The process that led to the formation of the continents. The Earth's crust is distinctively bimodal in thickness. Oceanic crust is normally about 4 mi (7 km) thick, varying mainly with the temperature of the mantle beneath the sea-floor spreading ridges when the crust was formed. In contrast, the typical 22–25-mi (35–40-km) thickness of continental crust is controlled ultimately—through the agents of erosion, sedimentation, and isostatic adjustment—by sea level. Oceanic crust is formed at spreading RIDges, continental crust at subduction zones. Both are recycled to the mantle, but oceanic crust, being less buoyant, is recycled about 30 times faster than continental crust. Consequently, continents, having a mean age of almost 2 billion years and a maximum age of 4 billion years, provide the only directly accessible record spanning most of Earth history. They are, however, structurally more complex than ocean basins because of their great antiquity and weak rheology. See also Earth crust; Mid-Oceanic Ridge.
Constructive processes
Subduction zones are the main factories for making continental crust (see illustration). The primary constructive processes are trench accretion, arc magmatism, and arc-continent collision. Mantle plumes and lithospheric stretching cause secondary magmatic additions to continental crust. See also Lithosphere; Magma; Subduction zones.
Recycling of continental crust at a subduction zone. Arrows show direction of movement.
Trench accretion occurs where an oceanic plate sinks beneath a continental plate or another oceanic plate; sediment scraped off the top of the descending plate accumulates as an accretionary prism at the leading edge of the overriding plate; Trench accretion is dominantly a process of crustal reworking, not crustal growth, because the bulk of the sediment is derived from the erosion of preexisting crust.
A volcanic arc is a surface manifestation of partial melting near the tip of the mantle wedge above the subducting slab. Melting in the wedge is induced by the infiltration of aqueous fluids, which lower the temperature required for melting to begin (hydration melting). Although the main source of arc magmas is the mantle wedge, sediment and ablated crust entrained in the slab make subordinate contributions to arc magmas. The mantle-derived fraction represents new crustal addition.
Where subduction occurs beneath continental lithosphere, trench accretion and arc magmatism add crust to the continent directly. Continental-type protocrust is also formed at subduction zones (often having complex developmental histories) situated entirely within oceanic lithosphere. Incorporation of such protocrust in a continent is accomplished by arc-continent collision.
Plumes are jets of anomalously hot mantle that partially melts as it reaches the lithosphere, causing the volcanism of Hawaiian-type islands and related seamount chains. Plumes also cause volcanism on continents, for example, Yellowstone in North America. In addition to surface volcanism, the melts may pond at or near the base of the crust, causing magmatic underplating. Seamounts and oceanic plateaus are fragmentarily accreted to continents at trenches. See also
Destructive processes
Subduction zones may also cause destruction of continental crust (see illustration). The destructive processes are sediment subduction and subduction ablation. Constructive and destructive processes may be at work simultaneously, and the net balance may swing one way or the other with time. Selective destruction of lower crust may occur in continent-continent collision zones as a result of convective dripping of tectonically thickened lithosphere.
Even at subduction zones with well-developed accretionary prisms, some of the sediments disappear beneath the deformation front of the prism. Some is accreted structurally to the base of the prism, and some is transferred to the magmatic arc by melting. Some escapes melting and sinks deeply into the mantle, where it constitutes an isotopically recognizable component in the source of plume basalts. Sediment subduction, being fed by surface erosion, preferentially destroys the upper crust.
It is postulated that crust and lithospheric mantle from the overriding plate may become entrained in the subducting slab, causing tectonic ablation. Like subducted sediment, ablated crust is potentially capable of melting as it passes beneath the convecting mantle wedge. Unlike subducted sediment, ablated material may include lower crust and lithospheric mantle as well as upper crust.
Where continental lithosphere is tectonically thickened, cold lithospheric mantle is forced downward into hotter convecting mantle. Lithospheric thickening generates lateral thermal gradients, which drive mantle convection. The thickened lithospheric mantle is therefore unstable and may drip away. Unlike sediment subduction, upper crust is not lost by dripping; and unlike subduction ablation, continental drips are not susceptible to hydration melting.
Continents and Earth history
The mean age of extant continental crust is about 2 billion years. The conventional interpretation is that there was little continental crust following a period of high impact flux and that continental growth was slow at first, then rose to a peak 2–3 billion years ago, after which it slowly tapered off. An alternative interpretation, however, holds that the volume of continental crust has been in a near steady state since the impact flux waned. The present age distribution is explained by assuming a secular decline in the rate of crustal recycling, presumably modulated by the decreasing vigor of mantle convection as the Earth cooled. The difference in interpretation hinges on the importance assigned to recycling of continental crust. In crustal growth models, there is little crust older than 3 billion years, because little was formed. In steady-state models, much crust was formed early on but little of it survives. The steady-state interpretation is consistent with isotopic data showing that the oldest crustal relics contain highly evolved as well as juvenile components, and that the contemporaneous mantle was also heterogeneous and included strongly depleted regions. Furthermore, the near steady-state alternative is supported by comparative planetology, which indicates that for the hot young mantle not to be thoroughly differentiated by near-surface melting is physically implausible. See also Continental drift; Earth; Fault and fault structures; Marine geology; Geophysics; Plate tectonics; Seismology.
