The Earth's crust constitutes the outermost rigid layer of the planet, fundamentally defining the physical surface upon which global geological processes occur. It is structurally heterogeneous, primarily divided into continental crust, composed mainly of felsic materials, and oceanic crust, which is predominantly mafic in composition. The thickness and composition of the crust vary widely depending on the tectonic setting, and its movement is integral to the theory of plate tectonics. These immense plates interact at boundaries, leading to geological phenomena such as vulcanism, metamorphism, and seismic activity, thereby shaping the planet's surface over vast geological timescales.