Field Trip to the Eifel Volcanic Fields - 27th October - 1st November 2012

Day 1, Sunday 28 October

Text Fiona Till & Gerd Weidemann, photos Fred Owen.

We woke up to a beautiful frosty morning, bitterly cold but otherwise perfect weather for the first day of our field trip to the Eifel Volcanic Fields. In spite of a few moans, Lord Bracknell (Mike Molloy) was adamant about an early start, so we met up for breakfast at 7 a.m. (apart from two of us who somehow managed to put their clocks back two hours instead of one (this being the weekend with the change from summer to winter time). The minibus collected us at 8 a.m. and we set off to our first site, the Eppelsberg Scoria Cone, located just south of Nickenich, about an hour’s drive away from our hotel.

As we were newcomers to OUGS ME, Mike kindly picked on us to be the scribes for the day. On the way, our excellent leader, Professor Hans-Ulrich Schmincke, an expert on Eifel volcanism, gave us a brief description of the area and an outline of the day’s agenda. We were to spend the day in the East Eifel Volcanic Field, his proposal being to visit a few outstanding outcrops, allowing us plenty of time to analyse each site.

The Quaternary Volcanic Fields in the Eifel region are divided into two main areas: The West Eifel Volcanic Field comprises about 240 volcanoes, 140 more than that of the East Eifel, but these volcanoes characterise the landscape to a lesser degree than those of the East Eifel due to their superior age and the higher elevation of the area, making it more prone to erosion through its greater exposure to rainfall than the drier East Eifel region. The origin of the Eifel volcanism is thought to be related to mantle plume activity, which has caused about a 200-metre uplift of a large lithospheric block known as the Rhenish Shield over the last 40 million years. The Eifel is therefore a typical example of continental intraplate volcanism.

The Eppelsberg Scoria Cone, which erupted about 220,000 years ago, has been extensively quarried for decades, allowing an insight into a cone development with several eruption phases (phreatomagmatic, phreatic, pyroclastic) and corresponding volcanic deposits, such as interbedded ash, lapilli and bombs. Scoria is used mainly to build roads and tennis courts and the industrial use of volcanic materials has long been of economic importance to the Eifel area.

The highlight of the day was to observe and touch a classic textbook example of a dyke with chilled margin, including a rarely-seen terminal fork representing the waning of the magma flow (Figure 1).

After looking at some tectonic-related features, such as faults and Graben structures (Figure 2), we moved on to our next site just a few kilometres away.

At the Wingertsberg quarry we were shown a rare example of a dyke feeding a lava flow (Figure 3).

Further on at the same locality, we observed a facies, which typically occurs next to a crater, including a massive ejectile (Figure 4).

For the first time we encountered the beautiful blue crystal hauyn, easily found in the pumice. Later on in the afternoon we drove towards the Rhine to a small pit near Kettig. Here we saw fine examples of tree holes, remaining evidence of warm climatic conditions at the time of deposition. However, the most interesting feature was the presence of a pumice flood wave deposit, formed after the breaching of the Brohl dam, which had developed as pyroclastic flows reached the Rhine, causing it to be blocked.

On the way back we took a short detour past the Maria Laach abbey, constructed using various volcanic rocks, and then stopped at a viewpoint to get a first impression of the Laacher See.

We novices feel privileged to have had the opportunity to spend time with one of the world’s most eminent volcanologists, Professor Schmincke, who willingly shared his knowledge in such a competent and entertaining way. Our thanks are extended to Mike and Elisabeth for making this wonderful trip possible.

 

Day 22, Monday 29 October

Text: Gillian Sheldrick, photos Fred Owen.

Today we focused on maars and their deposits in the southern part of the West Eifel volcanic region, on a cold and frosty day with several snow showers. We visited Meerfelder maar, which formed about 70ka, and Pulver maar (approximately 20ka), looking in both cases at the interior structure of the maar and at the deposits expelled from it. Finally we looked briefly at three maars at different heights above sea level, whose water tables pose a puzzle.

Meerfelder Maar is a slight outlier, to the southwest of the main field of maars. It is a typical maar: a roughly circular bowl around 1.5 km diameter, with steep sides and the base well below the surrounding ground level. The ‘bowl’ is the hole left after the violent explosive eruption which shattered the country rock and deposited it beyond the edges of the bowl, together with igneous deposits. Inside the enclosed bowl it feels peaceful and still, and very detached from the world outside. The ground is partially covered by a lake, typical of (but not an essential feature of) maars in this area.

