Response: If one were to look up the definition of isostasy, they will not come across as you have put it," the balance that must exist (in equilibrium) between the gravitational force pulling the mountain down, and the bouyant force of the mantle below the mountain pushing it up". So this is incorrect. Then you go on to say,"Secondly it doesn't say mountains balance the Earth's surface, in other words, it doesn't say the Earth's surface shakes less because of mountains, or shakes less near mountains".
But it clearly says,"In order to balance the weight of the earth's surface, much of the compressed rock is forced downward, producing deep "mountain roots"(See the Book of "Earth", Press and Siever page. 413). Mountains therefore form downward as well as upward (see isostasy)"
The statement above cleary says, "In order to balance the weight of the earth's surface.." So mountains do in fact balance the earth's surface.
What you are doing here is playing with words.
Mr Spinkles was explaining a process. The process of tectonic plates rubbing together. The fact that "isostacy" to you means something other than the process he was explaining does not mean that his description of the process is not true.
In fact his explanation matches up with observations, and doesn't leave any suspicious gaps. And there is nothing in what he was saying to indicate that mountains as a rule stabilize the regions around them.
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