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Large landslides lie low: Himalaya-Karakoram ranges

New Geology articles posted online ahead of print April 27 and May 7, 2015

Geological Society of America

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IMAGE: This is the Agham fluvial terrace, located roughly 400 m above the current river level of the upper Shyok valley, NW Himalayas, India. See related article by J.H. Blöthe and... view more

Credit: J.H. Blöthe and colleagues and Geology

Boulder, Colo., USA - Large landslides are an important process of erosion in the Himalaya-Karakoram ranges (HKR). These high-relief landscapes are characterized by steep slopes that are prone to frequent landsliding. By mapping nearly 500 large (greater than 0.1 km2) landslides in the HKR, Jan Henrik Blöthe and colleagues find that the vast majority of these mass movements lie in the lower portions of the landscape, whereas glaciers and rock glaciers occupy the higher elevations almost exclusively.

These findings suggest that different processes dominate the gross erosion in the HKR at different elevations: Large landslides appear to preferentially undermine the topography in response to incision along major rivers, whereas glacial erosion and/or more frequent and smaller slope failures, associated with intense frost action, compensate for this role at higher elevations.

In this study published in Geology, Blöthe and colleagues introduce a new method that they term excess topography (ZE) to identify the location of potentially unstable rock-mass volumes. They find that locations with high values of ZE are concentrated near or below the median elevation, which is also where the majority of the mapped landslides occur, and conclude that the HKR are characterized by two vertical domains of landslide and (peri-)glacial erosion that may respond to different time scales of perturbation

FEATURED ARTICLE

Large landslides lie low: Excess topography in the Himalaya-Karakoram ranges

Jan Henrik Blöthe et al., University of Potsdam, Potsdam, Germany; and University of Bonn, Bonn, Germany. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36527.1.

Other recently posted GEOLOGY articles (see below) cover such topics as

    1. Kelp forests;

    2. Coral islands;

    3. Gypsum caves; and

    4. Chinese stalagmite.

GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Kelp, cobbles, and currents: Biologic reduction of coarse grain entrainment stress

Claire C. Masteller et al., University of California, Santa Cruz, California, USA. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36616.1.

Kelp forests colonize a large portion of global coastlines and support diverse assemblages of organisms. To this end, their role in coastal ecosystems has been well studied. However, the impact of kelp on the morphology of the coastlines upon which it colonizes is not as well known. This study quantifies the effect of kelp on current driven coarse sediment transport. Authors Clair Mastellar and colleagues have developed a model for the motion of coarse grains coupled to kelp fronds, and show that kelp significantly enhances their mobility, allowing large grains to be transported at current velocities where their motion would typically be impossible. Their model predictions agree well with a year of underwater observations of kelp-assisted sediment transport from the Strait of Juan de Fuca. The results of this study highlight an example of a system where biology may play a significant role in sediment transport and landscape evolution.

Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll

P.S. Kench et al., University of Auckland, Auckland, New Zealand. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36555.1.

The geological stability and existence of low-lying atoll nations is threatened by sea level rise and climate change. Funafuti Atoll, in the tropical Pacific Ocean has experienced some of the highest rates of sea level rise of approximately 5.1 mm/yr, totaling ~0.30 m over the past 60 years. This study analyzed historical shoreline positions over the past 118 years, in 29 islands of Funafuti atoll to determine their physical response to recent sea level rise. Despite the magnitude of sea level rise results showed that no islands have eroded away, the majority have increased in size and there has been a 7.3% increase in net island area over the last century (1897-2013). Furthermore, there is no evidence of increased erosion over the past half century as sea level rise accelerated. Results also show that reef islands in Funafuti have continually adjusted their size, shape and position on the coral reef platform in response to variations in storms, sediment supply as well as sea level. This study suggests a more optimistic outlook for the habitability of atoll nations and the rates and styles of island change documented in this study will provide critical information to assist countries adapt to future environmental changes.

Gypsum caves as indicators of climate-driven river incision and aggradation in a rapidly uplifting region

Andrea Columbu et al., University of Melbourne, Parkville, Victoria, Australia. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36595.1.

Caves can be composed of underground natural tunnels that are carved by rivers at the level of the outside fluvial network. These caves are formed rapidly in gypsum rocks by fast dissolution and erosion. The chronological reconstruction of the formation of these cave levels can help in understanding how and when the landscape evolved during the last 200 thousand years.

Direct measurements of deglacial monsoon strength in a Chinese stalagmite

Ian J. Orland et al., University of Minnesota, Minneapolis, Minnesota, USA; and WiscSIMS (Wisconsin Secondary Ion Mass Spectrometer Laboratory) University of Wisconsin, Madison, Wisconsin, USA. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36612.1.

