Ocean floor gets recycled, but continents are nearly permanent
The oldest ocean floor on Earth is only about 170 million years old, because seafloor is constantly born at ridges and swallowed at subduction zones. Continents are too buoyant to sink far, so some of their deep roots have survived for more than three billion years.
The lithosphere is a planet's rigid outer shell. On Earth it includes the crust plus the top layer of the mantle, which behaves elastically over thousands of years or more. Beneath it lies the asthenosphere, hotter and weaker rock that can slowly convect. The boundary between the two is defined by how they respond to stress, not by chemistry: the lithosphere bends or breaks, while the asthenosphere flows. Geologists often place the boundary at roughly 1,000 degrees Celsius, where olivine, the upper mantle's weakest mineral, turns ductile. The lithosphere is broken into the tectonic plates.
The idea took shape early in the twentieth century. The English mathematician A. E. H. Love described a strong outer layer in 1911, and the American geologist Joseph Barrell coined the term lithosphere after inferring from gravity anomalies over continents that a solid layer must rest on a weaker, flowing one. Reginald Aldworth Daly expanded the concept in 1940.
Oceanic lithosphere starts thin at mid-ocean ridges, barely thicker than the crust, then thickens as it cools and moves away, reaching about 140 kilometres in its oldest parts; its growth roughly follows the square root of its age. After a few tens of millions of years it becomes denser than the asthenosphere below, and that instability makes it sink at subduction zones. Sinking slabs stay rigid enough to produce earthquakes down to about 600 kilometres, and some pieces appear to reach nearly 2,900 kilometres, close to the core.
Continental lithosphere is lighter and ranges from about 40 to perhaps 280 kilometres thick. It can be dragged only about 100 kilometres down before bobbing back up. Under ancient cratons, unusually thick, low-density mantle roots stabilise the crust, and rock fragments carried up in kimberlite pipes show some of these roots are over three billion years old.
Source: Lithosphere
