Kilometers Of Material Research Articles

Oceans of magma: Large Igneous Provinces (LIPS) and plumes

The Mt St Helens eruption of 1989 created world news and called forth expressions from journalists such as ‘the unbelievable power of Nature’ and ‘incredible powers of destruction’. Yet, this eruption pales into insignificance in the light of earlier volcanic events. Mt St Helens is estimated to have produced 1.2 cubic kilometres of solid material. Laki, in Iceland, is the most voluminous eruption in historic times and is estimated to have produced around 14 cubic kilometres of lava in 1783 to 1784. However, geologists know of older single eruptions that produced hundreds or even thousands of cubic kilometers of solid material and here I tell of a volcanic event that resulted in between one and ten million cubic kilometres of material in just a couple of million years or so. If this occurred at the present day it would almost certainly wipe out mankind and, indeed, earlier mass extinction events are ascribed to similar phenomena which have happened several times in the past. These events created ‘Large Igneous Provinces’ (LIPs) and here I concentrate on part of one of these to give an impression of their nature, examining what might be the cause of such enormous amounts of magma.

Observations of an aeolian landscape: From surface to orbit in Gale Crater

Landscapes derived solely from aeolian processes are rare on Earth because of the dominance of subaqueous processes. In contrast, aeolian-derived landscapes should typify Mars because of the absence of liquid water, the long exposure times of surfaces, and the presence of wind as the default geomorphic agent. Using the full range of available orbital and Mars Science Laboratory rover Curiosity images, wind-formed features in Gale Crater were cataloged and analyzed in order to characterize the aeolian landscape and to derive the evolution of the crater wind regime over time. Inferred wind directions show a dominance of regional northerly winds over geologic time-scales, but a dominance of topography-driven katabatic winds in modern times. Landscapes in Gale Crater show a preponderance of aeolian features at all spatial scales. Interpreted processes forming these features include first-cycle aeolian abrasion of bedrock, pervasive deflation, organization of available sand into bedforms, abundant cratering, and gravity-driven wasting, all of which occur over a background of slow physical weathering. The observed landscapes are proposed to represent a spectrum of progressive surface denudation from fractured bedrock, to retreating bedrock-capped mesas, to remnant hills capped by bedrock rubble, to desert pavement plains. This model of landscape evolution provides the mechanism by which northerly winds acting over ∼3Ga excavated tens of thousands of cubic kilometers of material from the once sediment-filled crater, thus carving the intra-crater moat and exhuming Mount Sharp (Aeolis Mons). The current crater surface is relatively sand-starved, indicating that potential sediment deflation from the crater is greater than sediment production, and that most exhumation of Mount Sharp occurred in the ancient geologic past.

Landslide‐generated tsunamis at Réunion Island

Landslides that occur on oceanic volcanoes can reach the sea and trigger catastrophic tsunamis. Réunion Island has been the location of numerous huge landslides involving tens to hundreds of cubic kilometers of material. We use a new two‐fluid (seawater and landslide) numerical model to estimate the wave amplitudes and the propagation of tsunamis associated with landslide events on Réunion Island. A 10 km3 landslide from the eastern flank of Piton de la Fournaise volcano would lift the water surface by about 150 m where it entered the sea. The wave thus generated would reach Saint‐Denis, the capital of Réunion Island (population of about 150,000 people), in only 12 min, with an amplitude of more than 10 m, and would reach Mauritius Island in 18 min. Although Mauritius is located about 175 km from the impact, waves reaching its coast would be greater than those for Réunion Island. This is due to the initial shape of the wave, and its propagation normal to the coast at Mauritius but generally coast‐parallel at Réunion Island. A submarine landslide of the coastal shelf of 2 km3, would trigger a ∼40 m high wave that would severely affect the proximal coast in the western part of Réunion Island. For a landslide of the shelf of only 0.5 km3, waves of about 2 m in amplitude would affect the proximal coast.

Calderas Bottom‐to‐Top: An Online Seminar and Field Trip

Caldera‐related processes have deep implications in Earth science and are important to the general public. To the geologist, calderas are surface manifestations of large magma reservoirs and the source of some of the largest eruptions on Earth. For society, these volcanoes represent not only an extreme volcanic hazard but also a potential source of abundant geothermal energy and metal ore deposits.During February–April 2009, an innovative, online course entitled “Calderas: Bottom‐to‐top” was offered across several universities to students interested in investigating caldera‐forming processes associated with “supervolcanoes.” Supervolcanoes have the ability to spew hundreds to thousands of cubic kilometers of material from large magma chambers pooled in the Earth's crust. A caldera‐forming eruption results from overpressurization of the chamber and leads to formation of large depressions, tens of kilometers in diameter.

