Miranda: An icy moon deformed by convection

(Press-News.org) Boulder, Colo., USA – Miranda, a small, icy moon of Uranus, is one of the most visually striking and enigmatic bodies in the solar system. Despite its relatively small size, Miranda appears to have experienced an episode of intense resurfacing that resulted in the formation of at least three remarkable and unique surface features -- polygonal-shaped regions called coronae.

These coronae are visible in Miranda's southern hemisphere, and each one is at least 200 km across. Arden corona, the largest, has ridges and troughs with up to 2 km of relief. Elsinore corona has an outer belt that is approx. 80 km wide, relatively smooth, and elevated above the surrounding terrain by approx. 100 m. Inverness corona has a trapezoidal shape with a large, bright chevron at its center. The northern hemisphere of Miranda was never imaged by the Voyager 2 spacecraft, so it is unknown whether additional coronae exist.

Using numerical models, Noah Hammond and Amy Barr show that convection in Miranda's ice mantle likely formed the coronae. During convection, warm buoyant ice rose toward the surface, driving concentric surface extension beneath the locations of the coronae, causing the formation of extensional tectonic faults. This style of resurfacing is similar to plate tectonics on Earth, in that convection is a primary driving force for surface deformation.

Hammond and Barr write that the internal energy that powered convection probably came from tidal heating. Tidal heating would have occurred when Miranda was in an eccentric orbit -- moving closer to and further from Uranus. This caused the tidal forces from Uranus to vary, periodically stretching and squeezing Miranda and generating heat in its ice shell. Hammond and Barr find that convection powered by tidal heating explains the locations of the coronae, the deformation patterns within the coronae, and the estimated heat flow during corona formation.

FEATURED ARTICLE Global resurfacing of Uranus's moon Miranda by convection
Noah P. Hammond and Amy C. Barr, Dept. of Geological Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, USA. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G36124.1.

Other recently posted GEOLOGY articles (see below) cover such topics as 1. The 2004-2008 Mount St. Helens eruptions;
2. The largest landslides on Earth; and
3. The East African Rift Valley.

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Pathways for degassing during the lava dome eruption of Mount St. Helens 2004–2008
H. Elizabeth Gaunt et al., Rock & Ice Physics Laboratory, Dept. of Earth Sciences, University College London, London WC1E 6BT, UK. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G35940.1.

Scientists studying the 2004-2008 Mount St. Helens eruptions found that gas dissolved in the magma escaped vertically through fractured rock near the outer rim of the volcano, acting as a valve to release pressure and reducing the likelihood of explosive eruptions. The findings contrast with previous models that showed gas escaping horizontally through volcano walls and so will help scientists better understand the different escape routes of gas and the important roles that permeability and direction of flow play. The explosive potential of volcanoes is primarily controlled by the quantity of gas in the magma and its ability to escape, often from solidified magma through a network of interlinked cracks. How easily gas escapes is termed "permeability." The study measured the permeability of rock samples taken across a lava spine, from core to outer rim. Rocks were more permeable at the outer rim compared to the core of the volcano and the level of permeability varied with direction of flow -- vertical values were up to 1000 times greater than horizontal values. This NERC-funded study was led by UCL Earth Sciences' Rock & Ice Physics Laboratory with support from UCL's Institute for Risk & Disaster Reduction and USGS Cascades Volcano Observatory.

Catastrophic emplacement of the gigantic Markagunt gravity slide, southwest Utah (USA): Implications for hazards associated with sector collapse of volcanic fields
David B. Hacker et al., Dept. of Geology, Kent State University, Kent, Ohio 44242, USA. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G35896.1.

Large-scale landslides that were formed by partial collapse of volcanic edifices are among the most hazardous events associated with volcanoes. However, wholesale collapse of large sections of a volcanic field containing multiple volcanoes is less well known, due partly to the huge scale of such gravity-slide features. This study by David Hacker and colleagues describes the geometry and internal features of the newly recognized Markagunt gravity slide of southwest Utah. The Markagunt gravity slide occurred 21 to 22 million years ago as a result of failure during magmatic doming of the southwest flank of the Oligocene to Miocene Marysvale volcanic field. The gravity slide is more than 90 km long, extends over more than 3,400 square kilometers, and advanced more than 30 km over the Miocene land surface. The presence of a basal zone of breccia with clastic dikes, and pseudotachylyte along secondary shear planes, provide strong evidence of catastrophic emplacement. The Markagunt gravity slide, along with the comparable Heart Mountain gravity slide in northwest Wyoming, constitutes a class of sector collapse structures not widely recognized within modern volcanic fields. Although gigantic catastrophic collapses of volcanic fields such as these are rare, they represent the largest known continental landslides on Earth and provide new insight concerning potential hazards associated with volcanic field development.

