Defining the Lithosphere
The lithosphere is the Earth's rigid, outermost shell, comprising the crust and the uppermost part of the mantle. It acts as a brittle, solid layer that is broken into large, moving pieces known as tectonic plates. This layer resists deformation over long geological periods, distinguishing it from the more ductile layers beneath.
Composition and Structure
The lithosphere varies significantly in thickness, ranging from approximately 5-10 kilometers under ocean basins (oceanic lithosphere) to 100-200 kilometers beneath continents (continental lithosphere). Its upper part is the Earth's crust, which can be either thin, dense oceanic crust (basaltic) or thicker, less dense continental crust (granitic). The rigid upper mantle portion of the lithosphere shares a chemical composition with the deeper mantle but is mechanically distinct due to its cooler temperature.
Role in Plate Tectonics
The fragmentation of the lithosphere into tectonic plates is central to the theory of plate tectonics. These plates 'float' and slowly move over the semi-fluid asthenosphere, a weaker and hotter layer of the upper mantle. The interactions at the boundaries of these moving plates are the primary drivers for most of Earth's major geological activities, including earthquakes, volcanic eruptions, and the formation of mountain ranges.
Lithosphere vs. Asthenosphere
It is important to distinguish the lithosphere from the asthenosphere. The lithosphere is characterized by its rigidity and solid behavior, while the asthenosphere, although still solid, is ductile and capable of slow flow over geological timescales. This mechanical difference, largely due to temperature gradients, is what allows the lithospheric plates to move across the Earth's surface.