last several decades and here we review the ramifications of convection in magma chambers. Multiphase flow (melt + crystals + bubbles) plays a particularly important role in the evolution of magma chambers. The large density differences between melt and discrete phases such as bubbles and crystals, and the resulting flow fields generated by buoyancy are shown to be an efficient mechanism to generate mixing in chamber systems. Finally we discuss integrative approaches between geophysics and geochemistry and future directions of research. Introduction The compositional diversity of melts that reach the surface of the Earth, and diversity in eruptive style, are largely determined through processing of these melts as they ascend and sometimes stall in the crust. Most eruptive products and intrusive suites have been modified substantially from their progenitor mantle magmas, either through preferential removal of crystal phases during fractionation, assimilation of crustal melts, or a combination of these processes (Daly, 1914; Anderson, 1976; Wyllie, 1977; Hildreth and Moorbath, 1988; DePaolo et al. , 1992; Feeley et al. , 2002). Much of the evolution of these magmas likely occurs where they spend the most time: where magma has either permanently or temporarily stalled in magma chambers. This accumulation is fundamental to the genesis of large eruptions, as the background flux from the mantle cannot explain the voluminous outbursts of magma at the surface of the Earth. Magma chamber dynamics largely control the compositional evolution of these magmas, and ultimately a better understanding of magma chambers may provide clues to the triggering of eruptions.

Anomalous K-Pg–aged seafloor attributed to impact-induced mid-ocean ridge magmatism

A global anomaly in seafloor structure is attributed to mid-ocean ridge magmatism triggered by the Chicxulub meteorite impact. Eruptive phenomena at all scales, from hydrothermal geysers to flood basalts, can potentially be initiated or modulated by external mechanical perturbations. We present evidence for the triggering of magmatism on a global scale by the Chicxulub meteorite impact at the Cretaceous-Paleogene (K-Pg) boundary, recorded by transiently increased crustal production at mid-ocean ridges. Concentrated positive free-air gravity and coincident seafloor topographic anomalies, associated with seafloor created at fast-spreading rates, suggest volumes of excess magmatism in the range of ~105 to 106 km3. Widespread mobilization of existing mantle melt by post-impact seismic radiation can explain the volume and distribution of the anomalous crust. This massive but short-lived pulse of marine magmatism should be considered alongside the Chicxulub impact and Deccan Traps as a contributor to geochemical anomalies and environmental changes at K-Pg time.

Basal control of supraglacial meltwater catchments on the Greenland Ice Sheet

Ice surface topography controls the routing of surface meltwater generated in the ablation zones of glaciers and ice sheets. Meltwater routing is a direct source of ice mass loss, as well as a primary influence on subglacial hydrology and basal sliding of the ice sheet. Although the processes that determine ice sheet topography at the largest scales are known, controls on the topographic features that influence meltwater routing at supraglacial internally-drained-catchment (IDC) scales (< 10s of km) are less well constrained. Here we examine the effects of two processes on ice sheet surface topography: transfer 5 of bed topography to the surface of flowing ice and thermal-fluvial erosion by supraglacial meltwater streams. We implement 2D basal transfer functions in seven study regions of the western Greenland Ice Sheet ablation zone using a suite of recent data sets. We find that ∼1-10 km scale ice surface features can be well-explained by bed topography transfer in regions with different long-term averaged ice flow conditions. We use flow-routing algorithms to extract supraglacial stream networks from 2-5 m resolution digital elevation models, and compare these with synthetic flow networks calculated on ice surfaces predicted 10 by bed topography transfer. Multiple geomorphological metrics calculated for these networks suggest that bed topography can explain general ∼1-10km supraglacial meltwater routing, and that thermal-fluvial erosion thus has a lesser role in shaping ice surface topography on these scales. We then use bed topography transfer functions and flow-routing to conduct a parameter study predicting how supraglacial internally drained catchment (IDC) configurations and subglacial hydraulic potential would change under varying multi-year averaged ice flow and basal sliding regimes. Predicted changes to subglacial hydraulic flow 15 pathways directly caused by changing ice surface topography are subtle, but temporal changes in basal sliding or ice thickness have potentially significant influences on IDC spatial distribution. We suggest that changes to IDC size and number density could affect subglacial hydrology primarily by dispersing the englacial/subglacial input of surface meltwater. Copyright statement. TEXT

Magmatic Landscape Construction

Magmatism is an important driver of landscape adjustment over ∼10% of Earths land surface, producing 103‐ to 106‐km2 terrains that often persistently resurface with magma for 1–10 s of Myr. Construction of topography by magmatic intrusions and eruptions approaches or exceeds tectonic uplift rates in these settings, defining regimes of landscape evolution by the degree to which such magmatic construction outpaces erosion. We compile data that span the complete range of magmatism, from laccoliths, forced folds, and InSAR‐detected active intrusions, to explosive and effusive eruption deposits, cinder cones, stratovolcanoes, and calderas. Distributions of magmatic landforms represent topographic perturbations that span >10 orders of magnitude in planform areas and >6 orders of magnitude in relief, varying strongly with the style of magmatism. We show that, independent of erodibility or climate considerations, observed magmatic landform geometry implies a wide range of potential for erosion, due to trade‐offs between slope and drainage area in common erosion laws. Because the occurrence rate of magmatic events varies systematically with the volume of material emplaced, only a restricted class of magmatic processes is likely to directly compete with erosion to shape topography. Outside of this range, magmatism either is insignificant on landscape scales or overwhelms preexisting topography and acts to reset the landscape. The landform data compiled here provide a basis for disentangling competing processes that build and erode topography in volcanic provinces, reconstructing timing and volumes of volcanism in the geologic record, and assessing mechanical connections between climate and magmatism.