The tephra pit at Gillenfeld
Written by Prof. Dr. H.-U. Schmincke

 

How the mysterious Eifel maars formed – the sunken, partly water-filled craters surrounded by ringed walls of tephra (loose volcanic material) – has long been disputed. In 1970 Volker Lorenz, Hans-Ulrich Schminke and their colleagues showed that the prevailing doctrine (formed by CO2 explosions) was not consistent with the evidence found in the field.

Detailed research showed that maars are formed by the powerful explosions caused when rising hot magma meets groundwater. This new interpretation, revolutionary for its time, was only made possible because numerous tephra pits in the Eifel (and elsewhere) allowed a precise reconstruction of the mechanism behind the eruptions.

One of the few pits still open in the Eifel is the pit on the south-western crater edge of the Pulvermaar at Gillenfeld. The Pulvermaar is a jewel amongst the Westeifel maars and not only because of its circular shape and the depth of the water. The only more recent maar is the Ulmener Maar, formed 11,000 years ago after the end of the Ice Age. The sediment that came from it was deposited over the 12,900 year old Laacher Lake tephra.


 

 

 

 

 

 

 

 

 



Image: Prof. Schmicke being inspirational

There are many reasons for preserving the pit at Pulvermaar. The structures and rocks of the grey tephra layers that form the 10m high, 50m long wall may not be that attractive to most of us, but if we look closer it is possible to trace the formation of a maar in a unique way. In the bottom 8m or so the layers alternate between coarse-grained and fine-grained, some of which have a dune-like pattern and there are large 1m blocks with dents from impact. The thickness of the strata and the diameter of the impact blocks decrease as you move west. This is a clear indication that the tephra material came from the maar, some of it hurled (blocks) and some carried by fierce ground currents (cross-stratification). The upper 2m or so are fine-grained and in even layers, particular the light stratum. Many of these deposits are fallout deposits (lapilli and ash particles that fell from the sky). In the eastern section of the wall rifts reflect vertical movenents (when the crater sank) in the later stages of the maar formation.

As in all maars up to 80-90% of the crater wall is composed of local rock particles, Devonian slate and sandstone. This high proportion of local rock is a key clue as to the how the maar formed. The contact of magma and groundwater causes the rock in the vent to burst explosively. The Pulvermaar and neighbouring maars typically have glistening, tuberous crystal deposits (mica, pyroxene, amphibole etc.), that were formed due to the slow crystallisation of the magma in magma chambers in the earth’s crust and rose through the rock with the magma. These rocks, on which only a limited amount of research has been done, are an important reason for the preservation of the tephra pits. However, there is no mantle rock (“olivine bombs”) at all. One more important reason: New studies (Schminke 2006) show that magmatic gases, and not just groundwater, contributed to the eruption, as the few lava fragments and the lava coating around the lumps of crystal are not transparent.

These tephra pits will certainly remain a mecca in the future for petrologists from around the world – and a clear example to help the rest of us better understand how the famous Pulvermaars were formed through hundreds of individual explosions (every layer is an explosion).