Evaporites, petroleum exploration, and the Cenozoic evolution of the Libyan shelf margin, central North Africa

The Mediterranean margin of Libya can be divided into three physiographic provinces, the Pelagian Shelf, the Sirt Embayment, and Offshore Cyrenaica. The petroleum potential of the Pelagian Shelf has been investigated but the Sirt Embayment and Offshore Cyrenaica are almost unexplored. During 2004–2005, new 2D pre-stack time-migrated seismic data were acquired and used to examine the large-scale structural, depositional, and salt tectonic features of the Libyan shelf and slope. The data cover approximately 38,000 line kilometers in water depths ranging from under 50 to over 3000 m. Cenozoic strata along much of the Libyan margin have a demonstrable progradational character punctuated by surfaces of erosion and margin failure. Within the Sirt Embayment the most visible retrogradational surface becomes seismically coincident with the top of Messinian unconformity. This retrogradational surface extends for over 700 km along strike and cuts both sides of the Sirt Embayment. Over 5000 cubic kilometers of material are missing from above this surface. There are two ideas for how retrogradation occurred. One idea relates the retrogradation to margin erosion during the Messinian salinity crisis. The other idea suggests that a large part of the Libyan margin experienced a cataclysmic failure during the late Miocene. Some existing models for offshore Libya have interpreted a widespread layer of halite lying within the Messinian which thickens basinward. This interpretation was probably based on the fact seismic reflection continuity was lost over much of the Sirt Embayment on older data beneath the top Messinian unconformity. The loss of good reflection character adversely affected exploration efforts by obscuring deep structures. Recent seismic data and a current understanding of salt behavior do not support the interpretation of thick halite within the Messinian section. Regional observations do not indicate any mobile halite present in the Sirt Embayment. There is a relatively thin, high amplitude and high velocity layer of non-halite evaporites (mainly anhydrite) which caps the Messinian section. Where this high amplitude and high velocity layer is absent or eroded, seismic continuity within the Messinian interval is restored. Limited available well data support this interpretation. True mobile halite is interpreted to exist in offshore Libya only in the far west of the Pelagian Shelf near the Tunisian border. Beneath the Pelagian Shelf are a series of tight contractional folds that are interpreted to be salt cored. Basins adjacent to the folds display geometries characteristic of salt withdrawal. The fold crests formed bathymetric highs which served as nucleation sites for nummulitic shoal development. These shoals are the principal reservoirs of the Pelagian Shelf.

Growth and collapse of the Reunion Island volcanoes

This work presents the first exhaustive study of the entire surface of the Reunion Island volcanic system. The focus is on the submarine part, for which a compilation of all multibeam data collected during the last 20 years has been made. Different types of submarine features have been identified: a coastal shelf, debris avalanches and sedimentary deposits, erosion canyons, volcanic constructions near the coast, and seamounts offshore. Criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible. Debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain. They have built four huge Submarine Bulges to the east, north, west, and south of the island. They form fans 20–30 km wide at the coastline and 100–150 km wide at their ends, 70–80 km offshore. They were built gradually by the superimposition and/or juxtaposition of products moved during landslide episodes, involving up to several hundred cubic kilometers of material. About 50 individual events deposits can be recognized at the surface. The landslides have recurrently dismantled Piton des Neiges, Les Alizes, and Piton de La Fournaise volcanoes since 2 Ma. About one third are interpreted as secondary landslides, affecting previously emplaced debris avalanche deposits. On land, landslide deposits are observed in the extensively eroded central area of Piton des Neiges and in its coastal areas. Analysis of the present-day topography and of geology allows us to identify presumed faults and scars of previous large landslides. The Submarine Bulges are dissected and bound by canyons up to 200 m deep and 40 km long, filled with coarse-grained sediments, and generally connected to streams onshore. A large zone of sedimentary accumulation exists to the north–east of the island. It covers a zone 20 km in width, extending up to 15 km offshore. Volcanic constructions are observed near the coast on both Piton des Neiges and Piton de la Fournaise volcanoes and are continuations of subaerial structures. Individual seamounts are present on the submarine flanks and the surrounding ocean floor. A few seem to be young volcanoes, but the majority are probably old, eroded seamounts. This study suggests a larger scale and frequency of mass-wasting events on Reunion Island compared to similar islands. The virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature. The increased number of known flank–failure events has to be taken into consideration when assessing hazards from future landslides, in particular, the probability of landslide-generated tsunamis.

