The Arensberg, one of the greatest geological attractions of Gerolsteiner Land, is the only significant volcanic peak from the Tertiary period. The rising magma has penetrated Lower Devonian claystones and sandstones, Middle Devonian limestones and marls and Triassic red sandstones. Numerous new minerals have formed in the entrained limestone blocks through heat contact with the molten lava. The crater can be entered through a tunnel. The access road is signposted in 54578 Zilsdorf (Basaltstraße).

Two volcanic eruptions created a mountain where there was none before. The first was about 32 million years ago: Rising magma made its way up through layers of shale, limestone and sandstone and swept away blocks of rock with tremendous force.

With great force, the fragments are hurled out of the vent and mix with the ash ejected. To this day, the lighter rock blocks amidst the darker ash deposits can be seen very clearly at the quarry's break-off edges. Further magma from the Earth's interior penetrates upwards into these still loose deposits and forms a pear-shaped basalt dome.

The second eruption about 24 million years ago is much quieter: Magma rises again and pushes itself under the solidified basalt of the first eruption. The Arensberg used to be much higher. While millions of years ate away at the old ash deposits of the first eruption, it was people who took the mountain's summit away in the 19th century. The ruins of a medieval church had to give way to the basalt quarry. Today, it is commemorated by a small chapel on the side of the path. Gradually, people penetrated deeper and deeper into the mountain and followed the vent into the depths. Today, the old quarry offers an impressive view into the interior of an extinct volcano.