East African lake evidence for Pliocene millennial-scale climate variability
Katy E. Wilson et al., Dept. of Earth Sciences, University College London, London WC1E 6BT, UK. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G35915.1.

The East African Rift Valley is often considered to be the "Cradle of Mankind," where hominin species evolved and diversified over the last few million years. Around 2 to 2.5 million years ago, the evolution of the genus Homo was accompanied by the use of stone tools and an increase in brain size. Ancient lake sediments from central Kenya indicate that around 2.5 million years ago, parts of the Rift Valley were occupied by large, deep-water lakes, which appeared and disappeared regularly through time. Authors Katy E. Wilson and colleagues look in detail at one of these lakes and find that the lake formed very quickly and disappeared more gradually. Variations in the chemical composition of nannofossils preserved in these sediments indicate that the lake itself varied greatly on millennial timescales. Wilson and colleagues interpret these fluctuations as changes in regional climate and in the strength of the local rainy seasons. The timing of these changes is very similar to the timing of events observed in ice core records from Greenland since the last ice age. These results show that this key period of human evolution likely coincided with a time when the local climate was highly variable.

Magmatic life at low Reynolds number
Allen F. Glazner, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G36078.1.

Many eye-catching features in granites, such as mineral layering and accumulations of large crystals, have traditionally been explained by analogy to layering in sedimentary rocks; that is, as a result of crystals being swept along by currents in a magma chamber. However, simple physical arguments and comparison to the physical environment of microorganisms demonstrate that this cannot be true. Commonplace granitic Remnants of ancient Australia in Vanuatu: Implications for crustal evolution in island arcs and tectonic development of the southwest Pacific
Janrich Buys et al. (corresponding: Carl Spandler), School of Earth and Environmental Sciences, James Cook University, Townsville 4811, Australia. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G36155.1.

Volcanic island chains located far from continental landmasses are considered idea sites to study how new crust is formed, as the influence of pre-existing continental crust is assumed to be negligible. In this study, Janrich Buys and colleagues use the age of zircon grains included in volcanic rocks from Vanuatu to show that ancient continental material is part of the basement geology of the Vanuatu island arc. This continental material was transported thousands of kilometers from northern Australia to Vanuatu sometime prior to the Cenozoic Era. Incorporation of this old continental material into magmas that subsequently transit through the crust can modify the chemical composition of the magma in a manner that would normally be ascribed to recycling of crustal components due to subduction processes. Therefore, these results highlight the need for caution in interpreting the geochemistry of magmas formed in oceanic settings. The recognition of Australian crust in Vanuatu also impacts on tectonic models of the southwest Pacific, as it implies that fragmentation of the east Australian margin of Gondwana occurred earlier than previously thought.

Neoarchean disaggregation and reassembly of the Superior craton
Jean H. Bédard Geological Survey of Canada, 490 de la Couronne, Québec, Québec G1K 9A9, Canada; Lyal B. Harris, Institut National de la Recherche Scientifique, Centre Eau Terre Environnement, 490 de la Couronne, Québec, Québec G1K 9A9, Canada. Published online ahead of print on 16 Sept. 2014; http://dx.doi.org/10.1130/G35770.1.

A non-uniformitarian model is proposed to explain the architecture and origin of the Superior craton, Canadian Shield. The model proposes that a mantle overturn event starting at approx. 2.8 billion years ago partially disaggregated an older composite craton. The up- and outwelling mantle flow tore ribbon-continents away from the southern margin of this older craton, creating narrow oceanic tracts where uncontaminated mantle-derived melts could erupt to form volcanic belts like the Abitibi. The model proposes that the mantle flow field changed at about 2.72 billion years ago, driving the northernmost remnant continental block of this older craton towards the south and reaccreting the detached fragments and oceanic tracts to its leading edge.

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