The Eifel is a geologically very complex and also very scenic area. It is the Mittelgebirge (low mountain range) north of the Moselle, stretching from the Rhine to Luxembourg. In the north, it ends at the North German Plain, or more precisely the Kölner Bucht (Cologne Bay), a bulge of the plain to the south in which the city of Cologne is located. There are several contradictory subdivisions of the Eifel for landscape and historical reasons. We have divided it into three regions that are geologically very different. In the west lies the Kalkeifel, which is built up of limestone, in the east the Vulkaneifel, which is characterised by volcanism, and in the north and centre the rump mountains of the Variscan orogen, with shales, metamorphic rocks and ores.
The Kalkeifel (Limestone Eifel) belongs to the South German Scarplands, i.e. it is not part of the Rhenish Massif like the rest of the Eifel. Although it consists of karstified limestone, it has few spectacular caves to offer. On the other hand, it has a large number of karst springs, many of which have built up limestone tuff deposits. Small primary caves are often found in this geologically very young rock.
The Nordeifel (Northern Eifel) is the northern spur of the Rhenish Massif. A rump mountain range, the last remnant of the Variscan orogeny. We don't know for sure, but it was probably a mountain range like the Alps today, and over millions of years it was eroded away until only its core was left, The core are folded crystalline and metamorphic rocks, but also intrusions of magma and various mineral resources which were enriched by hydrothermal processes, for example. There are various show mines, but many are dedicated to the mining of basalt and slate. In the Schneifel and to the north, however, there are large deposits of lead ore and even three lead show mines.
The Vulkaneifel (Volcanic Eifel) offers the most interesting geological sights. There are the famous Eifelmaare, mostly circular lakes with a diameter of a few tens to a few hundred metres, surrounded by a crater rim. These are the vents of volcanoes, usually one-off explosive eruptions triggered by groundwater meeting the hot magma in the depths, causing a kind of steam explosion. One volcano that covered half of Germany with volcanic ash during its eruption and covered the surrounding area with 30 m high basalt flows is Lake Laach. Even today, volcanism is not extinct, as shown by several carbon dioxide (CO2) sources. The carbon dioxide is of volcanic origin and is produced during the degassing of molten magma. The risk of a volcanic eruption in Germany is very low, but if was an eruption it would most likely be here. The area has been massively developed in recent years, is a UNESCO GeoPark and has a volcano park with various museums and mines where volcanic rock was mined.