Hyperion: Rotation, Shape, and Geology from Voyager Images

Voyager 2 images have been used to measure the instantaneous rotation state of Hyperion during the period of the spacecraft's 1981 flyby of Saturn, to define the satellite's shape, and to map geological features. For the 38 hr of image coverage at resolutions better than 17 km/pixel, control point calculations yield a rotation rate of 72°/day, with a pole direction of RA = 226 ± 4°, Dec = 35 ± 4°. Rotation was essentially about the long axis of Hyperion. A shape model with 5° × 5° spacing is developed using stereo, limb, and terminator data. About 95% of the surface of Hyperion is constrained within 20 km by the control point and lower resolution images. An ellipsoidal fit to this model gives principal axes (radii) of 164, 130, and 107 km. The shape model has a mean radius of 133 ± 8 km; the model moments of inertia (I/MR 2) are A = 0.32 ± 0.02, B = 0.46 ± 0.02, and C = 0.52 ± 0.02. G. J. Black et al . (1995, Icarus) have used the shape model and the instantaneous rotation state to calculate the expected rotation of Hyperion throughout the Voyager 2 encounter. They predict significant changes in the rotation pole orientation, but only minor changes in rotation rate during the 18 days of low-resolution, disk-resolved imaging of Hyperion. We compare the Voyager-observed lightcurve of Hyperion to lightcurves predicted from the dynamical model. During this 18-day period the dynamical model matches the observed lightcurve very well; the best lightcurve fit is for a rotation rate of 72 + 3 - 4°/day (identical to the control point solution) at the time of the first of the high-resolution images. Much of the Hyperion lightcurve amplitude derives from wobble rather than from the spin. The good agreement of the dynamical model with observed behavior suggests that Hyperion's moments of inertia are well modeled by the shape and that it has no substantial internal density contrasts. The overall shape of Hyperion is typical for icy objects smaller than 200 km mean radius. Small icy satellites appear to have less structural control of their shapes than do rocky satellites and asteroids. A 300-km-long arcuate scarp and a more subdued, nearly parallel scarp suggest spallation of several kilometers of material from nearly 1/4 the area of the satellite due to the formation of a 140-km-diameter crater.

Massive Siliciclastic Deposits Juxtaposed Against Massive Carbonate Bodies: Paradox of Eastern Gulf of Mexico: ABSTRACT

The eastern Gulf of Mexico is the location of a major and dramatic transition from siliciclastic to carbonate deposits and deposition. The massive lutites that make the Mississippi Cone are juxtaposed against the equally massive carbonates intercalated with evaporites that make the western portion of the Florida Platform. The region now occupied by the continental margin of the eastern Gulf has been cut off from clastic sedimentation since Late Jurassic or Early Cretaceous time, and over 5,000 m (16,404 ft) of predominantly carbonate sediments have built up. Surficial sediments of the carbonate portion of the transition zone are called the West Florida Lime Mud Facies. A foraminifera-coccolith ooze is still being deposited in water depths as shallow as 200 m (656 ft); this facies covers over 40,000 km2 (15,444 mi2) and in many respects resembles a deep-sea ooze. However, in some places shelf carbonate debris, including whole valves of mid-shelf oysters, have cascaded over the shelf-slope break and are incorporated into the slope ooze. Composition, rates of accumulation, and relatively shallow deposition suggest that this sedimentary body may be an analog of some of the chalk deposits of northwestern Europe. It has long been considered axiomatic that planktonic foraminifera and coccolith oozes are deep-sea deposits that accumulate at rates of a few tens of millimeters per thousand years. However, radiocarbon dates show that the eastern Gulf margin ooze has accumulated rapidly. Prior to the last rise in sea level, deposition rates were as high as 65 cm/1,000 years (25 in./1,000 years); and ever since sea level began to rise sedimentation rates have still been as high as 20 cm/1,000 years (8 in./1,000 years). Such a rapid buildup of the carbonate sediments has led to widespread mass wasting and all forms, from creep to massive multiple slide deposits containing thousands of cubic kilometers of material, are represented in our set of high resolution seismic profiles. The actual contact between carbonates and siliciclastics occurs in a variety of styles. Carbonate rubble is found in a terrigenous lutite matrix at the base of the West Florida Escarpment. Farther west on the Mississippi Cone the brown and yellow-brown siliceous lutites contain carbonate turbidites with provenance the West Florida slope. Another type of contact is shown in some piston cores with carbonate ooze in normal and alternating depositional End_Page 453------------------------------ contact with Mississippi siliceous lutite. The position and timing of alternating carbonate and fine terrigenous deposition, as well as the loci of carbonate buildups on the slope, must be affected and perhaps even controlled by the positioning of the Loop Current, a major precursor to the Gulf Stream which, depending upon factors as yet not totally understood, irregularly advances into or retreats from the eastern Gulf. When the loop is up and sweeps from north to south along the West Florida slope, it blocks or deflects Mississippi sedimentation. When the loop is restricted to the area of the Florida Straits, fines derived from the Mississippi River can be deposited at the base of the escarpment and even on the slope. End_of_Article - Last_Page 454------------