I haven't kept up with all the conversation on this topic but the latest posts about the composition of the continental rocks vs. the oceanic crust is a topic we should all know a little about. If you do a search on "isostasy" you will find many wonderfully clear diagrams showing the continents "floating" on the oceanic crust like great icebergs, their roots mostly submerged, but with part of their mass standing above the ocean floor. This is all simply a matter of the relatively lower density of the continental material compared to the oceanic rocks. The idea that rock can deform and adjust itself in this way over long periods of time always seemed amazing to me. But somewhat less so now, after I came across a stone vault in a graveyard where one of the originally flat slabs of hard limesone on the roof had sagged into a pronounced swale over the course of a hundred years, simply under the force of gravity. Under the heat and pressure below the surface of the earth the rocks become more plastic, and given the long time spans of geologic history they are capable of a considerable amount of deformation.
The concepts of plate tectonics provide great explanations for how the processes that separate the earth's crust into moving segments also results in partial melting of the ocean rocks at subduction zones and the separation of lower density liquid to form continental type rock . It is now the only widely accepted scientific explaination of how the continents were formed.
Every time I see a story about the "lost continent of Mu" which sank into the Ocean the first word I think of is isostasy. Continents can't sink, and even if they were created at the level of the ocean floor they would rise up. They have no choice.
__._,_.___
No comments:
Post a Comment