Two volcanic eruptions created a mountain where there was none before. The first eruption took place about 32 million years ago: Rising magma forced its way up through layers of slate, limestone and sandstone and with immense power dragged blocks of rock up with it.

The pieces of rock were ejected from the vent with explosive force and became mixed with volcanic ashes which were being ejected at the same time. Until today, where rock is exposed in the sides of the quarry, the lighter-coloured blocks of rock can be clearly seen in the midst of the darker ash deposits. More magma pushed its way up from inside the Earth into these deposits, which were still loose, and formed a pear-shaped basalt dome.

The second eruption about 24 million years ago was rather quieter: Magma rose up again and pushed through until it was just beneath the solidified basalt which had been left by the first eruption. The Arensberg used to be a lot higher. While the layers of ash deposits of the first eruption were eroded away over millions of years, it was people who took away the top of the mountain in the 19th century.  Even the ruins of a mediaeval church had to make way for the basalt quarry. Now, a small wayside chapel is a reminder of it. People quarried further and further into the mountain and down into the conduit. Today, the old quarry offers an impressive view of the interior of an extinct volcano.