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It is situated just above the Rhine-valley. Together with the Drachenfels directly opposite, the Rodderberg narrows the Rhine-valley. This gorge portion is the southern limit of the lowland-bay of Cologne.
The Rodderberg detonated in a phreatomagmatic eruption about 250,000 years ago. This happened when the magmatic plume reached the groundwater level of the Rhine (about 50m higher than today). After the explosion, the crater was filled with water that formed a maar of about 90 m depth. In the last 250,000 years this feature filled with dust and soil until it was filled up to today's level.
At present, all that is left of the old Volcano is just a small hill which has a shallow craterous form on its surface - if one doesn't know that it is a volcano, you would not see it.