Shield (geology)
A shield in the geological sense is a larger area that is tectonically stable over a long period of time , in which the surface geology is dominated by crystalline , Precambrian rocks, some of which are intensely tectonically deformed . All of the larger volumes of (inevitably younger) sedimentary rocks have either been removed to this day in such a region or, due to a permanent topographical elevation, significant sedimentation never occurred .
Shield and blackboard
The basement is largely exposed in the shields, as the continental crust there has a low density and high thickness and is therefore constantly isostatically uplifted . The rocks visible on the surface of the earth today come from greater depths. As a rule, they have therefore undergone a metamorphosis , which has often made them harder and more resilient. If sediments are present, they are usually discordant on the basement , as the former had undergone a mountain formation and thus a phase of erosion before the deposition of the table sediments . Accordingly, there is a time gap between the youngest rocks of the shield and the oldest (furthest below) rocks of the sedimentary cover.
In contrast, tablets ( platform ) are cratons with younger cover so that the basement is not visible on the surface. In the area of the panels, the shields are covered with undeformed sediments ( overburden ). Since the crust of the basement of the tablets has a smaller thickness or a higher density than that of the shields, it has less upwelling and could therefore be covered several times by an epicontinental sea over the past 500 million years , which is ultimately the cause of the deposition of the table sediments was.
The term was coined in 1888 by the Viennese geologist Eduard Suess , who used it to define a pre-Cambrian mainland core that was welded together through orogenetic and metamorphic processes.
Occurrence
Is known u. a. the Canadian Shield , which makes up a third of North America around the Hudson Bay (see picture). It consists of crystalline and eruptive rocks of the Archean and Proterozoic . Even Africa has a number of large shields; some are still at a considerable altitude today .
In Europe there is the Baltic Shield , which covers most of Scandinavia via Finland to the Kola Peninsula . The core of the Baltic Shield, firmly welded by Precambrian folds and metamorphoses , has been rising several centimeters a year since the end of the Ice Age ( postglacial land elevation ), which is clearly visible in the changes in the Baltic Sea coasts.
Other shields are the Ukrainian Shield , which covers parts of central Ukraine , the Siberian Shield (also Angara platform) in Northeast Asia, the Yangtze Craton in China or the West and North Australian Shield in Australia (all in orange in the picture). The Russian tablet under part of the Eastern European Plain is not a sign, but a tablet.
literature
- André Cailleux, The Unknown Planet. Earth anatomy . Kindler's University Bibl., Paris / Munich 1968.
- Erich Schwegler et al .: Geology in brief . Verlag Ferd. Hirt, 3rd edition, Kiel 1969.
- Hans Murawski: Geological dictionary , Ferd.Enke-Verlag Stuttgart 1977.