The lithosphere is the rigid outer layer of the solid Earth — the crust plus the uppermost, mechanically rigid part of the mantle — and it is the actual layer broken into tectonic plates, not the crust alone. **Thickness varies by type:** oceanic lithosphere ranges from essentially zero thickness at a mid-ocean ridge to roughly 100 km at its oldest; continental lithosphere is thicker and more variable, commonly 100–200 km and locally deeper beneath old, stable continental interiors ("cratons"). - **Defined thermally, not just compositionally:** the lithosphere–asthenosphere boundary is fundamentally a mechanical/thermal transition — rock above a certain temperature threshold (roughly 1300°C for mantle peridotite) becomes weak enough to flow over geological timescales, defining the top of the asthenosphere (Section 4.1). This is why oceanic lithosphere thickens predictably with age (Section 3.9): it is simply the depth to which the surface has had time to conductively cool below that threshold. - **Rigid but not indestructible:** the lithosphere behaves elastically on short (seismic) timescales and brittlely under sustained stress, which is why it fractures into discrete plates and produces earthquakes at their boundaries, rather than deforming smoothly the way the underlying asthenosphere does. Plates interact along three fundamentally different kinds of boundary, each with a distinct tectonic and volcanic signature. A striking asymmetry in Earth's rock record follows directly from the mechanisms above: the oldest oceanic crust anywhere on the planet is only around 200 million years old, while the oldest continental crust exceeds 4 billion years — a difference explained entirely by plate tectonic recycling, not by any difference in how the two crust types originally formed. - **The half-space cooling model:** newly formed oceanic lithosphere at a ridge is thin and hot; as it moves away from the ridge, it cools conductively from below, thickening and becoming progressively denser (and, correspondingly, subsiding — older ocean floor sits systematically deeper than young ocean floor, a relationship that closely follows a square-root-of-age law and is one of the best-confirmed quantitative predictions in all of plate tectonics). - **Why oceanic crust disappears but continental crust doesn****'****t:** oceanic lithosphere's increasing density with age is exactly what eventually makes it dense enough to subduct (Section 3.5); continental crust is too buoyant to subduct efficiently regardless of age (Section 3.4), so it persists, gets deformed and reworked, but is rarely destroyed outright — explaining why the compositional-extraction process described in Section 10.3 (continents built by progressively concentrating incompatible elements out of the mantle) has been able to accumulate continental material over the planet's entire history, while the oceanic crust doing much of that same extraction today is continuously destroyed and recycled within a couple hundred million years. ## 3.10 Hazards and Human Relevance The same processes covered above as abstract geophysics are also the direct cause of two of the most consequential natural hazard categories humans face. - **Earthquakes** are concentrated almost entirely along plate boundaries (Section 3.4), because that is where accumulated elastic strain in the brittle lithosphere is released; the deepest earthquakes (down to roughly 700 km) occur specifically within subducting slabs, well below the depth at which ordinary brittle rock fracture is normally possible, in a still-debated mechanism related to mineral phase transitions within the cold, fast-sinking slab itself.

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