Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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the surface form basic plutonic rocks such as gabbro that become layer 3 in oceanic crust. Melts intruded into near vertical fractures above the chamber form the gabbroic‐basaltic parallel “sheeted” dikes that become layer 2b. Lavas that flow onto the ocean floor commonly form basaltic pillow and sheet lavas that become layer 2a. The marine sediments of layer 1 are deposited atop the basalts as they spread away from the ridge axis. In this way layers 1, 2, and 3 of the oceanic crust are formed. The underlying mantle consists of ultramafic rocks (layer 4). Layered ultramafic rocks form by differentiation near the base of the basaltic‐gabbroic magma bodies, whereas the remainder of layer 4 represents the unmelted, refractory residue that accumulates below the magma bodies.

Schematic illustration of the formation of oceanic crust along the ridge axis generates layer 2 pillow basalts and dikes and layer three gabbros of the oceanic crust (blue) and layer 4 mantle peridotites (gray).

      Because the ridge axis marks a divergent plate boundary, the new sea floor on one side moves away from the ridge axis in one direction and the new sea floor on the other side moves in the opposite direction relative to the ridge axis. More melts rise from the asthenosphere and the process is repeated, sometimes over >100 Ma. In this way ocean basins grow by sea floor spreading as though new sea floor was being added to two slowly moving conveyor belts that carry older sea floor in opposite directions away from the ridge where it forms (Figure 1.8). Because most oceanic lithosphere is produced along divergent plate boundaries marked by the ridge system, these boundaries are also called constructive plate boundaries.

      As sea floor spreads away from the ridge axis, the crust thickens from above by the accumulation of marine sediments and the lithosphere thickens from below by a process called underplating that occurs as the solid, unmelted portion of the asthenosphere spreads laterally and cools through a critical temperature below which it becomes strong enough to fracture. As the entire lithosphere cools, it contracts, becomes denser, and sinks, so that the floors of the ocean gradually deepen away from the thermally elevated ridge axis. As explained in the next section, if the density of oceanic lithosphere exceeds that of the underlying asthenosphere, subduction occurs.

Schematic illustration of model depicts the production of alternating normal (colored) and reversed (white) magnetic bands in oceanic crust by progressive sea floor spreading and alternating normal and reversed periods of geomagnetic polarity (a–c).

      Source: Courtesy of USGS.

Schematic illustration of world map showing the age of oceanic crust; such maps confirmed the origin of oceanic crust by sea floor spreading.

      Source: From Lamont Doherty Earth Observatory.

      1.5.3 Convergent plate boundaries

       Subduction zones

Schematic illustration of convergent plate boundary, showing trench-arc system, inclined seismic zone and subduction of oceanic lithosphere.

      The surface expressions of subduction zones are large trench‐arc systems of the kind that encircle most of the shrinking Pacific Ocean (Isacks et al. 1968). Trenches are deep, elongate troughs in the ocean floors marked by water depths that can exceed 11 km. They are formed as the downgoing slab forces the overriding slab to bend downward forming a long trough along the boundary between them.

      Because the asthenosphere is mostly solid, it resists the downward