Sinh quyển của Trái Đất đã có thay đổi đáng kể bầu khí quyển và các điều kiện vô cơ khác, tạo điều kiện thuận lợi cho sự phổ biến của các vi sinh vật ưa khí cũng như sự hình thành của tầng ôzôn-lớp bảo vệ quan trọng, cùng với từ trường của Trái Đất, đã ngăn chặn các bức xạ có hại và chở che cho sự sống. | Exhumation and Cratonization 33 Cratonization Although cratons have long been recognized as an important part of the continental crust their origin and evolution is still not well understood. Most investigators agree that cratons are the end product of collisional orogenesis thus they are the building blocks of continents. Just how orogens evolve into cratons and how long it takes however is not well known. Although studies of collisional orogens show that most are characterized by clockwise P-T-t paths Thompson and Ridley 1987 Brown 1993 the uplift-exhumation segments of the P-T paths are poorly constrained Martignole 1992 . In terms of craton development the less than 500 C portion of the P-T-t path is most important. Using a variety of radiogenic isotopic systems and estimated closure temperatures in various minerals it is possible to track the cooling histories of crustal segments and when coupled with thermobarometry the uplift-exhumation histories. Results suggest wide variation in cooling and uplift rates most orogens having cooling rates 2 C My whereas a few such as southern Brittany cool at rates 10 C My Fig. . In most cases it would appear to take a minimum of 300 My to make a craton. Some terranes such as Enderby Land in Antarctica have had long exceedingly complex cooling histories lasting more than 2 Gy. Many orogens such as the Grenville orogen in eastern Canada have been exhumed as indicated by unconformably overlying sediments reheated during subsequent burial and then reexhumed Heizler 1993 . In some instances postorogenic thermal events such as plutonism and metamorphism have thermally overprinted earlier segments of an orogen s cooling history such that only the very early high-temperature history 500 C and perhaps the latest exhumation record 300 C are preserved. Fission track ages suggest that final uplift and exhumation of some orogens such as the Trans-Hudson orogen in central Canada may be related to the early stages of .