Optoluminescence dating gives a date of around 70 ka, making it the youngest maar dated in the area; it is also the largest maar in the Eifel region*. Its size has raised a puzzle as yet unresolved: the volume of clastics (fragmented country rock) found surrounding the maar is not nearly sufficient to fill the hole from which they came. One theory is that the maar formed within an existing valley or depression, but this has not been demonstrated.

 

After viewing the maar from inside, we then drove up its steep sides to view it from above (illustration 1). We also visited a viewpoint tower with excellent views over the landscape, and a good view of the elevated plateau of the Rhenish shield. Its surface is uneven: as well as the maars and scoria cones, it is cut by many river valleys, which have cut deeper as the plateau was, and continues to be, uplifted. The river terraces have been used to date the uplift, which accelerated at around 800ka. Volcanism in the region began at around 650 ka, so a causal connection is postulated; perhaps both events were due to mantle material accumulating around 30km below ground level. This is supported by other observations, such as the doubling of the moho discontinuity in this area.

At Leyendecker quarry at Deudersfeld, we observed the deposits expelled from the Meerfelde Maar, which have been studied in detail by a PhD student of Professor Schminke’s, as part of an investigation into whether CO2 (as well as H2O) plays a role in determining into how highly explosive is an eruption leading to Maar formation; minerals rich in CO2 have indeed been found here.

The layers of material ejected from the maar are broadly horizontal . They include a layer of fine white sand, about 10 to 12cm thick above which is a thin (1cm) black layer with some charcoal, originating from trees ignited by the explosive heat (either in situ or close by). This plus moulds of tree trunks and branches suggest a warm wet climate at the time of the eruption. Above this are many layers of volcanic material mixed with clasts of country rock, the lower layers coarsening upwards and the higher ones fining upwards. In the finest layers some cross bedding was seen. Overall, there is considerably greater volume of country rock clasts than new volcanic material, estimated at 95% or more of the whole.

The upper layers, representing the later phases of the eruption after much of the country rock had been ejected, consist of up to 50% new material. Nodules ejected from the magma chamber were also found with a lava crust which when split revealed a crystalline core including dark green clinopyroxene, olivine (light green) Chromite (black). All these deposits, 20 or 30 metres thick, were laid down in a relatively short time; the eruption is estimated to have lasted only a few weeks.

 

After lunch we studied the Pulver Maar and its deposits in the south east of the maar field. We began in a quarry immediately adjacent to the Pulver Maar. The maar deposits have not been dated but are believed to be more recent than Meerfelde Maar (perhaps 20ka). The deposits are in very clearly defined horizontal beds fining upwards (the lowest part of the deposition is not visible here). There is also some faulting (probably extensional), with a small amount of movement on the decimetre scale. No plant remains or tree holes have been found in this location, suggesting that it took place during a period of glaciation.

Here we discussed the duration of the eruption: the face seen was perhaps 20 to 30 m high, with another 10m or so beneath our feet, and was probably emplaced during an eruption lasting no longer than a few weeks. Professor Schminke also explained how he has tried to estimate the total energy of the eruption using the wavelength and amplitude of the shallow dune formations seen in some of the finer layers (which here had a wavelength of around 8 - 10 m, amplitude ~0.5 m).

We then drove into the Pulver Maar itself to see the source of the deposits. This maar is particularly steep and deep, and has the deepest water (70 – 90 m deep) though Professor Schminke emphasised that not all maars contain water.Finally we visited a viewpoint over Three Maars. Gemüdener, Schalkenmahrener and Weinfelder maars, all contain water but it lies at different levels (see illustration 3). The water level in the first two, which lie at 406 and 420 m above sea level is at approximately the same, which may relate to the underlying water table. But Weinfelder maar, lying much higher (484 m) is also water filled with its own much higher water table. Since the rocks of which it is composed are porous, this is not readily explained.

Naturally there were non-maar- related highlights as well. These included:

- a warm hotel which kindly gave us shelter from the snow and allowed us to eat our packed lunch

 - Professor Schminke’s purchase of about 50 person-week’s worth of toilet paper felicitously branded “Happy End toilet paper”, to supplement the hard and nonporous variety on offer in our hotel

 - Two picturesque medieval castles facing each other across a low valley, the results of a longstanding enmity between the adjacent territories of the Archbishop Electors of Trier and the Manderscheid family of Luxembourg

 - Vertical crystals of (H2O) ice 5cm or more long growing up in batches like fungi in a Quarry floor

Day 3, Tuesday 30 October

Text and photos: Fred Owen, OUGS North-West

Applying his local meteorological experience Prof Hans Schmincke announced that the very wet, cloudy weather which greeted us today could be avoided by going east, towards the Rhine. Instead of the planned trip to the West Eifel, we would be going to examine the variety of deposits of the major Laacher See phreatomagmatic eruptions, which occurred 11ka ago. Arriving at the Wingertsberg Quarry an hour later it had stopped raining, the clouds had lifted and it was a little warmer than previous days.

