Plate Tectonics and Volcanism
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This chapter presents three great natural hazards: earthquakes, volcanic eruptions, and tsunamis. These three natural hazards pose increasing risks as economic activity and population continues to grow around the Pacific and eastern Indian Oceans, which are surrounded by convergent plate margins. The concept of convergent plate margin is similar to the concept of a convergent plate boundary except that the former encompasses broader regions on either side of the boundary itself. These are generally associated with subduction zones and pose a severe risk to nearby human populations. The chapter outlines basic plate tectonic concepts, and the most important features of convergent plate margins and subduction zones. The most spectacular surficial manifestations of a convergent margin are the trenches, which mark the plate boundary and where the greatest depths in any ocean are found. Another distinctive feature of most convergent plate margins is a chain of great volcanoes, parallel to and set back from the trench.
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The formation of a global network of plate boundaries surrounding a mosaic of lithospheric fragments was a key step in the emergence of Earth’s plate tectonics. So far, propositions for plate boundary formation are regional in nature but how plate boundaries are being created over 1000s of km in short periods of geological time remains elusive. Here, we show from geological observations that a >12,000 km long plate boundary formed between the Indian and African plates around 105 Ma with subduction segments from the eastern Mediterranean region to a newly established India-Africa rotation pole in the west-Indian ocean where it transitioned into a ridge between India and Madagascar. We find no plate tectonics-related potential triggers of this plate rotation and identify coeval mantle plume rise below Madagascar-India as the only viable driver. For this, we provide a proof of concept by torque balance modeling revealing that the Indian and African cratonic keels were important in determining plate rotation and subduction initiation in response to the spreading plume head. Our results show that plumes may provide a non-plate-tectonic mechanism for large plate rotation initiating divergent and convergent plate boundaries far away from the plume head that may even be an underlying cause of the emergence of modern plate tectonics.
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Abstract Plate tectonics, the grand unifying theory of geology, and its relation to the Earth is explained in this chapter. The planet transforms through time by means of the movement of rigid plates carrying the continents riding on the plastic material in the Earth’s upper mantle. Three major plate boundaries are divergent margins, where new ocean floor is being created along mid-ocean ridges and plates separate from one another; convergent margins, where the material is subducted and consumed as different types of plates collide, creating trenches, island arcs or mountain ranges, and transform boundaries; and where plates slide past one another. Besides the three predominant boundaries, hot spots caused by mantle plumes and diffuse boundaries make up additional dynamic forces in tectonics. Beyond these categories, geologists still are learning about tectonics; some boundaries are unknown or speculative. Plate tectonics explains why many of the Earth’s hazards are found where there are. Earthquakes trace many plate margins, as do volcanoes. The area around the Pacific Ocean is called the “Ring of Fire” because of the many volcanoes related to subducting plates. Tectonics accounts for why certain rocks are located where they are; for example, all rock types are found at convergent margins. The theory also predicts where valuable mineral and economic deposits are located.
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Petrologic and geochemical studies of basaltic rocks in the Yucca Mountain region are currently focused on understanding the evolution of volcanism in the Crater Flat volcanic field and the mechanisms of polycyclic volcanism at the Lathrop Wells volcanic center, the youngest center in the Crater Flat volcanic field. Geochemical and petrologic data indicate that the magma chambers which supplied the volcanic centers in Crater Flat became situated at greater crustal depths as the field evolved. Deep magma chambers may be related to a waning magma flux that was unable to sustain upper crystal magma conduits and chambers. Geochemical data from the Lathrop Wells volcanic center indicate that eruptive units identified from field and geomorphic relationships are geochemically distinct. The geochemical variations cannot be explained by fractional crystallization of a single magma batch, indicating that several magma batches were involved in the formation of the Lathrop Wells center. Considering the low magma flux in the Yucca Mountain region in the Quaternary, the probability of several magma batches erupting essentially simultaneously at Lathrop Wells in considered remote. It is more likely that the Lathrop Wells center was formed by a series of eruptions that took place over many thousands of years. Themore » geochemical data from Lathrop Wells is consistent with the concept of a complex, polycyclic volcano, which was originally proposed based on geomorphic and soil-development data.« less
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Abstract The coexistence of divergent (spreading ridge) and convergent (subduction zone) plate boundaries at which lithosphere is respectively generated and destroyed is the hallmark of plate tectonics. Here, we document temporally- and spatially-associated Neoarchean (2.55–2.51 Ga) rock assemblages with mid-ocean ridge and supra-subduction-zone origins from the Angou Complex, southern North China Craton. These assemblages record seafloor spreading and contemporaneous subduction initiation and mature arc magmatism, respectively, analogous to modern divergent and convergent plate boundary processes. Our results provide direct evidence for lateral plate motions in the late Neoarchean, and arguably the operation of plate tectonics, albeit with warmer than average Phanerozoic subduction geotherms. Further, we surmise that plate tectonic processes played an important role in shaping Earth’s surficial environments during the Neoarchean and Paleoproterozoic.
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<p>The formation of a global network of plate boundaries surrounding a mosaic of lithospheric fragments was a key step in the emergence of Earth&#8217;s plate tectonics. So far, propositions for plate boundary formation are regional in nature but how plate boundaries are being created over 1000s of km in short periods of geological time remains elusive. Here, we show from geological observations that a >12,000 km long plate boundary formed between the Indian and African plates around 105 Ma with subduction segments from the eastern Mediterranean region to a newly established India-Africa rotation pole in the west-Indian ocean where it transitioned into a ridge between India and Madagascar. We find no plate tectonics-related potential triggers of this plate rotation and identify coeval mantle plume rise below Madagascar-India as the only viable driver. For this, we provide a proof of concept by torque balance modeling revealing that the Indian and African cratonic keels were important in determining plate rotation and subduction initiation in response to the spreading plume head. Our results show that plumes may provide a non-plate-tectonic mechanism for large plate rotation initiating divergent and convergent plate boundaries far away from the plume head that may even be an underlying cause of the emergence of modern plate tectonics.</p>
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