The sinking of Beleriand: a detailed inquiry

[wsx04321]

How, Why and When did Beleriand sink?

 

[Licky Linguist]

Reading the Beleriand wiki page may give you a bit of info 😊

 

[Olorgando]

How, Why and When did Beleriand sink?

No part of your question can be answered to the satisfaction of detail-obsessives (to which I all to often belong).
The "when" part at least seems to have a "latest date", the end of the First Age. When all of it had sunk, and the western coastlands of Middle-earth must have at least approached what is shown in the maps in LoTR. When which pieces of it, say the very western Andrast, then others began to sink - speculation.

The "how" part - well, Melkor / Morgoth was still bodily in Middle-earth, and the Host of the West certainly had superhuman participants.
Whose actions might have seemed to (far) lesser beings as "clumsy". Say causing earthquakes that these superhumans would hardly notice, in contrast (in the real world) to what happened in Croatia just recently. Which kind of also might answer the "why" question ...