A sign, old sign or platform is a large, geologically - tectonically stable area (Kraton ) within a continent that is not or hardly covered by younger sediments. Most or all the younger sediments were removed so that the crystalline Precambrian basement and its mountain building are open at the surface, while their little altered sedimentary cover is present only in the peripheral areas. In contrast, panels (including platform ) cratons with recently uncovered, so the basement is not visible on the surface.
Most shields were also raised in these processes, so that the now visible at the surface rocks come from greater depth. As a rule, they have therefore a metamorphosis behind him, which they often were harder and more resistant.
The term was coined in 1888 by the Viennese geologist Eduard Suess, who sent a Precambrian continental core defined, which was welded together by orogenic ( orogenic ) and metamorphosis operations.
It is known, inter alia, the Canadian Shield, which accounts for one-third of North America around the Hudson Bay ( see picture). It consists of crystalline and eruptive rocks of the Archean and Proterozoic. Africa also has a number of large shields; some are today in considerable altitude.
In Europe, there is the Baltic shield, which covers most of Scandinavia via Finland to the Kola Peninsula. The solid welded by Precambrian folds and metamorphoses core of the Baltic shield stands out since the end of the Ice Age several centimeters a year, which is clearly visible in the changes of the Baltic coasts.
More shields are the Ukrainian shield that covers parts of central Ukraine, the Siberian shield (also Angara platform) in Northeast Asia, the Yangtze craton in China or Australia, the Western and Nordaustralische shield (all orange in the picture). The Russian panel under part of the East European Plain is no sign, but - as the name suggests - a blackboard.