Known as maars, these bowl shaped craters form when groundwater meets rising lava, resulting in an explosion. The tuff rings from surge deposits are clearly visible at the crater's rim, now covered in forest. The eruption occurred in early summer, and the initial ash plume is thought to have risen up to 35Km high. Phreato-magmatic explosions then created pyroclastic flows and surges, that filled all the valleys with tephra within a 10Km radius, and probably wiped out most life within a circle of 50Km. They estimate around 6.5 cubic kilometres of lava were erupted. The resultant deposits are used as marker beds to correlate stratigraphy by geologists and archaeologists throughout west-central Europe.
The deposits also dammed the nearby Rhine, creating a 140 square Km lake. When the dam broke a huge flood surged downstream, leaving evidence of its passing as far north as Bonn. The phonolitic (high alkali/medium-high silica) lava had a high sulphur content, which cooled the atmosphere for several years afterwards, so the next few summers would have been pretty miserable. Among the deposits are ejecta of rare intrusive carbonatite lavas, the youngest plutonic carbonatites worldwide, attesting to some pretty complex geochemistry churning down below. Human culture was set back, as the local communities disappeared, and were replaced some years later by less advanced ones (in terms of tool quality and the like, not necessarily culture).
It is the only caldera in central Europe, as the magma chamber underneath collapsed after emptying. Located on the Rhine Graben (a downfaulted block resulting from extensional tectonics), the magma is probably related in some complex way to the collision of Africa and Europe, that also built the Alps and Pyrenees. Part of the East Eifel volcanic field, these sporadic eruptions have been ongoing for several million years, and their timing seems to correlate with the cyclic accumulation and melting of ice sheets.
The field is still active, lava is gently bubbling down below, as thermal anomalies under the lake and degassing of CO2 into the region's lakes (including Laachersee) and rivers testify. Some rare minerals were formed by this volcanism, and Hauyne was first identified in these German volcanic fields. While still active, there are no current signs of imminent eruption, and the British newspaper the Daily Mail was forced to retract a story last year that a super-eruption that would devastate Europe was imminent.
A Benedictine monastery lies on its shores (visible at the lower left of the photo), and the tuff was quarried for centuries as building stones and to make querns for grinding corn.
Image credit: IMAGE BROKER/IMAGO/SIPA