<P> The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic - the main exception to this being recent igneous intrusions . Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer immediately beneath it . </P> <P> Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries . Additionally, continental crustal material is transferred to oceanic crust by sedimentation . New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes . Also, material can be accreted horizontally when volcanic island arcs, seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements . Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones . Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction . </P> <P> It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time . One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount . By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Ga ago . The growth of continental crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time . </P>

Where are the youngest rocks in the earth's crust found