Earth Materials. John O'Brien

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.

Figure 1.8 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). Sediment deposition atop these rocks produces layer 1 (yellow) of the crust. Sea floor spreading (black arrows) carries these laterally away from the ridge axis in both directions.

      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.

The formation of oceanic lithosphere by sea floor spreading implies that the age of oceanic crust should increase systematically away from the ridge in opposite directions. Crust produced during a period of time characterized by normal magnetic polarity should split in two and spread away from the ridge axis. New crust formed during the subsequent period of reversed magnetic polarity will form between the two areas of normally polarized crust and the reversely magnetized crust will also split in two. As indicated by Figure 1.9, repetition of this splitting process produces oceanic crust with bands (linear magnetic anomalies) of alternating normal and reversed magnetism whose age increases systematically away from the ridge, as initially explained by Vine and Matthews (1963).

Figure 1.9 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). The age of such bands should increase away from the ridge axis.

      Source: Courtesy of USGS.

Sea floor spreading was convincingly demonstrated in the middle to late 1960s by paleomagnetic studies and radiometric dating which showed that the age of ocean floors systematically increases in both directions away from the ridge axis, as predicted by sea floor spreading (Figure 1.10).

Hess (1962), and those who followed, realized that sea floor spreading causes the outer layer of Earth to grow substantially over time. If Earth's circumference is relatively constant and Earth's lithosphere is growing and being extended horizontally at divergent plate boundaries over long periods of time, then there must be places where it is undergoing long‐term horizontal shortening of similar magnitude. As ocean lithosphere ages and continues to move away from oceanic spreading centers, it cools, subsides, and becomes denser over time. The increased density eventually causes the strong ocean lithosphere to become denser than the underlying, weak asthenosphere. As a result, a plate carrying old, cold, dense ocean lithosphere begins to sink downward into the asthenosphere under a more buoyant plate edge, creating a convergent plate boundary.

Figure 1.10 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

Convergent plate boundaries occur where two plates are moving toward one another relative to their mutual boundary (Figure 1.11). The scale of such processes and the features they produce are truly awe inspiring.

       Subduction zones

The process by which the leading edge of a denser lithospheric plate is forced downward into the underlying asthenosphere is called subduction. The downgoing plate is called the subducted plate or downgoing slab; the less dense plate is called the overriding plate or slab. The area where this process occurs is a subduction zone. The subducted plate, whose thickness averages 100 km, is generally composed of dense oceanic lithosphere. Subduction is the major process by which oceanic lithosphere is destroyed and recycled into the asthenosphere and deeper Earth at rates similar to its creation along the oceanic ridge system. For this reason, subduction zone plate boundaries are also called destructive plate boundaries.

Figure 1.11 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