A teoria da isostasia mostra que existe um equilíbrio isostático da litosfera sobre a astenosfera, refletido pelas altitudes relativas dos diversos segmentos da litosfera, dependendo de sua espessura e densidade do material que a compõe. A primeira implicação da existência desse equilíbrio é mostrar a importância da gravidade na dinâmica da Terra. Muitas vezes subestimada, a gravidade tem um papel fundamental em toda a história dinâmica da Terra. Nos estágios de formação do sistema solar controlou os processos de acresção de planetasimais e diferenciação primitiva do planeta em camadas de diferentes composições. Na dinâmica atual, controla a precipitação pluviométrica, o fluxo das águas continentais, o movimento das geleiras, a sedimentação, e outros tantos processos, além dos movimentos verticais das massas continentais e oceânicas.
Mas como a teoria da isostasia pode explicar os movimentos verticais? Acontece que qualquer modificação dos parâmetros anteriomente mencionados provoca desequilíbrio isostático causando movimentos verticais no sentido de recuperar a condição de equilíbrio. Esses movimentos, ditos de ajuste isostático, podem ocorrer quando uma região é sobrecarregada com algum material, causando movimentos de subsidência, ou descarregada, causando soerguimento. Uma outra possibilidade é a modificação de densidade do material litosférico. Quando uma região encontra-se em desequilíbrio isostático a magnitude desse desequilíbrio reflete-se na magnitude de anomalias gravimétricas. Os principais processos responsáveis pelos desequilíbrios isostáticos são glaciações e a sedimentação/erosão.
quarta-feira, 14 de outubro de 2009
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