Craton

A craton (Greek: κράτος kratos “strength”) is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as a few hundred kilometers into the Earth’s mantle.

The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons can be described as shields, in which the basement rock crops out at the surface, andplatforms, in which the basement is overlain by sediments and sedimentary rock.

The word craton was first proposed by the German geologist L. Kober in 1921 as “Kratogen”, referring to stable continental platforms, and “orogen” as a term for mountain or orogenic belts. Later authors shortened the former term to kraton and then to craton.

Formation

The process by which cratons are formed from early rock is called cratonization. The first large cratonic landmasses formed during the Archean eon. During the Early Archean, Earth’s heat flow was nearly three times higher than it is today because of the greater concentration of radioactive isotopes and the residual heat from the Earth’s accretion. There was considerably greater tectonic and volcanic activity; the mantle was much more fluid and the crust much thinner. This resulted in rapid formation of oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at subduction zones. There are at least three hypotheses of how cratons have been formed: 1) surface crust was thickened by a rising plume of deep molten material, 2) successive subducting plates of oceanic lithosphere became lodged beneath a proto-craton in an under-plating process, 3) accretion from island arcs or continental fragments rafting together to thicken into a craton.

Earth’s surface was probably broken up into many small plates with volcanic islands and arcs in great abundance. Small protocontinents (cratons) formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones.

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.

One evolutionary perspective of how the cratonization process “might” have first begun in the Archean is given by Warren B. Hamilton:

Very thick sections of mostly submarine mafic, and subordinate ultramafic, volcanic rocks, and mostly younger subaerial and submarine felsicvolcanic rocks and sediments were oppressed into complex synforms between rising young domiform felsic batholiths mobilized by hydrous partial melting in the lower crust. Upper-crust granite-and-greenstone terrains underwent moderate regional shortening, decoupled from the lower crust, during compositional inversion accompanying doming, but cratonization soon followed. Tonalitic basement is preserved beneath some greenstone sections but supracrustal rocks commonly give way downward to correlative or younger plutonic rocks… Mantle plumesprobably did not yet exist, and developing continents were concentrated in cool regions. Hot-region upper mantle was partly molten, and voluminous magmas, mostly ultramafic, erupted through many ephemeral submarine vents and rifts focussed at the thinnest crust…. Surviving Archean crust is from regions of cooler, and more depleted, mantle, wherein greater stability permitted uncommonly thick volcanic accumulations from which voluminous partial-melt, low-density felsic rocks could be generated.

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2 Responses to Craton

  1. alfy says:

    Hi, Jim. A classic post for Phil, who is just now into fundamental global geology. The final paragraph from Hamilton is a classic. We need a glossary of all the specialist terms. I counted eight terms incomprehensible to me.

    I had hoped that your post might answer the issue I suggested to Phil in our discussions. In answer to his question, “How did cratons arise?” I suggested that the early global pool of magma contained a wide range of complex metallic silicates and as cooling progressed, there was a separation of a “lighter” (less dense) fraction towards the top and a “heavier” fraction below it. The lighter fraction cooled into cratons and began the process of accumulating into continental tectonic plates which to this day are less dense than the material on which they float.

    The lower fraction provided the magma which created sea-floor spreading and the denser oceanic crustal plates. It also provided material for the rising hot plumes for making offshore volcanic archipelagoes.

    This seems to be a simple explanation but I am not certain that it is correct as I am no geologist. It may be so simple as to be perfectly obvious to a professional who would be much more interested in precise mechanisms and details of crystalline rock types observable in cratons and oceanic crusts.

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