How wide was the narrow sea between the western continent and adjacent volcanic islands 20 million years ago?

The theory of plate tectonics is nowadays more or less universally accepted by geologists. the best known divergent boundary is the mid-atlantic ridges. the rate may seem low by human standards , but because this process has been going on for almost 20million years it has resulted to a plate movement of thousands of kilometers.

The enviroment with hedge hogs