system
Given the following post, respond as if you're commenting on Reddit:
human
Historically, how tall have the tallest mountains on Earth been, relative to Mt. Everest?

I know that upheaval and erosion are constant processes, which suggest that which mountains are rising up and which mountains are wearing down is constantly changing.  But I have never come across a good answer for this.  Is Mt. Everest extraordinarily tall, geologically speaking?  Or has there been something much bigger in the distant past?
gpt
There might be a kilometer in it, but we're certainly not talking about 20km peaks.

There are several limits, and it's generally accepted that the Himamlaya are approaching those limits.

First of all you need a very rapid collision of continents to push material up quickly - the aim is to force the mountains up before the continental root can develop to form isostatic equilibrium.  If you collide slowly you form the root at the same rate you build the mountain, which means you need 4 or 5 times the amount of material entering the collision belt to generate a given height of mountain.  You also need this collision to happen quickly because the higher a mountain gets the more subject to erosion it is.  

We have rocks exposed now which were once part of mountain belts, and the rocks exposed now show metamorphic changes consistent with being 20km (or more) below the surface.  That does not mean the mountains were 20km tall.  It is a result of isostatic compensation; You add 20 km of material on top, and the whole plate sags down into the mantle.  The final mountain will only be a few km tall, the rest of the material has been buried and subsided.