Plate tectonics..................Do you guys reckon theres an infinite supply of magma etc , or will the earth stop growing after a few billion years after the continental **** reaches its zenith?
Growing? Uh... Earth doesn't grow, people stopped believing that early last century.
To answer your question; yes, the Earth is cooling, slowly but surely. Once this cooling has reached a certain stage [we're talking a billion years or two], plate tectonics will effectively cease, and continents will no longer drift. But remember, continental drift is not culminating in anything, meaning there is no 'zenith', it's a constant process that has been going for as long as there have been plates, and will continue until the aesthenosphere has cooled enough to disallow movement.
It's worth noting this is already seen on Mars, as she is a far older planet than Earth, and has cooled to a degree that plate tectonics has all but ceased. I believe that explains the size of Olympus Mons as well; if I remember correctly, it is theorised that the mountain is situated over a hot-spot [much like Hawaii], and with the plates not drifting at all, the volcano formed has just grown and grown to the size it is now.
Aaaaah, knowledge is a truly wonderful thing.