The chemistry of a cave formation (e.g., stalagmite) can record past changes in the surface environment above a cave. Recent work determined that stalagmites from a number of Chinese caves contain records of cyclical changes in the Asian Monsoon over the past few hundred thousand years. Some of the important details, however, of the link between stalagmite chemistry and seasonal dynamics of the monsoon were untested because conventional analytical techniques typically averaged 10 or more years of stalagmite growth. In this study by Ian J. Orland and colleagues, a new analytical technique is used to make seasonal-resolution measurements of the chemistry in an annually-banded stalagmite from northeast China. The new data establish that this stalagmite, and likely other Chinese stalagmites, record year-to-year changes in both the annual proportion of regional monsoon rainfall and the amount of monsoon rainfall across southern Asia. The new analytical approach was also applied to a portion of the stalagmite that grew during a rapid warming event 11,500 years ago. The data reveal a large, rapid shift in rainfall chemistry that occurred in just 14 years over northeast China, likely as a result of an abrupt change in atmospheric circulation that is documented across the Northern Hemisphere.

Mantle heterogeneity controls on small-volume basaltic volcanism

Lucy E. McGee et al., Centro de Excelencia en Geotermia de los Andes (CEGA) and Department of Geology, Universidad de Chile, Santiago, Chile. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36590.1.

An exciting correlation between deep melting in the mantle and the resultant size of small ("monogenetic" volcanoes which erupt only once to form fields of small volcanic hills and lakes) continental volcanoes has been found in a volcanic field in northern New Zealand situated in the country's most populated city, Auckland. The study by Lucy McGee and colleagues takes three examples of volcanoes ranging from the smallest to the largest and compares various chemical parameters which give clues as to the origin of their magmas and the way in which these ascended to the surface. They found that the smallest volcanoes have a chemical signature suggesting that melting initiated very deep, within discrete domains of a source which is not only more fusible, but of a different composition to the surrounding source material. As rates of melting increased, write McGee and colleagues, "We can trace in the chemistry the involvement of different, more abundant sources and correlate this with the volume erupted at the surface." The difference in source compositions and density of magmas suggests that the "small volcano" magmas rise to the surface much quicker than the "large volcano" magmas, a feature that could be important for future hazard scenarios in populated centers such as Auckland.

Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29-3.23 billion years ago

Donald R. Lowe, Stanford University, Stanford, California, USA; and Gary R. Byerly, Louisiana State University, Baton Rouge, Louisiana, USA. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36665.1.

The Earth formed 4.5 billion years (byr) ago through the accretion of large asteroidal bodies and its early history continued to be profoundly influenced by large impacts. Based on the lunar cratering record, it has long been thought that the period of frequent large impacts ended about 3.8 byr ago. However, recent discoveries have shown that there is a geologic record here on Earth of early large impacts and that the Earth continued to be bombarded by asteroids from 20 to perhaps 100 km across until at least 3.2 byr-ago. This paper by Donald Lowe and Gary Byerly argues that at least two of these late impacts at about 3.29 to 3.23 byr-ago were large enough to have superheated the atmosphere, boiled the surface layer of the ocean, and evaporated tens to perhaps 100 m of sea water. These impact-related thermal events would have temporarily changed the world's ocean into a single, giant boiling hot spring and profoundly affected early crustal development and stability, the global surface environment, and biological evolution toward low-temperature microbial communities until 3.2 byr-ago or even later.

Graphic-textured inclusions in apatite: Evidence for pegmatitic growth in a REE-enriched carbonatitic system

Krisztina Pandur et al., University of Saskatchewan, Saskatoon, Saskatchewan, Canada. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/G36613.1.

This paper by Krisztina Pandur and colleagues documents the petrographic and chemical characteristics of a possibly unique set of graphic inclusion textures in apatite of the Hoidas Lake REE deposit, northern Saskatchewan, Canada. Graphic intergrowths of quartz and K-feldspar are commonly described in pegmatites of granitic systems, and several studies have focused on understanding these features, and provide a significant component in our knowledge of the evolution of felsic melts. Similar graphic-like features, however, are absent from pegmatites that are related to carbonatitic or alkaline magmatic systems. This study reports on the first study of recently discovered graphic-textured inclusions in an unusual setting, namely in apatite of the Hoidas Lake REE deposit, which, based on paragenetic relationships and chemical variations, is inferred to represent a dike/vein system related to an unusual carbonatitic or alkaline magmatic source. Characterization of the inclusions includes description of their textural features and unusual chemical variations. Possible explanations are provided for their genesis, based on analogy with classic graphic quartz-K-feldspar intergrowths in granitic pegmatites, and the controlling factors of these processes are discussed.

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