26 mayo 2007

La Tectónica de Placas

Son muchas las evidencias que muestran que la litosfera, la capa más externa, no es contina, sino que está fragmentada en grandes bloques, las placas.
Las placas son fragmentos de litosfera. Están formadas, por tanto, por corteza terrestre y la parte del manto superior que está en contacto con aquella. Según la composición de la litosfera, podemos distinguir las placas continentales, que están formadas exclusivamente por litosfera contiental; las placas oceánicas, formadas sólo por litosfera oceánica, y las placas mixtas, constituidas por los dos tipos de litosfera.




La teoría de la Tectónica de placas, que surgió en 1968, permite explicar globalmente los procesos dinámicos que se producen en la Tierra: formación de cordilleras, distribución geográfica de los movimientos sísmicos y volcanes, etc. Según esta teoría, que reune las ideas sobre la deriva continental y los estudios sobre el fondo oceánico, la litosfera está dividida en fragmentos denominados placas litosféricas que se deplazan sobre la capa inferior, la astenosfera, formada por material más plástico. Las placas litosféricas se desplazan lentamente como un bloque rígido sobre la astenosfera, a velocidad muy lenta (entre 1 y 20 cm al año), y su tamaño y forma cambian a lo largo de millones de años. El "motor" que provoca el movimiento de las placas, son las corrientes de convección. Estas son corrientes cíclicas en el manto, formadas por material caliente, que al ser menos denso asciende (corriente ascendente) y que al enfriarse y aumentar su densidad, se hunde hacia zonas más profundas (corriente descendente). Allí se vuelve a calentar y se cierra el ciclo. El desplazamiento horizontal de este material en la corriente cíclica arrastra las placas.





Esta es una Teoria que explica algunos de los fenómenos geológicos que acontecen en la Tierra, de la cuál algunos científicos discrepan. Me gustaría que dejarais vuestros comentarios acerca de vuestra opinión sobre este tama ¿Os parece adecuada la explicación de las corrientes de convección?,¿Pensais que puede haber alguna otra explicación posible?

________________________________________________________Nacho

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