Plate Tectonics and Macrogeomorphology

2021 ◽  
pp. M58-2021-12
Author(s):  
Michael A. Summerfield

AbstractThe plate tectonics revolution was the most significant advance in our understanding of the Earth in the 20th century, but initially it had little impact on the discipline of geomorphology. Topography and landscape development were not considered to be important phenomena that deserved attention from the broader earth-science community in the context of the new model of global tectonics. This situation began to change from the 1980s as various technical innovations enabled landscape evolution to be modelled numerically at the regional to sub-continental scales relevant to plate tectonics, and rates of denudation to be quantified over geological time scales. These developments prompted interest amongst earth scientists from fields such as geophysics, geochemistry and geochronology in understanding the evolution of topography, the role of denudation in influencing patterns of crustal deformation, and the interactions between tectonics and surface processes. This trend was well established by the end of the century, and has become even more significant up to the present. In this chapter I review these developments and illustrate how plate tectonics has been related to landscape development, especially in the context of collisional orogens and passive continental margins. I also demonstrate how technical innovations have been pivotal to the expanding interest in macroscale landscape development in the era of plate tectonics, and to the significant enhancement of the status of the discipline of geomorphology in the earth sciences over recent decades.

Tonalites, including trondhjemite as a variety, played three roles through geological time in the generation of Earth’s crust. Before about 2.9 Ga ago they were produced largely by simple partial melting of metabasalt to give the dominant part of Archaean grey gneiss terranes. These terranes are notably bimodal; andesitic rocks are rare. Tonalites played a crucial role in the generation of this protocontinental and oldest crust 3.7- 2.9 Ga ago in that they were the only low-density, high-SiO 2 rocks produced directly from basaltic crust. In the enormous event giving the greenstone-granite terranes, mostly 2.8-2.6 Ga ago, tonalites formed in lesser but still important proportions by partial melting of metabasalt in the lower regions of down-buckled greenstone belts and by remobilization of older grey gneisses. Tectonism in the Archaean (3.9- 2.5 Ga ago) perhaps was controlled by small-cell convection (McKenzie & Weiss 1975). Little or no ophiolite or eclogite formed, and only minor andesite. Plate tectonics of modern type (involving large, rigid plates) commenced in the early Proterozoic. Uniformitarianism thus goes back one-half of the age of the earth. Tonalites compose about 5-10 % of crust generated in Proterozoic and Phanerozoic time at convergent oceanic-continental margins. They occur here as minor to prominent members of the compositionally continuous continental-margin batholiths. A simple model of generation of these batholiths is offered: mantle-derived mafic magma pools in the lower crust above a subduction zone reacts with and incorporates wall-rock components (Bowen 1922), and breaches its roof rocks as an initial diapir. This mantle magma also develops a gradient of partial melting in its wall rocks. This wall-rock melt accretes in the collapsed chamber and moves up the conduit broached by the initial diapir, the higher, less siliceous fractions of melting first, the lower, more siliceous (and further removed) fractions of melting last. The process gives in the optimum case a mafic-to-siliceous sequence of diorite or quartz diorite through tonalite or quartz monzodiorite to granodiorite and granite. The model implies that great masses of cumulate phases and refractory wall rock form the roots of continentalmargin batholiths, and that migmatites overlie that residuum and underlie the batholiths.


Marginal basins are common features of present-day plate tectonics. Whereas some may represent trapped segments of normal ocean floor, many owe their origin to extensional seafloor spreading behind active volcanic arcs. They exhibit a variety of forms. Some are completely intraoceanic; others develop at continental margins, where back-arc spreading may lead to the detachment and dispersal of continental fragments. Marginal basins can be recognized in the early stages of formation; others have developed through more than one pulse of back-arc extension, and some have aborted shortly after formation. Closure of marginal basins may result in preservation of part of the basin floor as obducted ophiolite. Although the reasons why seafloor spreading occurs behind volcanic arcs are still imperfectly understood, all suggested mechanisms invoke a strong link with subduction. Thus if subduction occurred in the past it is logical to expect that fossil marginal basins may be preserved in the geological record. However, allowing for the gradually evolving thermal and chemical nature of the Earth’s mantle, ancient marginal basins need not necessarily duplicate every feature of modern ones. This contribution examines possible Phanerozoic, Proterozoic and Archaean marginal basin analogues in the light of the geological features shown by modern basins and attempts to assess their importance for crustal development.


