Geology

Convergent boundaries. Plates may converge directly or at an angle. Three types of convergent boundaries are recognized: continent‐continent, ocean‐continent, and ocean‐ocean.

Continent‐continent convergence results when two continents collide. The continents were separated at one time by oceanic crust that was progressively subducted under one of the continents. The continent overlying the subduction zone will develop a magmatic arc until the ocean floor becomes so narrow that the continents collide. Because the continents are less dense than the oceanic crust, they will not be pulled down the subduction zone. One continent may override the other for a short distance, but the two continents eventually become welded together along a geologically complex suture zone that represents the original line of collision. The crust is thickened along the suture zone, resulting in isostatic uplift, mountain‐building, and thrust faulting.

Ocean‐continent convergence occurs when oceanic crust is subducted under continental crust. This forms an active continental margin between the subduction zone and the edge of the continent. The leading edge of the continental plate is usually studded with steep andesitic mountain ranges. Earthquakes occur in the Benioff zones that dip underneath the continental edge.

Magmatic arc is a general term for belts of andesitic island arcs and inland andesitic mountain ranges that develop along continental edges. These mountain ranges (also called volcanic arcs) are underlain by crust that has been thickened by intrusive batholiths that were generated by partial melting along the underlying subduction zone. The Sierra Nevada in California and Nevada is a volcanic arc. Volcanic arcs result from isostatic processes, compressional forces along the leading edge of the continent, and thrust faults that move slices of mountain‐belt rocks inward over the continental interior, creating backarc thrust belts. The additional weight of these rocks downwarps the inland area, forming a foreland basin. The foreland basin fills with eroded material from the mountain ranges or occasionally with marine sediments if it becomes submerged.

Ocean‐ocean convergence occurs when two plates carrying ocean crust meet. One edge of ocean crust is subducted beneath the other at an ocean trench. The ocean trench curves outward toward the subducting plate over the subduction zone. Data from earthquakes along the subducting plate show that the angle of subduction increases with depth. Subduction probably occurs to a depth of at least 670 kilometers (400 miles), at which point the plate probably becomes plastic.

Andesitic volcanism often forms a curved chain of islands, or island arc, that develops between the oceanic trench and the continental landmass. Modern‐day examples of island arcs are the Philippines and the Alaska Peninsula. Geologists think that at a depth of about 100 kilometers (60 miles) the asthenosphere just above the subduction zone partially melts. This mafic magma may then assimilate silicious rocks as it moves up through the overlying plate, forming a final andesitic composition that vents to form the island arc. The distance the island arc forms from the oceanic trench is dependent on the steepness of the subduction zone—the steeper the angle of subduction, the more quickly the subducted material reaches the magmaforming depth of 100 kilometers, and the closer the arc will be to the oceanic trench.

The trench becomes filled with folded marine sediments that slide off the descending plate and pile up against the wall of the trench. This accumulation is called the accretionary wedge or subduction complex. The accretionary wedge is continuously pushed up to form a ridge along the surface of the trench over the subducted crust. The forearc basin is the relatively undisturbed expanse of ocean floor between the accretionary wedge and island arc; the area on the continental side of the arc is called the backarc.

The backarc basin, the basin that occurs between the island arc and the continental mass, is occasionally split by new extensional forces into two parts that migrate in different directions ( backarc rifting). In other words, a “mini” spreading center develops as an equilibrium response to changes in the way the plate is being subducted. This backarc spreading can push the island arc away from the continent toward the subduction zone. If it develops along the continental edge, it can also split off a strip of the continent and push it seaward toward the subduction zone—Japan is a modern‐day example. The rifting may be caused by a mantle plume that has come near to the surface and is spreading out, creating convection currents that stretch the crust to the point of breakage.

The locations of oceanic trenches shift gradually with time, a phenomenon thought to be caused by the force of the leading edge of the overlying plate, which pushes the trench back over the subducting plate. This is because the overlying plate has a forward tectonic force and a gravitational force that bears down on the subducting plate. Some geologists believe the subducting material sinks at an angle that is steeper than that of the subduction zone, which would tend to pull the subducting plate away from the overlying plate, allowing the overlying plate to again move forward and push the oceanic trench back over the subducting plate.

Divergent boundaries. A divergent plate boundary is formed where tensional tectonic forces result in the crustal rocks being stretched and finally split apart, or rifted. The central block drops to form a graben, and basaltic volcanism is abundant along the rift's faults. The rise of hot mantle material beneath the rift zone pushes the rift valley farther apart (Figure 1). Today's active divergent boundaries are midoceanic ridges (sea floor spreading centers). Divergent boundaries can also develop on land, as did those that broke up Pangaea about 200 million years ago. Continental rifting can end before the crustal mass has been fully separated. These failed rifts then become seas or large basins that fill with sedimentary material. An example of a failed rift is the approximately two‐billion‐year‐old midcontinental rift in the United States, which extends from the Great Lakes area southward to below the Great Plains. The rugged topography of the rift was filled with coarse‐grained sediments and volcanic flows and has since been buried by thousands of feet of sedimentary rock deposited under Paleozoic oceans.

Figure 1

Divergent Plate Development

Geologists have debated for years whether uplift causes rifting or whether rifting causes uplift. Some scientists feel that rifting thins the crust, reducing the amount of pressure it can exert; the reduced pressure allows deeper, more pressurized rocks to ascend, causing uplift (similar to unloading and dome structures). Most geologists agree that uplift occurred after the rifting that resulted in the Red Sea in the Middle East.

Eventually the crust is totally split by continued divergence along the rift, and the two parts are separated by a new sea that floods the rift valley. New, basaltic oceanic crust continues to build up along the rift, causing high heat flows and shallow earthquakes. The Red Sea is at this stage of divergent separation.

Rivers do not discharge into the new ocean because the continental edges have been uplifted by the rising mantle material and slope away from the ocean. As divergence continues, the sea widens and the midoceanic ridge continues to grow. Eventually the continental edges subside as the underlying rocks cool and are further lowered by erosion. Rivers begin to flow into the sea forming deltas, and marine sedimentation begins to form the continental margin, shelf, and rise.

Transform boundaries. A transform boundary is a fault or a series of parallel faults (fault zone) along which plates slide past each other via strike‐slip movements. As previously discussed, transform faults connect offset midoceanic ridges (including the rift valleys). The motion between the two ridge segments is in opposite directions; beyond the transform fault, crustal movement is strike‐slip in the same direction. Thus, the transform fault “transforms” into a fault that has different motions along the same fault plane. Transform faults can connect diverging and converging boundaries or two converging boundaries (such as two oceanic trenches). Transform faults are thought to form because the original line of divergence is slightly curved. As an adjustment to mechanical constraints, the tectonic forces break the curved or irregular plate boundary into a series of pieces. The segments are separated by transform faults that are parallel to the spreading direction, allowing the ridge crest to be perpendicular to the spreading direction, which is the easiest way for two plates to diverge. Transform faults allow the divergent boundary to be in a structural equilibrium.