Bob, Mountains are parts of larger dynamic systems. They do not erode completely away in millions of years. In many of the cases these rocks have been recovered by other sediments and rocks, folded, buried, and exposed again at the surface. So in most cases they were not continually at the surface of the earth. The Guinea shield for example is a Precambrian aged metamorphic rock sequence that has been exposed by erosion at the surface. This is where the tepuis are, where Angel Falls tumbles off, etc. These refer to rocks or bands of rocks that have not be melted by the general rock cycling process since they initially formed. For most rocks they have been at one of the edges of many of the tectonic plates that have existed for the past 4 billion years. The rocks at the edge are typically subducted by the edge of one tectonic plate being forced under the adjacent plate. These rocks are drug down under the other plate, where they remelt and return to the surface through volcanoes. This is what is going on in places like the Philippines, Japan, the pacific northwest. These volcanoes are being fed by remelted crust material. (their water content makes them melt at a lower temperature). Eventually almost all of the crustal material has been recycled, except for some of these rare cases. Places like the Guyana shield have never been subducted. The other thing is that the mountains have a root, much like the hidden part of an iceberg, that extends deep into the crust. As the top erodes away this portion floats higher. So some of these deposits are from ancient mountains, but represent the "roots" of the mountains.
Ed "The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science." - Albert Einstein --~--~---------~--~----~------------~-------~--~----~ Eastern Native Tree Society http://www.nativetreesociety.org Send email to [email protected] Visit this group at http://groups.google.com/group/entstrees?hl=en To unsubscribe send email to [email protected] -~----------~----~----~----~------~----~------~--~---