Location 1: Wingertsberg Quarry, 1 km south of Laacher See represents the proximal deposits of the Mendig Fan shown in Photo 1. This wall did not exist in 1971 and is now preserved as an important tephra teaching and studying exhibit.

When facing the wall. Laacher See eruptive centre lies one km beyond it to the north, so the direction of flow was towards the camera. Prof Schmincke explained that there was a further 6m of deposits beneath what could be seen and that individual layers represent different styles of eruption in rapid succession over a thirty nine day period.

Zircon analysis of the sequence shows three major compositional units in the succession The lower, mafic, one contains 2300 ppm zircon, the middle one contains 1000 ppm at the base and 450 ppm at the top, while the Upper one, being very crystalline, is completely different. This shows that the initial magma was highly evolved and during the eruptions was replaced by pulses of fresh magma. Indeed, input of fresh magma may well have triggered the initial eruption. There are also distinct differences in flow regime between these three units:

Lower - is very complex, contains ballistic surges, 6/7 pyroclastic flows, surge and overbank deposits.

Middle - is comprised mainly of pyroclastic flows

Upper - comprises flow, fallout and dune deposits parallel to flow direction – see Location 2. Some layers contain up to 50 per cent lithics, of underlying shale and sandstone of Devonian origin, transported in low velocity, close to the ground, flows, while the topmost layers are explosive Maar deposits formed by classical phreatomagmatic eruptions.

Location 2 – the same deposit is shown in Photo 2, but at right angles to the wall seen at Location 1, with the eruptive centre to the left. Here it is possible to distinguish between the flow-regimes. It is impossible to describe them all here but the most impressive, and rare, are high energy “chute and pool” features. A good example can be seen in the centre of the photo. These occur at velocities greater than required to form anti-dunes and can be identified by a rising bedding plane, which cross-cuts lower beds, almost pinches out at the top and is followed by a downward dipping slope to form a thickening bed, as in a pool.

At the top of the sequence are so-called 'B' layers containing coarse deposits of crystal-rich lava and above them fining-up layers in a classical turbulent, granular flow regime.

It was noted that accretionary lapilli and pumice are absent here.

Compositionally the magma is 16% alkali oxides, 23% alumina and 55% silica, reinforcing that it was highly evolved. Its high viscosity would have inhibited eruption without interaction with ground water on its rise to the surface.

Location 3: Krufter Ofen Quarry, Photo 3, is 1 km SE from the Laacher See eruptive centre.

Photo 4 shows the deep pit where the lower layers of pumice lapilli are being quarried.

The deposits in the pit contain many imprints of leaves stripped from trees in the early phase of the eruption. The whole land surface area in the region of Krufter Scoria cone has been stripped of 7m of lapilli beds for building material.

Photo 5, looking towards the town of Mendig, clearly shows the Neuwied tectonic basin, which started to form 20 Ma ago and resulted in the volcanic activity that formed the numerous scoria cones in the Eifel.

Lunch was enjoyed in the open air at Maria Laach on the SW shore of Laacher See. Afterwards, while walking to Location 4 we saw clear evidence that there was already an eruptive centre here before the major explosive eruption 11ka ago. Steep sided basalt lava was draped over by the tephra of the early stages of the major eruption. Indeed some of the lower beds of tephra contain boulders of this basalt amongst the lithic fragments within them.

Location 4: Laacher See SE shore shown in Photo 6. There is a 200m zone where mantle produced carbon dioxide, with constant He3/ He4 ratio, bubbles to the surface via lithospheric faults and crustal cracks in the Rhine valley rift system. Similar escapes were seen in the moffette fields in W Bohemia visited on the 2010 OUGSME field trip.

A major fault runs N-S through Laacher See and was the source of the several eruptive centres. Evidence from the thickness, composition, and form of the deposits reveals that the eruption progressed from south to north of the present Laacher See position.

Location 5: Kunkskopf Quarry is near to Wassenach upwind (north) of the eruptive centre. The deposits here, shown in Photo 7, are typical of a scoria cone, containing a variety of sediments including lots of accretionary lapilli up to 2 cm diameter, airfall blocks, fragments of bombs shattered in mutual collisions, plastically deformed blocks and some haloes in the surrounding rocks caused by the high temperature of the fallout. Compositionally the rock is a potassic basanite, the same as seen at Eppelsberg on Day 1.

