An extinct ridge exists in western Iceland, leading to the theory that the plume has shifted east with time. The oldest crust of Iceland is more than 20 million years old and was formed at an old oceanic spreading center in the Westfjords (Vestfirðir) region. The westward movement of the plates and the ridge above the plume and the strong thermal anomaly of the latter caused this old spreading center to cease 15 million years ago and lead to the formation of a new one in the area of today's peninsulas Skagi and Snæfellsnes; in the latter there is still some activity in the form of the Snæfellsjökull volcano. The spreading center, and hence the main activity, have shifted eastward again 7-9 million years ago and formed the current volcanic zones in the southwest (Reykjanes, Hofsjökull) and northeast (Tjörnes). Presently, a slow decrease of the activity in the northeast takes place, while the volcanic zone in the southeast (Katla, Vatnajökull), which was initiated 3 million years ago, develops. The reorganisation of the plate boundaries in Iceland has also been attributed to microplate tectonics.