Will there come a point in the future at which all land that we have now will have gone back into the Earth due to subduction, and be replaced by new land ?
It’s not guaranteed that it all will be recycled, though much of it will. There are parts of Australia that are over 4 billion years old, and the crust there dates to when the moon was formed.
Isn’t the land being subducted, by and large, oceanic crust? Oceanic crust originates in spreading zones, and consists largely, I believe, of basalt, which is heavier than craton constituents. Thus the oceanic crust subducts and the (relatively) lighter continental crust rides up over the top. Based on this, I would venture a guess that continental cratons have relatively intact for quite a while.
Doing this off the top of my head, so I could be mistaken in some of it.
Subduction is for ocean beds - I’d forgotten that obvious point.
You are correct. Pretty much only oceanic crust gets subducted. Where oceanic crust collides with continental crust, the oceanic crust will be subducted and the lighter continental crust will ride over it. Where continental crust collides with continental crust, as in the Himalayas, continental crust may be subducted to some extent, but most of it will be pushed up as mountains. Since new continental crust is formed by volcanism, the size of the continents has grown over geological time as new crust has been accreted to the ancient cratons that form the cores of continents.
Doesn’t that eventually lead to a collapse of the cratons? We can’t keep making the continents larger and higher. Eventually, they’ll have to collapse in on themselves and start the process again.
The continents get larger in area but not higher on average, because volcanoes and mountains pushed up by collisions erode. (The faster they are pushed up the faster they erode.) The cratons are actually very low relief after billions of years of erosion. (They are commonly known as “shields” because of this.)
Here’s a map of the major cratons. The green areas have been accreted to the continents since the Precambrian. You can see that the major mountain areas are around the edges of cratons rather than within them.
Oceanic crust doesn’t tend to stick around long, geologically speaking. The oldest currently existent crust is only Jurassic, and most of it is a lot younger: https://www.ngdc.noaa.gov/mgg/ocean_age/data/2008/image/age_oceanic_lith.jpg
As for “recycling” the exact fate of the subducted slabs isn’t exactly clear. There’s some evidence that they tend to just pile up in “slab graveyards” at the mantle-core boundary at 660 KMs: Crustal recycling - Wikipedia
At any rate, the mantle is VASTLY larger volume-wise than the crust (particularly since we’re only talking oceanic crust), so the chance that any particular bit of matter would subduct, recycle through the mantle and then be regurgitated out of a mid-oceanic spreading center are pretty slim.
That’s not strictly-speaking true. That’s the oldest ocean crust that’s still in the ocean basins. Relict bits of ocean crust (ophiolite) do get emplaced along with continental crust. This has been happeningfor the last 3.8 billion years, at least. Ophiolites are a big component of some greenstone belts. I’ve been to the Barberton greenstones, for instance, and seen the pillow lavas and sheet dykes, which are characteristic of ophiolite sequences.
To answer the OP - I think the fact that we still have un-subducted rocks around from 3.8 Ga says no, not all land will be recycled.