 

Location 6.

Professor Schmincke explained that a major pyroclastic flow raced NNE down Brohltal to reach the R Rhine at Brohl, near Andernach, where it blocked the river with a 30 m dam to form a 140 sq km lake. The map, Photo 9, shows the route the flow took from the end of the finger to the Rhine at the top of the map, and Photo 10 shows the actual valley.

Location 7: Bad Tonisstein, Photo 11. Here we saw massive ignumbrite deposits of the Laacher See pyroclastic flows, containing rounded pumice and some Devonian lithics in an ash matrix, which filled some of the side valleys of Brohltal to a depth of 60 m. The deposits, known as ‘Trass’ were extensively quarried to make cement, much of which was used to build the dikes in Holland because it could set under water. The deposits become denser with increasing distance from the source. Passing though a short tunnel we saw the unconformable contact, Photo 12, between the moderately dipping Devonian basement rocks and the overlying Laacher See ignimbrites.

 

 

Day 4, Wednesday 31 October 2012

Text and photos: Dee Edwards

On the last fieldwork day of the trip the morning dawned frosty and misty but promising good weather; although we were deep in the countryside the closeness of 'civilisation' was obvious in the aircraft con-trails in the blue sky shown here:

Misty morning at Weidenbach
Fig. 1 Misty morning at Weidenbach (Eifel)
Topographic map of the volcanic features near Gerolstein
Fig. 2 Topographic map of the volcanic features near Gerolstein (shows the location of Betteldorf, Walsdorf and Rockeskyller Kopf)

All the localities this day were in the West Eifel Volcanic Field (WEVF), which is about 50km across, contains about 240 volcanoes, the maars being closely spaced in the central part of the field. Extract of the maps of the area (eg Figure 2 Topographic map of the volcanic features of the Gerolstein area) do not show a large number of lakes because maars do not have to contain a lake. The Vulkanologische Karte of the West and Hocheifel is interesting

Geological map of the Wartgesberg-Strohn area
Fig. 3 Geological map of the Wartgesberg-Strohn area

One side is a topographic map on to which the various tuff and scoria rings etc have been annotated by hand. The reverse is the geological map, which is quite complicated so being able to turn over to the simpler topographic map was very helpful.

To explain the multitude of volcanological features of the area, it is thought that the lithosphere beneath the WEVF, like that of the nearby Rhine graben, is around only 50km thick, ie thinner than the 90km thickness in other areas of the Rheinischer Shield. Consequently mantle material rose higher, and because it was under less pressure, became partially molten. As the molten rock was less dense than the surrounding material it continued to rise through the Devonian sediments, sometimes to the surface.

Spindle bombs
Fig. 4 Spindle bombs in the Wartgesberg quarry
Reddened Devonian palaeosurface in the Wartgesberg quarry
Fig. 5 Reddened Devonian palaeosurface in the Wartgesberg quarry, showing the near-vertical bedding

The first locality was a large quarry worked by Scheyren Lava Co in a cinder scoria cone called the Wartgesberg Volcano, that is owned by the village of Strohn and which as a result is quite rich, choosing to spend money on elaborate roundabouts.

Our first task was to draw the exposed quarry face that represented a section of the cone and to work why it was asymmetric. There were spindle bombs visible on the sides of the roadway, shown in Fig. 4 (next page). We then walked down through the quarry to the base of the face. The original country rock, Devonian sediment, was visible in the sides of the bottom of the quarry. The Devonian was reddened and the attitude was almost vertical. This represented a sloping palaeosurface, probably a valley or canyon. Above was a thick layer of black lapilli of volcanic glass, representing a basaltic eruption.

Agglutinate at Wartgesberg quarry
Fig. 6a Agglutinate at Wartgesberg quarry
Agglutinate at Wartgesberg quarry
Fig. 6b Agglutinate at Wartgesberg quarry

The top bench of the quarry was similar to the first location on Sunday (Epplesberg), showing phreatomagmatic eruption layers. The whole section showed:

- Finer layers on top (fall out layer)

- Couple of black lapilli layers

- Fine layers (perhaps lake sediment in maar)

- Coarse blocks at the bottom, basal deposits of quenched basalt

- Reddened Devonian on sids of the palaeosurface on to which the volcano was erupted

So the original face and sketches we drew represented half of the central crater or cone, the slope into the crater being steeper than the outside flank.

