By Corina Fiore Updated Aug 30, 2022
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Plate tectonics describes the slow, but relentless, motion of the Earth's lithospheric plates. Convection currents in the mantle create divergent, convergent, and transform boundaries. At divergent margins, hot magma rises, creating mid‑ocean ridges and new crust. Convergent margins see denser oceanic lithosphere sink into the mantle, generating trenches, fold‑and‑thrust belts, and volcanic arcs. Transform boundaries, such as the San Andreas Fault, allow plates to slide past one another.
Igneous rocks form when magma cools, either beneath the surface (intrusive) or at the surface (extrusive). Divergent boundaries flood the ocean floor with basaltic magma that crystallizes into fine‑grained basalt. Convergent boundaries subduct sedimentary layers; as they are heated, the rock melts and rises, erupting as magma that solidifies into volcanic basalt or andesite. Intrusive bodies like dikes and batholiths also form when magma intrudes pre‑existing rock and cools slowly.
Metamorphism requires temperatures and pressures high enough to reorganize mineral structures without melting. At convergent margins, the immense pressure of colliding plates transforms limestone into marble or shale into schist—a process called regional metamorphism. At divergent and convergent zones, the heat of ascending magma induces contact metamorphism, producing greenschist or amphibolite facies around volcanic intrusions. Large mountain ranges, such as the Himalayas, are dominated by metamorphic lithology.
For a deeper dive, see the USGS Plate Tectonics Guide or the Wikipedia page.
