Spreading and subduction

Morphologically, mid-ocean ridges are the most complex and geologically active terrains on the seabed. They are now recognized as the places where the Earth’s oceanic crust is constructed (starting with the seminal paper of Vine and Matthews, 1963). Characterized by high topographic relief, the mid-ocean ridges are dominated by volcanic and tectonic processes. Globally, over 60,000 km of mid-ocean ridges produce ∼35 km

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of new volcanic crust every year. This new crust is welded to the retreating edges of the older crust as the Earth’s plates move apart. Some of these plates will collide, one plate sliding below another (a process known as subduction). This oceanic crust is destroyed in deep-ocean trenches, characterized by extreme topographic relief and dominated by tectonic and sedimentary processes. Spreading and subduction are the two extreme stages of the evolution of oceanic crust, corresponding to its creation and its recycling. Since the recognition of plate tectonics in the late 1960s, there has been much theoretical and practical work on their respective mechanisms. This chapter aims at presenting the main structures that can be observed on the seabed, and how they can be related to specific volcanic, tectonic, or sedimentary processes.