Exposed stratigraphic thicknesses in Archean greenstone belts, high P-T mineral assemblages in Archean granulite-facies terranes, and K, Rb, and Sr geochemical polarity indices in Archean volcanic rocks suggest that large portions of the Archean crust were ≥25 km in thickness, and corresponding depths to subduction zone were ≥85 km prior to the emplacement of widespread Archean granitic rocks at 2.5 to 3.0 b.y. The polarity indices also suggest that the crust thickened to ≳30 km during emplacement of Archean granite. Compositional data suggest that at least some of the granitic rocks are not intrusive equivalents of greenstone volcanic rocks (at the same silica level).
The thickness and seismic velocity distribution in the present crust do not appreciably change as a function of mean crustal age from terranes ≲225 m.y. to those >2,500 m.y. in age. A decrease in the range of crustal thickness with age is observed, however, probably reflecting an increasing degree of cratonization with age. Most data suggest that the Archean crust closely approached its present-day thickness before or during emplacement of Archean granitic rocks and that the crust has not significantly changed in thickness since that time.