Magmatic Rifting And Active Volcanism

A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a ten-day period, c. 2.5 km3 of magma were intruded along a 60 km-long dyke separating the Arabian and Nubian plates. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. This activity led to a renewed international focus on the role of magmatism in rifting, with major international collaborative projects working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This work has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.

The magma-rich East African Rift System (EARS) is a modern rift zone that provides an opportunity to constrain the existing relationship between magmatism and strain in continental rifts. Within the EARS, the broadly rifted Turkana Depression has been identified as an important locus of strain. This study focused on the magmatic events in the Turkana Depression that began in the Pliocene (ca. 4 Ma) as part of a pulse of basaltic activity known as the Stratoid Phase - the last pulse of basaltic magmatism before strain was accommodated in the axial region of the Turkana Basin. This basaltic pulse began with the eruption of the widespread Gombe Stratoid Series and is followed by less voluminous basaltic activity in the form of shield volcanoes emplaced on top of the Gombe lava flows. We find that the Gombe Stratoid Series is representative of a high flux event with a complex magma plumbing system that was produced by large degrees of decompression melting of the upper mantle during a broad stretching event in Turkana. In contrast, the waning of magma flux resulted in the development of shield volcanoes subsequent to the eruption of the Gombe Stratoid Series. The timing of magmatic activity of some of the shield volcanoes suggests that there was a migration of magmatism towards Lake Turkana. Unlike the Gombe Stratoid Series lavas, we find that shield-volcano lavas require a component derived from metasomatized enriched lithospheric mantle. The contribution of this enriched component is more pronounced towards the east, coincident with a younging of magmatism in this direction. We suggest that the presence of easily fusible material in the continental lithospheric mantle in the vicinity of the Lake Turkana Basin may result in the generation of localized lithospheric melt following the broad plate stretching event that yielded the Gombe Stratoid Series. Lithospheric melts will promote the focusing of formerly broadly distributed plate stretching into a more localized region where plate melting may have occurred. We suggest that melting of enriched lithospheric mantle may account for the modern localization of strain and migration of magmatism and rifting towards the currently active volcanic centers in Lake Turkana Basin.

This dissertation examines the interplay between tectonic and magmatic processes that influence the nature of volcanically active regions. In particular, I address how faulting, fluid transport, and magma intrusion interact in a variety of settings, including continental rifts, large igneous provinces (LIPs), and monogenetic volcanic fields. In the East African Rift (EAR), I combine structural measurements and field observations with geochronology, geochemistry, and seismicity data to investigate how continental breakup initiates and evolves. Analyses of volcanic cone lineaments illustrate variations in the geometries and distributions of dike intrusions along the rift. In early stage basins (10 Ma), dikes begin to accommodate extension along the entire length of the basin. Measurements of diffuse gas flux in the EAR demonstrate that significant volumes of magmatic CO2 (>4 Mt yr−1) are rising along faults from upper mantle/lower crustal magma bodies. Over the entire EAR, this tectonic degassing (~71+/-33 Mt yr−1) contributes significantly to Earth's natural CO2 budget. Moreover, the release of magmatic volatiles assists strain localization and the development of new rift segmentation within existing half-graben depressions. Newly developing segments become kinematically linked with existing rift segments through the reactivation of rift-oblique basement fabrics. Resulting transverse faults postdate rift-parallel fault systems, indicating that preexisting weaknesses play a critical role at all stages of rift development. A regional synthesis of sill-fed dikes of the Ferrar LIP, Antarctica, and field and remote-sensing analysis of the Hopi Buttes volcanic field, Arizona, highlights the role of interconnected dike-sill systems in feeding eruptions. Measurements reveal a previously unrecognized feeder system to LIP eruptions----analogous to a "cracked lid" atop a sill network----that may be the archetypal model for LIPs that intrude sedimentary basins undergoing negligible regional extension. At Hopi Buttes, several characteristic shallow feeder systems are identified: (1) dike-dominated, (2) sill-dominated, and (3) interconnected dike-sill networks. Dike-fed eruptions are common, but sills also modulate the flow of magma toward and away from eruptive conduits, providing a novel mechanism for lateral vent migration and changes in magma-water ratios that lead to explosive volcanism.

This volume contains papers giving an interdisciplinary review of 12 major rift systems from North and South America, Africa, Europe and Asia. These papers are written by an international group of academic and industrial specialists each of whom is most knowledgeable about the respective rift. The analyzed rifts were selected on the basis of availability of an as-complete-as-possible geological and geophysical data base. Thirteen papers deal with geodynamic processes governing the evolution of rifts. A comprehensive digest of the available stratigraphic, structural, geophysical and petrological data, together with an extensive list of references, is provided for each of the analyzed rift systems. The megatectonic setting and dynamics of evolution of each basin is discussed. Geodynamic models are tested against the record of the analyzed rifts. The question of "active" as against "passive" rifting is addressed. The rifts analyzed range in age from Precambrian to Recent and cover a wide spectrum of megatectonic settings. There is discussion of the evolution of rifts in a plate-tectonic frame. The case histories are followed by discussions addressing the global setting of rifts and geodynamic processes active during the development of rifted basins.

This memoir is the first to review all of Antarctica’s volcanism between 200 million years ago and the Present. The region is still volcanically active. The volume is an amalgamation of in-depth syntheses, which are presented within distinctly different tectonic settings. Each is described in terms of (1) the volcanology and eruptive palaeoenvironments; (2) petrology and origin of magma; and (3) active volcanism, including tephrochronology. Important volcanic episodes include: astonishingly voluminous mafic and felsic volcanic deposits associated with the Jurassic break-up of Gondwana; the construction and progressive demise of a major Jurassic to Present continental arc, including back-arc alkaline basalts and volcanism in a young ensialic marginal basin; Miocene to Pleistocene mafic volcanism associated with post-subduction slab-window formation; numerous Neogene alkaline volcanoes, including the massive Erebus volcano and its persistent phonolitic lava lake, that are widely distributed within and adjacent to one of the world’s major zones of lithospheric extension (the West Antarctic Rift System); and very young ultrapotassic volcanism erupted subglacially and forming a world-wide type example (Gaussberg).

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Explores the complex physico-chemical processes involved in active volcanism and dynamic magmatism Understanding the magmatic processes responsible for the chemical and textural signatures of volcanic products and igneous rocks is crucial for monitoring, forecasting, and mitigating the impacts of volcanic activity. Dynamic Magma Evolution is a compilation of recent geochemical, petrological, physical, and thermodynamic studies. It combines field research, experimental results, theoretical approaches, unconventional and novel techniques, and computational modeling to present the latest developments in the field. Volume highlights include: Crystallization and degassing processes in magmatic environments Bubble and mineral nucleation and growth induced by cooling and decompression Kinetic processes during magma ascent to the surface Magma mixing, mingling, and recharge dynamics Geo-speedometer measurement of volcanic events Changes in magma rheology induced by mineral and volatile content The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.