The Upper Rhine Plain was formed during the Early Cenozoic era, during the Late Eocene epoch. At this time, the Alpine Orogeny, the major mountain building event that was to produce the Alps, was in its early stages. The Alps were formed because the continents of Europe and Africa collided. It is thought that because the collision was irregular, the initial contact between the two continents resulted in the formation of dilational (extensional) structures in the foreland basin to the north of the Alps. The result was substantial crustal thinning, forming a major extensional graben and causing isolated volcanic activity. The stretch factor is estimated to be ~2.
To both the east and west of the Rhine Plain, two major hill ranges have formed that run the length of the basin. To the west, in France, these hills are known as the Vosges mountain range and in the east, in Germany, the hills comprise the Black Forest. These ranges exhume the same types of rocks in their cores, including deep crustal gneiss. Both ranges correspond to uplifts of more than 2,500 metres, much of which has since been eroded. This uplift has occurred because of the isostatic response associated with the formation of an extensional basin. As a consequence, the highest mountains exist immediately adjacent to the margin of the basin, and become increasingly low outwards. The boundaries between the hill ranges and the Rhine Graben are defined by major, normal fault zones.
The extension induced by the formation of the Alps was sufficient to thin the crust and provide suitable dilational conduits for magmatic and volcanic activity to occur. This resulted in the emplacement of mafic dykes, which follow the general structural trend of the extensional faults. In addition, isolated volcanoes such as the Kaiserstuhl were formed.
The Kaiserstuhl (literally "Emperor's Chair") is a cluster of volcanic hills to the northwest of Freiburg, within the Rhine Graben. The highest point of this small, isolated volcanic centre is the Totenkopf (557 metres). Volcanic activity was most prevalent in the Miocene epoch, some 15 million years ago. Today, the Kaiserstuhl volcano is extinct.