The lithosphere is the brittle outer layer of the solid Earth. The plates of plate tectonics are segments of the lithosphere.
The lithosphere is basically the layer of the Earth that is strong enough to flex rather than flow. Its top is easy to seeit's at the Earth's surfacebut the base of the lithosphere is a transition rather than a sharp boundary. The details of that transition are an active area of research.
Flexing the Lithosphere
The lithosphere is not totally rigid, but slightly elastic. It flexes when loads are placed on it or removed from it. Ice-age glaciers are one type of load. In Antarctica, the thick ice cap has pushed the lithosphere well below sea level today, for example. In Canada and Scandinavia, the lithosphere is still unflexing where the glaciers melted about 10,000 years ago. Here are some other types of loading:
- Construction of volcanoes
- Deposition of sediment
- Rise in sea level
- Formation of large lakes and reservoirs
Here are other examples of unloading:
- Erosion of mountains
- Excavation of canyons and valleys
- Drying up of large water bodies
- Lowering of sea level
The flexing of the lithosphere from these causes is relatively small (usually much less than a kilometer), but measurable. We can model the lithosphere using simple engineering physics, as if it were a metal beam, and get an idea of its thickness. (This was first done in the early 1900s.) We can also study the behavior of seismic waves and place the base of the lithosphere at depths where these waves begin to slow down, indicating softer rock.
These models suggest that the lithosphere ranges from less than 20 kilometers in thickness near the mid-ocean ridges to about 50 km in old oceanic regions. Under the continents the lithosphere is thicker, from around 100 to as much as 350 km.
These same studies show that underneath the lithosphere is a hotter, softer layer of solid rock named the asthenosphere. The rock of the asthenosphere is viscous rather than rigid and deforms slowly under stress, like putty. Therefore the lithosphere can move across or through the asthenosphere under the forces of plate tectonics. This also means that earthquake faults are cracks that extend through the lithosphere, but not beyond it. (more on faults)
The lithosphere includes the crust (the rocks of the continents and the ocean floor) and the uppermost part of the mantle beneath the crust. These two layers are different in mineralogy but very similar mechanically, so for the most part they act as one plate. Although many people refer to "crustal plates," it's more accurate to call them lithospheric plates.
It appears that the lithosphere ends where the temperature reaches a certain level that causes average mantle rock (peridotite) to grow too soft. But there are many complications and assumptions involved, and we can only say that the temperature would be from about 600º to 1200ºC. A lot depends on pressure as well as temperature, and of course the rocks vary in composition too thanks to plate-tectonic mixing. It's probably best not to expect a definitive boundary. Researchers often specify a thermal, mechanical or chemical lithosphere in their papers.
The oceanic lithosphere is very thin at the spreading centers where it forms, but it grows thicker with time. As it cools, more hot rock from the asthenosphere freezes onto its underside. Within about 10 million years, oceanic lithosphere becomes denser than the asthenosphere beneath it. Therefore, most of the oceanic plates are ready for subduction whenever it happens.
Bending and Breaking the Lithosphere
The forces that bend and break the lithosphere come mostly from plate tectonics.
Where plates collide, the lithosphere on one plate sinks downward into the hot mantle. In that process of subduction, the plate bends downward as much as 90 degrees. As it bends and sinks, the subducting lithosphere cracks extensively, triggering earthquakes in the descending rock slab. In some cases (such as in northern California) the subducted part can break off completely, sinking into the deep Earth as the plates above it change their orientation. Even at great depths, subducted lithosphere can be brittle for millions of years, as long as it is relatively cool.
The continental lithosphere can split, with the bottom part breaking off and sinking. This process, called delamination, is a hot topic in plate tectonics. The crustal part of the continental lithosphere is always less dense than the mantle part, which in turn is denser than the asthenosphere beneath. Gravity or drag forces from the asthenosphere can pull the crustal and mantle layers apart. Delamination allows hot mantle to rise and yield melt underneath parts of a continent, causing widespread uplift and volcanism. Places like California's Sierra Nevada, eastern Turkey and parts of China are being studied with delamination in mind.