Do Tectonic Plates Merge?

Question

I had a question concerning the nature of continental plates.

When looking at the Eurasian Plate today, for the most part, there is one solid fragment of the Earth's crust. However, having had a look at maps detailing prehistoric Earth, this isn't always the case. Throughout the entire Phanerozoic Eon, Eurasia has been a collection of different landmasses gradually merging together. Baltica, Kazakhstania, all the various regions of East and South-East Asia. Yet, today, only India and Arabia are still situated upon separate plates. All others no longer possess any divisions between them.

Why is this the case? I was under the assumption that it was only oceanic plates that submerge into the mantle due their density. I though that when continental plates collide, neither continent submerges, forming mountains. Shouldn't there still be distinct tectonic plates?

Any answers would be greatly appreciated.

Answer 1

Continental collision as Indian Plate with Eurasia one finish in a merge, but the process is slow. An example of merged plates is Iberian Subplate that is presently considered Eurasian Plate.

The collision creates mountain chains as Himalaya. Presently Indian Plate is a separated system because it has still a big granite root under the mountains. While erosion happens, the new geophysical echillibrium bounces new relief to the chain and the root starts to be sutured. There is stil some decenes of kilometers of granite root under the Himalaya, so the chain is still a plate limit.

Answer 2

This is more a problem with how you are trying to categorize things than any concrete problem.

A "plate" is not a completely hard and fast concept, there is some interpretation involved, it is a way of categorizing a group of natural objects with similar properties. When continental crust merges they do not become homogeneous, much like how individual boards can be used to make a piece of furniture a continental plate can be made of many joined plates. Continental plates are one of those things that don't follow fine categorization well, continental plates can be composed of many smaller plates and the distinction between two plates can be vague. Generally once two plates are moving together they are treated as one, the definition of "Plate" much like the definition of species is one of those things can van vary quite bit depending on how it is being used.

Most continental plates are built of many smaller plates that have become fused together, these pieces often become distinct provinceswithin the plate. It is possible to reconstruct past individual past plates in many continental plates

Imagine if you have two sheets of clay on a table, you can slide each piece around until you push them together with force, and the edges stick together, then they fuse into one large sheet and you can't move one without moving the other so you can safely treat them as one thing.