Aufdickel Quarry

On the fourth day we had to travel 40 km South West from our base in East Eifel for our only visit to the West Eifel. Our first destination was the Aufdickel Quarry. We enjoyed a brief respite from the heat and sun of the previous days; it rained lightly and the fragrant herbs in the disused quarry smelled wonderful in the damp air.

The only phonolite in the entire West Eifel area was to be found here. There were two clearly defined units in the quarry, an upper massive one, crosscutting the dipping layers of a lower, bedded one. In fact, both units were made of the same components, the only difference being that the upper unit was unsorted into graded layers.

The components we found were yellowish-grey phonolitic pumice, more dense phonolitic particles, carbonatite fragments, sanidine crystals and the usual lithics from the Devonian basement. Somebody even found some limestone, including a fossilized ‘oriental shoe’ coral. In the Devonian this area had been an archipelago of islands with basins in between.

Aufdickel Quarry Examination of the outcrop face. We were told some very large sanidine crystals could be found here. Our trip leader had previously found one that weighed 2 kg.

The sanidinised rocks had been formed by metasomatism in the carapace of a magma chamber, where volatiles and alkalis were exchanged. The Devonian country rock had been changed into sanidine crystals, but the sedimentary layering of the host rock had been retained.

Sanidine is the hightemperature form of potassic feldspar that often occurs as phenocrysts in volcanic rock such as rhyolite and trachyte. In Aufdickel quarry, the bulk composition was very K-rich; 13% by weight. Paul had at one time found a sanidine crystal here that weighed 2.5 kg, but he had sacrificed the beauty to a crusher in order to age the rocks!

The age of the deposits, at 465 – 470 Ka, was within about 10-20 Ka of that of a bed of identical-looking phonolite pumice from 452 Ka that we had seen at Kärlich in East Eifel, on our first day. The location of the source eruption(s) of these two sites is unknown, but the eruption at Kärlich, which had a volume of ejecta of 2-3 km³, was the second largest in the entire history of East Eifel volcanics. Faults and the dip of the bedding in the lower units here in the Aufdickel Quarry indicated that we were either very close to the vent or that there had been very steep morphology underlying the deposits.

The Aufdickel area has been mapped as maar deposits, overlain by phonolites. A maar is a phreatomagmatic crater, whether filled with water or not. Many are filled with sediments. The upper unit might have been a lahar or a pyroclastic flow. To decide which, it would be necessary to map it, to see if the unit stuck to valley bottoms or graded laterally into overbank ash deposits. The lower bedded unit was probably the result of phreatomagmatism, since the eruption fines had not been transported away from the volcano. This was an indication that water had almost certainly been involved.

On the journey here, we had seen an enormous scoria cone not very far away, called Rockeskiller Kopf. Melilite and nephelinite from that cone had been used in K/Ar dating to give an age of either 360,000 or 620,000 (some more work needed to constrain the dates further, I think!). There the composition was similar to that at Herchenberg, very undersaturated (not basanite, which is the composition of the less undersaturated magmas).

Late Pleistocene Maar Volcanism — Dauner Maare

After a short walk beside a soggy cornfield to return to the bus, we voyaged on to the Dauner Maare, comprising Weinfelder Maar, Shalkenmehrener Maar (also known as Töten Maar) and Gemündener Maar. There was a visitor board by the road, with topographical information about the maars and a cross section through them. Shalkenmehrener Maar was a double crater, one of which was now dry, having emptied during the last ice age, around 50-60,000 years ago. Monks now own the land and water and sell the latter to a brewery.

Close by there is a group of scoria cones called the Mauseberg group. These black scoriaceous deposits lie above weathered Devonian basement, as usual in the Eifel.

Meerfelder Maar

Leyendecker Quarry Looking for mantle nodules in the quarry. The nodules are covered by the orange coloured mineral idingsite which is the weathered product on the outside of the nodules.

The rest of the morning was spent in the Leyendecker Quarry, examining tuff ring deposits that were part of the Meerfelder Maar. It was originally thought that the eruption that created the maar had just been of CO₂, because nobody could find the magma. Then Kamura came along in the sixties and found deposits that reminded him of those he knew from Japanese maars. They were phreatomagmatic base surge deposits, containing xenoliths and mantle nodules of peridotite, pyroxenite and amphibolite.

We had great fun here picking out the abundant crumbly nodules with weathered brown exteriors and cracking them open to show bright green crystalline interiors. These olivine nodules were not a high percentage of the bulk, at 9-15%, but could be found from bottom to top of the section we saw. They fractured easily, possibly because of the quenching of the magma as it erupted. I was told that a puzzling sharp-edged crust on one olivine nodule that I found was the result of it having behaved brittly and fracturing before eruption.

Some of the mantle nodules were of hornblendite, predominantly black hornblende, olivine and pyroxene. The mantle material most common here is spinel lherzolite, which would leave a residue of hartzburgite in the mantle. There were also massive inclusions, denser than the magma, which had been brought up very rapidly, probably with the aid of water. There was no evidence of contact metamorphism, which would have been finegrained. We also saw vacant holes left by tree trunks in the upper layers.

Maars tend to be form in preexisting valleys and scoria cones form on hills. In the Meerfelder area there are regional tectonics with faults & zones of weakness, which result in valleys where the magma comes up preferentially. The chance of hitting water in fault systems is high, so most of the eruptions have been phreatomagmatic. These eruptions in the West Eifel were very recent, with the Ullmener Maar being the very youngest, at 9,000 years old. The Meerfelder Maar deposits have been dated from charcoal (¹⁴C) to 29,000 years old.

It is thought that the same persistent, intraplate mantle plume underlies both the East & West Eifel regions, which are 50 to 70 km apart. The upwelling is 100 km wide and 400km deep and it will erupt again.

There might possibly be a superstructure at greater depth, linked with the Auvergne in France, with the separate plumes rising like fingers from a larger plume beneath. In some papers it has been suggested that the plume that feeds Iceland also feeds the Eifel, and that it is flowing laterally, also to N. Africa, the Mediterranean, and central Europe. The deeper into the earth you go, the more surface volcanics you can include.

The eruptions are episodic on a 10,000-year cycle but the eruptions in the Miocene have suggested another, larger, cycle. They are all from the same persistent upwelling, under Palaeozoic blocks. A hot spot track has been suggested but the age progression doesn’t fit. There doesn’t appear to be any systematic plate movement. The rifting of the Rhine graben may be linked to the volcanism, with both phenomena being related to the same cause, i.e. plume activity. The plume might be lifting up the Rhenish shield, causing extension and rifting.

On the return journey we stopped at a viewpoint to look at the romantic twin castles of Manderscheid. This gave us a glimpse of Devonian basement in the road cutting behind us.

Manderscheid Castle

We had a free afternoon and people went their separate ways. Several of us went into the nearby town of Andernach to do some shopping. Others visited the Maria Laach Monastery.

That evening, by special arrangement, we were guests at an amusing and enjoyable wine tasting session at the Vulkan Eifel Museum in Mendig. Our host was the owner of the museum, so his main interest was geology, but he made great efforts to inform us about the different merits of the local Mosel wines and many of us came away with a bottle or two.

Lynn Everson

(Photos by Kirsty Crocket)