Thanks to Eifel Tourismus for the map

Twenty of us assembled in the Naturfreundehaus, LaacherSee, on the evening of Saturday July 31 2004; half via nearby Hahn airport, the rest by car. Ten of us came from England, three from Belgium, two each from the Netherlands and Spain, one each from Israel, France, and Jersey (not necessarily of those nationalities). In geological expertise we ranged from those who had got OU firsts last year to those who had not done a course for some years. We occupied various size rooms on one floor of the Naturfreundehaus, one of a chain of not -quite- Youth Hostels in Germany. On our arrival it was full of excited children having a theatrical performance and then a last-night barbecue. We discovered the next day that the children were from Chernobyl, and they were replaced later in the week by another batch on holidays offered by the town of Mendig nearby.

View into Kärlich Quarry. Blue clay layer is visible.

Our trip leader was Paul van den Bogaard, currently working at IFM-GEOMAR in Kiel, who had done a great deal of the research on the Quaternary Eifel volcanics, and led us patiently through the complex tectonic and volcanic history of the area.

Volcanism both began and ended in the East Eifel, centred on the Laacher See, so we were extremely well situated, with only one half-day in West Eifel about 40 kilometres to the southwest. The area is a typesite for 'maars': craters, often occupied by a shallow lake, produced by the explosive interaction of volcanic magma with groundwater, and surrounded by a low rampart of the ejected mixture of country rock and highly fragmented ash. West Eifel had about 240 eruptions compared to about 100 in the East, was more mafic, and has many more water-filled maars, but the East is more varied because more evolved magmas were erupted. The lavas in both areas changed somewhat after 200Ka, becoming more basaltic and basanite in composition. They are the typical potassium-rich, silicaundersaturated lavas found in an intra-plate continental rift valley situation, such as the East African Rift.

Kärlich Quarry - Loess layer with Paul for scale.

The East Eifel volcanics lie near Koblenz to the west of the Rhine Graben, which in the Oligocene, probably as a result of Alpine deformation, reactivated earlier fractures going back to Permian extensions in Pangaea. The Rhine Graben cut through the Rhenish Shield or Massif, forming a triple junction, and the resulting doming led to considerable Quaternary uplift of the area to the west and south of it, of about 200 metres and which is still rising at a rate of over 1 mm a year. Whether a mantle plume is responsible for the NE-SW extension and rising of the mantle does not seem to have been finally settled. It seems likely from geophysical and geochemical evidence – Paul said that there is a plume about 100 km2 in extent below the surface, corresponding roughly to the volcanic area, but also cynically that geophysicists always find mantle plumes.

Nearly all the sites visited were in quarries, as quarrying or mining lava for millstones and building materials, and these days, scoria cones for road-building, has, with agriculture, been the main local industry for centuries. Paul had had to get permission in advance for our visits. The light - coloured phonolite quarries in August are extremely dusty and hot, especially wearing hard hats, but the cut quarry faces exposed such varied and colourful stratigraphy as to make them extremely worth while. Phonolite is a finegrained, porphyritic, extrusive igneous rock consisting of alkali feldspar, found in continental regions subject to upwarping and rifting. The name refers to the fact that it rings like a bell when struck with a hammer. Throughout the week Paul tried to teach us to look at the strata much like sedimentary deposits: bedding, grain size, sorting and composition.

Kärlich Quarry - A lone biotite crystal, picked out of the loess deposit (but it didn’t originate from there!).

Our first site on Sunday was Kärlicher Quarry to the SE of Laacher See. This was one of the older eruptions, at 360Ka, and was on the outer rim of the volcanic area, overlooking the Rhine Graben. Later eruptions, such as the final Laacher See eruption, at 11Ka, bigger than the 79 A.D. Vesuvius eruption, obliterated much of the earlier eruptions. Just below us, as we looked over the valley of the Rhine and Koblenz, was the Mannheim-Kärlich nuclear power station, which Paul said operated for a few days only before it was realised that it had been built on an active fault and volcanic vent.

