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5.10: Oceanic Lithosphere and Basins

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    Oceanic Lithosphere and Basins

    Origin of oceanic lithosphere

    At mid-ocean ridges (spreading centers), lithospheric plates move apart. This creates space for magma to flow upward into the newly created fractures. Over time, more and more fractures form, fill with magma, and then cool and fracture. This process generates new oceanic lithosphere (ocean crust).

    Zones of active rifting along mid-ocean ridges are typically 12 to 18 miles (20 to 30 km) wide. In some locations, the very hot, fluid lava migrating upward from the asthenosphere (upper mantle) reaches the surface of the seafloor resulting in formation of undersea volcanoes. These undersea eruption produce pillow basalts - pillow-shaped pods of basalt rock formed where the hot lava cools rapidly when exposed to seawater.

    As new lithosphere forms, it gradually moves away from the mid-ocean ridge crest beyond the zone of active rifting and volcanism. Over time, the cooling crust gets denser and isostatically sinks lower where it is floating on the asthenosphere. Oceanic sediments gradually blanket the aging oceanic crust as it moves away from the spreading center. The layer of sediment grows thicker and thicker as it moves away from the mid-ocean ridge.

    Structure of the oceanic lithosphere

    Four distinct layers of oceanic lithosphere (combined are called an ophiolite sequence)
    Ophiolite sequence • Layer 1: sequence of unconsolidated sediments
    Consists mostly of plankton remains and fine dust blown in from distance sources including from land (continental deserts sources) and meteorite dust.
    • Layer 2: consisting of pillow lavas
    Forms from basaltic lava erupting on the surface of the seabed, rapid cooling from exposure to seawater creates the pillow-like structure of the lava beds on the sides of underwater volcanoes.
    • Layer 3: interconnected dikes called sheeted dikes
    Newly cooled igneous rock formed at depth shrinks and fractures as it cool, allowing more magma to inject upward into new fractures in the expanding rift zone along the axis of a spreading center
    • Layer 4: gabbro (like basalt but slowly cooled at depth)

    Formation of oceanic lithosphere
    • Magma (liquid material carrying crystals with it) originates from partially melted mantle forms Layer 4.
    • Magma injected into fractures above the magma chambers creates the sheeted dike complex, forming Layer 3.
    • Pillow basalts are from basaltic lava that is flash cooled in seawater, forming Layer 2.
    • Sediments deposit on top of pillow basalts, forming Layer 1.

    Aging of oceanic lithosphere results in chemical and physical changes
    Once new oceanic lithosphere forms it begins to cool. The lithosphere is very warm relative to the cold ocean water above it. Large quantities of seawater sink into the new ocean crust and chemically reacts with it. These chemical process are a form of metamorphism. Ultramafic rock (rocks enriched in magnesium and iron) that formed deep in the upper mantle and oceanic lithosphere can gradually be altered into serpentinite. Large amounts of serpentinite are exposed in the Coast Ranges of northern California where old ocean crust has been pushed up and exposed in the mountain ranges.

    Formation of an Ophiolite sequence gabbro Pillow basalts at Avila Beach, CA Serpentinite outcrop near San Jose, CA
    Figure 5.25. Formation of an ophiolite sequence (ocean crust) near mid-ocean ridges. Figure 5.26. Gabbro, a dominant crystalline igneous rock type formed in oceanic lithosphere. Figure 5.27. Pillow basalts exposed near Avila Beach, central California. Figure 5.28. Serpentinite (the State Rock of California) is a metamorphic rock derived from altered oceanic lithosphere (mostly Layer 4). This outcrop is part of a large mountain-sized block of oceanic crust exposed in the mountains near San Jose, California.
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