Final answer:
Continental crust cannot be subducted because it is thicker and less dense than oceanic crust, thus more buoyant and unable to sink into the asthenosphere. Subduction typically involves the denser and thinner oceanic plates moving beneath other plates at subduction zones.
Step-by-step explanation:
Continental crust cannot be subducted because it is too buoyant compared to the denser oceanic crust and the asthenosphere. The continental crust, primarily composed of granite, is less dense and thicker than the basaltic oceanic crust. Subduction zones, where one plate moves under another, typically involve the denser, thinner oceanic plates being thrust into the upper mantle at these zones. The lighter continental plates, on the other hand, tend to rise higher on the mantle rather than being pushed down. The recycling of Earth's crust occurs through this subduction process, with oceanic plates sinking and melting eventually, contributing to mantle convection currents that are part of the planet's tectonic activities.
In contrast, the continental crust has an average density of about 2.7 g/cm³, making subduction impractical. This density differential is a critical factor that prevents continental crust from subducting beneath oceanic crust or other continental crust. Furthermore, the process of subduction is facilitated by the mechanical properties of the lithosphere and the underlying more ductile asthenosphere, which allows the more rigid oceanic lithosphere to bend and subduct.
Therefore, the balance of new crust formation at rift zones and the destruction at subduction zones does not include the subduction of continental crust. Instead, these massive landforms may collide and form mountains through the process called orogeny, but they do not easily succumb to the subduction process.