A question I had while listening to Lecture 2.2, Does the quantity of mountains/ landmass keep increasing as the earth gets older and tectonic motion continues? cause When divergent rift occurs the material from the Asthenosphere is pushed up, and when convergent rift occurs while there are many possibilities one possibility that could reduce the landmass is when a plate submerges beneath another, but it is explained a little later that this submerged rock comes back up in the form of magma/lava. So is my assumption true or is there another process by which earth reduces the landmass in the lithosphere
I don’t know the answer, but one thing to keep in mind is that while mountains form typically where tectonic plates collide, they can also be eroded over time: according to 2.3, the Appalachian were likely as tall as the Himalayas at some point. So maybe we’re in a sort balance, with mountains appearing in some places, but disappearing in others, adding up to a somewhat constant overall “quantity” of mountains over time.
@Mervyn The New York Times did a short piece on processes that might contribute to a change in the mass of the Earth a couple years ago in their ‘Good Question’ section!
Anyhow, as far as I know there hasn’t been been much change in the total volume of the lithosphere, for at least a few billion years or so - it just keeps getting recycled through various geological processes.