The exposed quarry face showed the Devonian red sandstone basement (as at Stonehaven, the 'old red continent' of Laurussia), and slates and schists from the collision of the Cornish-Rhenish terrane in the Variscan orogeny. This orogeny created the high Rhenish Massif which had been eroded, resulting in a 320 million year unconformity until the next sedimentary deposition in the Oligocene, when it was overlain by brown to yellow clay, and later by a striking layer of midnight-blue clay of volcanic origin, rich in cobalt, which had been quarried since the 19^th century, as it is fire-resistant and is now used in high-tech ceramics. We were asked to avoid walking on it as any clastic inclusions from our boots would cause it to explode during baking.

Above the cobalt lies Tertiary tuff, compacted volcanic ash deposits, generated by pyroclastic eruptions or water-deposited, with grain size < 2mm, overlain by fluvial deposits, then clay in lumpy volcanic deposits, called brockentuff, – basaltic ash of the earliest (465Ka) eruptive phase of phonolite enriched in incompatibles like niobium, melilite, nepheline and pumice. These components, with Devonian fragments, shales and sandstones, together with Tertiary clay deposits, were picked up below the surface by the magma before the phreatomagmatic eruption. There are many layers of fragmented country rock with magmatic material which break easily because they were fractured by cooling quickly, and have a cauliflower/ breadcrust surface because they were quenched rapidly. In a slower eruption there would have been time for the silica to be absorbed by the silica-undersaturated magma. There was no vesiculation. The brockentuff also contained layers attributed, by their lithics and very large orthoclase crystals, to a volcano near Bonn, 60 km away.

Kärlich Quarry - Blue clay layer and JCB digger. Blue clay is very carefully excavated to avoid contamination and sold by the kilo.

The succession also contains Rhine terrace gravels, distinguishable from Moselle gravels by their lithics and heavy minerals present.

Above the brockentuff lies a thick layer of loess – unconsolidated, wind-deposited sediment composed largely of silt-sized quartz particles. This area was not glaciated during the various Ice Ages, but was affected by permafrost. The loess contains some poorly-sorted lapilli (not rounded, so not fluvial), often in sagged beds, and in places is discoloured by more recent soil having been washed into it.

We then drove across the Rhine at Andernach, and northward along the steeply-sloping Rhine riverbank, with Variscan folds, to the Ariendorf Gravel Quarry north of Bad Honningen. There we saw another Pleistocene succession topped by a loess layer, of alternating paleosol, tephra, paleosol, and Rhine terrace layers, all containing volcanic fragments and Devonian materials. That soils had had time to form indicated fresh eruptions rather than pulses of one eruption, and the clays in the paleosols retained water better than the sandy ash of the volcanics, so looked darker. The volcanic tephra (a collective term applied to all pyroclastic particles or fragments ejected from a volcano, irrespective of size, shape or composition, but usually applied to air-fall rather than pyroclastic-flow deposits) layers from the airfall material contained pumice clasts, lithic fragments and crystals of sanidine, leucite, titanium, garnet, biotite, and sphene as well as volcanic glass. These mainly feldspathoid minerals indicate the phonolitic nature of the eruptions. The volcanic layers have been dated to 417Ka, 400Ka and 215Ka. On top of the quarry there were interesting present day mud flakes on polygonal mud cracks.

My map showed no bridges over the Rhine for a considerable distance north of Andernach, but we headed for the river bank at Rheinbrohl, and took a free, small, fast ferry across the river, which gave an interesting view of the steep slopes at this point. A drive westwards up the Brohl valley brought us to the Kempenich Tuffring and lapilli pit, north-west of Laachersee. This was the site of possibly the oldest of the Quaternary eruptions at 500-450Ka where we saw palagonite tuffs, lowtemperature alteration products similar to basaltic glass due to the interaction of groundwater percolating in a subaerial environment, needing only time, water and an unstable component. There were also clay minerals, and vesicles in the tuff cemented with carbonate, very few black magmatic clasts, many brown altered lithics and much white pumice.

On the way back to Laachersee we stopped at Perler Kopf which was said to be the oldest volcanic centre in the East Eifel, the only outcrop we saw the whole week, presumably not of interest to quarrymen. It had been a melilitenephelinite lava flow forming a leucite-phonolite dome.

Ann Cripps