2017 ◽  
Author(s):  
◽  
Philip Tschirhart

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The Anthropocene is a geological and temporal designation that represents the end of an 11,000 yearlong geological time period and marks the beginning of a new epoch characterized by humans' deleterious relationship with the earth. Although scientists are still contemplating the Anthropocene's official designation, environmental activists are increasingly appropriating the designation in ways that complicate and challenge existing environmental discourses related to nature, technology, and politics. The Anthropocene, as a rhetorical invention, works to temporalize space and give agency to the collective self-organizing processes and kairologics that make imagining human and ecological universals necessary. In other words, the Anthropocene invokes a temporal meta-consensus that constitutes new universalisms. I analyze eight manifestos that take up the Anthropocene's implications. My analysis details how these contemporary environmental manifestos negotiate the discursive hegemony of the Anthropocene's universal postulates. I detail three universalisms entangled in the Anthropocene's rhetorical invention: First, the Anthropocene constitutes a transnational and collective human subjectivity. Second, the Anthropocene posits an all-encompassing and linear progression of time. And third, the Anthropocene presumes a totalizing earthly geometry and omnipresent ecology. In contrast, the manifesto as a genre works as a constitutively particularizing media. Manifestos emerge from the margins to challenge and politicize universals by juxtaposing their particular perspective with the status quo's totalizing universalisms. In the manifestos analyzed, the Anthropocene is temporalized and eventualized as a moment of meta-consensus, a space of appearance and pre-figuration, a moment to initiate movement. The manifestos rhetorical exigence is to politicize the spatial-temporality opened by the Anthropocene. In this way, the manifestos are kairopolitical. Each manifesto produces a counter temporality, a reading that posits a critical intervention in one or more of the universal imaginaries. The disparate counter narratives of the Anthropocene offer different cuts in its linear progression of time and attempt to find spaces to escape from its totalizing geometry. A diffractive reading of the Anthropocene as a 'kairotope' (McAlister, 2010), a temporally and spatially figurative rhetoric, challenges these universals and poses the question; what does a counter-public appropriation of the Anthropocene's rhetorical invention look like? Can the Anthropocene be made to condition a space for imagining alterity and opening up the commons to more sustainable and equitable relations of living? To answer that question it is necessary to read the paratextual criticisms and challenges that circulate around the contemporary Anthropocenic manifestos analyzed. I aim to problematize the Anthropocene's appropriation without denying its implications. Rather than suggest that, as humans', we might "rhetoric our way out of it" this project affirms the Anthropocene's global ecological exigence in effort to theorize how such a challenging and conflictual consensus might postulate new relations of solidarity and sustainability that begin with the interobjective materialism of the earth itself.


1993 ◽  
Vol 30 (2) ◽  
pp. 261-277 ◽  
Author(s):  
Jacob Verhoef ◽  
Walter R. Roest

The emergence and wide acceptance of plate tectonics has had a profound influence on the way we look at the Earth. Starting as a theory to explain similarities in coast lines across the Atlantic, plate tectonics has become a unifying theory in the earth sciences. In this paper, we describe the role of staff of the Geological Survey of Canada in the developing and refining of this theory. At the same time, we illustrate the effect plate tectonics has had on our understanding of the evolution of offshore eastern Canada. Of critical importance in this development was the unique data set collected by systematic surveying of this region, largely by the Geological Survey of Canada, making the Grand Banks of Newfoundland one of the best-studied offshore areas in the world. Plate tectonic theory not only offers a framework for the evolution of ocean basins, continental margins, and their sedimentary basins, but also for the assemblage of continents.


2020 ◽  
Vol 42 (3) ◽  
pp. 271-282
Author(s):  
OLEG IVANOV

The general characteristics of planetary systems are described. Well-known heat sources of evolution are considered. A new type of heat source, variations of kinematic parameters in a dynamical system, is proposed. The inconsistency of the perovskite-post-perovskite heat model is proved. Calculations of inertia moments relative to the D boundary on the Earth are given. The 9 times difference allows us to claim that the sliding of the upper layers at the Earth's rotation speed variations emit heat by viscous friction.This heat is the basis of mantle convection and lithospheric plate tectonics.


Sign in / Sign up

Export Citation Format

Share Document