Craton: Difference between revisions

Cratons have thick lithospheric roots. Mantle [[seismic tomography|tomography]] shows that cratons are underlain by anomalously cold mantle corresponding to [[lithosphere]] more than twice the typical 60-mile (100&nbsp;km) thickness of mature oceanic or non-cratonic, continental lithosphere.<ref name=SN24>Petit (2010) p.24</ref> At that depth, craton roots extend into the [[asthenosphere]].<ref name=SN24/> Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy, and a low intrinsic [[isopycnic]] density. This low density offsets density increases due to [[geothermal contraction]] and prevents the craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere&mdash;up to 4 billion years versus 180 million years.<ref>Petit (2010) p.25</ref>

An associated class of inclusions called [[eclogite]]s, consists of rocks corresponding compositionally to oceanic crust ([[basalt]]) that has [[metamorphosed]] under deep mantle conditions. [[Isotope|Isotopic]] studies reveal that many eclogite inclusions are samples of ancient oceanic crust subducted billions of years ago to depths exceeding 90&nbsp;mi (150&nbsp;km) into the deep kimberlite diamond areas. They remained fixed there within the drifting tectonic plates until carried to the surface by deep-rooted magmatic eruptions.

If peridotite and eclogite inclusions are of the same temporal origin, then peridotite must have also originated from spreading sea floor ridges billions of years ago, or from mantle affected by [[subduction]] of oceanic crust. During the early years of Earth's existence, when the planet was much hotter, greater degrees of melting at spreading oceanic ridges generated oceanic lithosphere with thick crust, much thicker than 12 miles (20&nbsp;km), and a highly depleted mantle. Such a lithosphere would not sink deeply or subduct, because of its [[buoyancy]] and the removal of denser melt that in turn increased the density of the residual mantle. Accordingly, cratonic mantle roots are probably composed of buoyantly subducted [[Subduction|slabs]] of a highly depleted oceanic lithosphere. These deep mantle roots increase the stability, anchoring and survivability of cratons and makes them much less susceptible to [[tectonic theory|tectonic thickening]] by collisions, or destruction by [[sediment]] subduction.

There were no large continents in the Early Archean, and small protocontinents were probably the norm in the Mesoarchean because they were prevented from coalescing into larger units by the high rate of geologic activity. These [[felsic]] protocontinents (cratons) probably formed at hot spots from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Although the first continents formed during the Archean, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5-40% of the present [[continental crust]] formed during the Archean.<ref>Stanley (1999)</ref>