Introduction to the Marine Environment from a Geological Perspective

PRATOLONGO, P.; PAN, J.; CUADRADO, D.

Marine Biology A Functional Approach to the Oceans and their Organisms

CRC Press -Taylor and Francis

Lugar: Miami; Año: 2022; p. 3 - 20

The age of the ocean is about 4,000 MY. Considered the largest habitat for life on Earth (97% of the biosphere by volume), oceans cover 71% of its surface and harbor 97.5% of its superficial water.The oceans produce about 40% of the annual global primary productivity, and their average salinityis 34.48. The oceanic crust is ~ 8 km-thick, and geologically younger (≤ 200 MY) in relation to continental crust. Ocean basins are formed by virtue of the oceanic crust being made up of basaltic material, which is denser than the granitic rocks that continental crusts are made of. The hypotheses of isostasy explains differences in thickness in Earth?s lithosphere. The lighterlithosphere floats over the denser underlying material of the asthenosphere. Isostatic equilibriumoccurs where the crust and mantle settle into place in absence of disturbing forces. There are twomain hypotheses (Airy?s and Pratt?s) for isostatic adjustment. The hypothesis of continental drift has been incorporated in the Theory of Plate Tectonics, which explains that continents move as part of lithospheric plates on top of the asthenosphere, as a result of the underlying convection cells in the upper mantle. Where two plates meet, there might be divergent boundaries in the case of two lithospheric plates being pulled apart from each other. For instance, at mid-ocean ridges, new oceanic crust and lithosphere is created by seafloor spreading; at zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form. On the other hand, convergent boundaries occur where two plates slide toward each other to either form a subduction zone or a continental collision. Finally, in transform or conservative boundaries, two lithospheric plates slide along transform faults, where plates are neither created nor destroyed. The Wilson cycle explains the cyclical opening and closing of ocean basins. Active continental margins are associated with convergent or transform plate boundaries; a narrow continental shelf quickly drops off into the depths of the subduction trench. On the other hand, passive margins occur away from plate boundaries, tectonic activity is minimal, and there is sediment accumulation on the wider continental shelves.