During the day we visited the Wingertsberg scoria cone complex dated at 150k or 200k. This is regarded as 13,000 years ago. We first visited the Korretsburg scoria cone where pumice is mined. This 5-6 m section consisted one of the possible eruption centres of the Niedermendiger lava which we had seen on Thursday. It was covered by Laacher See (LS) tephra dating from 13 k, and we could determine the initial blast of the LS tephra as well as the end of the LS eruption near the vent.

The Wingertsberg scoria cone.

By now we were thoroughly expert (!) in recognising the differences between phreatomagmatic and dry eruptions and between ash flows, surges and fallout tephra, which are mostly manifested in a jumble of scoria, bombs, lapilli and dust. Today we were due to see bedding, bedding and yet more bedding. The day was mainly devoted to a study of the Laacher See eruption about 13,000 years ago. We first visited the Korretsburg scoria cone where pumice is mined. This 5-6 m section consisted of well bedded ash layers intermixed with layers of pumice. Estimates of the ac tual numbers of layers varied, beginning with estimates of 50 beds, reducing through Paul’s prompting to 20, and then to 10 until Paul finally persuaded us that there were in fact 3 main layers. Some of us also had to be persuaded about the difference between well sorted and poorly sorted bedding. The apparently well sorted grey ash layers were in fact poorly sorted and the layers containing lumps of pumice and ash were actually well sorted.

The Korretsburg scoria cone.

The bottom layer was the Lower Laacher See Tehphra (LST) a well sorted, poorly bedded layer showing some coarsening upwards. The next layer, the Middle LST consisted of alternating layers of poorly sorted, well bedded ash and well sorted pumice. The top layer was also Middle LST and consisted of well sorted larger fragments (dislodged using that useful geological tool, an extended walking pole) of pumice and lapilli. The bottom layer of Lower LST also contained some Devonian slate fragments. There were also some examples of tubular vescicles flattened by the pressure above and the ash layers above showed the influence of external water. The amount and size of vescicles decreased towards the top of the three layers giving a higher bulk density at the top. The phonolite of which the tephra was composed was less evolved towards the top, indicating that it came from a lower part of the magma chamber.

Our next stop was to see the Wingertsbergwand, where quarrying has been halted so that the layers of the Laacher See eruption can be preserved in a park as a section 20-30 m thick. We could identify the lithic rich pumice layer at the top of the Lower LST but could not see the base. The brownish beige of the Middle LST changed to the greyish ash of the Upper LST. The Middle LST flows have flat tops with the bottom part filling up the topography and is recognised as a sustained Plinian eruption, the column reached 40 km in height. From the rate of flow and the volume of material erupted, calculations have given estimates that indicate that the Plinian eruption lasted about 10 hours. This compares with the 9 hour duration of Vesuvius in AD 79 (from eye witness accounts) which has a similar composition and eruption sequence to Laacher See. Discussion about the accounts of the Vesuvius eruption and the remains of the victims found there led us to ponder about the victims of this eruption, not humans but probably small animals and birds. By comparison with a recent well publicised eruption Mount St Helens, which produced 1 cubic km of material, the Laacher See eruption produced 5 cubic km of material. This site was about 2.5-3km from the eruption centre but light ash from the eruption reached as far north as Sweden.

Standing wave in the Wingertsbergward.

At this site we also saw a standing wave due to rapid flow of ash causing turbulent deposition which had been ‘frozen’ in the ash.

The next site was In Den Dellen, where we saw that the metamorphic contact of the wall of the magma chamber with the country rock had produced cordierite. Other minerals indicated that this was the bottom of the magma chamber.

During the afternoon we took a walk by the Laacher See, which was the site of two large vents from which the material erupted and is now the site of a large lake, about 2 km in diameter. We passed by a preexisting scoria cone dating from 200K which had been blasted by the Laacher See eruption and also saw bubbles of carbon dioxide emerging from the surface of the lake proving that there is still ‘something down there’.

CO2bubbles in Laacher See.

Eileen Lawley