On the other side of the quarry was a thick sequence of pale grey 'agglutinate' that resembled a lava flow (some of us incorrectly identified it as a flow, but Prof Schmincke was adamant that it was not. (My excuse is that we were 100m away when we tried to identify it, but even closeup it looked like a flow). The difference between a flow and an agglutinate is that a lava flow moved but an agglutinate is an accumulation in situ of spatter, bombs etc and usually is interpreted as a time when the eruption rate increased. The bombs etc welded together to form an agglutinate, shown in Figure 6. The composition of this agglutinate is basinite (an alkaline igneous rock of olivine/ pyroxene/ nepheline).

volcanic 'bomb' at Strohn
Fig. 7 Largest volcanic 'bomb' at Strohn

We then drove to the village of Strohn for lunch at the museum Vulkanhaus Strohn, designed by Hans-Ulrich Schmincke, and that had a small café attached. Afterwards we saw the ‘largest volcanic bomb’ shown in Figure 7 (an aggregate or agglutinate of material, not strictly a bomb, despite the large display board saying so) that had originally rolled to a location near the village & that was moved to its current position on a metal plate over ice/snow in the winter of 1980-1.

Geological map of the Bettelsdorf, Walsdorf and Rockeskyller Kopf area
Fig. 8 Geological map of the Bettelsdorf, Walsdorf and Rockeskyller Kopf area

The bomb has a diameter of 5m and an estimated mass of 120-130 tonnes. Lunch was followed by a brief stop at a farm specializing in goat’s cheese (very clean goats and lots of them) en route to Betteldorf.

The three later localities, Betteldorf, Walsdorf and Rockeskyller Kopf are shown on the geological map Figure 8 (though the exact localities are difficult to pinpoint) and also on the topographic map Figure 2.

Bettelsdorf quarry
Fig. 9 Bettelsdorf quarry
Dyke in the Walsdorf lava grube
Fig. 10 Dyke in the Walsdorf lava grube, opening to a tulip shape at top, and volcanic layers

Betteldorf quarry had spectacular planar layers of bedded reddish tephra and lapilli (shown in Figure 9) & about 15m above these, larger clasts representing a phreatomagmatic episode.

There were also multiple small faults visible in the layers. Prof Schmincke told us that the first layers on top of the Devonian are extremely widespread and can be correlated but the origin of the material is not yet known. There are then cycles of phases of magmatic, phreatomagmatic and then scoria ejection.

A 'Neptunian' dyke was also visible in the face, representing material falling into a crack that was probably tectonically generated, with finer material on the outside and coarser in centre. We noted that some layers were cemented with calcite (Dave got to use his acid bottle at last).

Next was a quarry called ‘lava grube’ near Walsdorf, with inner and outer craters showing crater unconformities. But the best feature here was a dyke cutting through the volcanic layers and that opened to a ‘tulip’ shape at the top, shown in Figure 10.

The final locality of the day was Rockeskyller Kopf quarry, shown in Figure 11, showing a basal cone which is part of a larger cone.

The layers shown on the left of Figure 11 (see next page) are coarser at the base and there are finer layers towards the top, interpreted as phreatomagmatic eruption, with inverted layers and round lapilli at the top.

This was part of a larger edifice. There is a transition to spatter with slightly welded fallout material continuous over the crest. On the right a grey-coloured rock can be seen, cross-cutting the ash layers and above the welded spatter. I identified this on my sketch as a lava flow but notice that in Prof Schmincke’s book there is a photo of this face and the caption clearly says that it is an agglomerate (‘kein lava see!’) but I correctly identified that there should be a crater to the right. My excuse is that we couldn’t get close to the outcrop to examine the rock properly.

quarry at Rockeskyller Kopf
Fig. 11 Rockeskyller Kopf quarry

Prof Schmincke thought that the layered (RHS) part could have been 50m higher originally but that this had been eroded. This was a spectacular last quarry.

We finished the day by examining a nearby ploughed field for sanidine (high temperature potassium feldspar) crystals that might be dated (using 40Ar/39Ar), as exact dates for the volcanic events in this area are problematic. Although the exact source of the crystals was not known they were likely to be reasonably local, and any date obtained would be useful. We were watched by a herd of handsome cows, curious as to why these strange people were trampling over the farmer’s planted field. (Mike Molloy says it was lucky we weren’t in Bavaria as the farmer would be out with his shot gun.)

It was a great privilege to be led over this ground by a world expert, who I had known by reputation for many years but never met. It was especially valuable to visit working quarries, as this is where the best exposures can be seen, and are not usually accessible to the casual visitor. Prof Schmincke’s obvious close relationships with the local quarry operators and local groups was a huge advantage for us. I really enjoyed the trip (sore shoulder excepted!) and I learned a lot, for example that I seen too many lava flows in the field & not enough agglomerates!

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