The Eifel Volcanic Fields 31 July - 7 August 2004
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.
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.
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.
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 19th 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.
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.
Photos by Kirsty Crocket
Eifel Field Trip - Day 3
The programme indicated visits to 7 localities plus 1 optional today, but due to the interest shown we only managed to visit the 4 most important ones.
Huttenberg Sand Pit
Our first visit was to the Huttenberg Sand Pit in the rim of the Huttenberg crater. This was a very interesting location as the Huttenberg tephra is very poorly exposed, with only 3 exposures in the area.
The pyroclastic rocks were cross bedded throughout, mainly very well bedded in massive layers with flat tops and bottoms; indicating a horizontal turbulent flow, the direction of which was not evident as no perpendicular face available to view. Flow was, in fact, west to east.
The pyroclastic flow was rich in ash with the fine ash having risen several kilometres and drifted in the wind so that it was decoupled from the main flow. Changes in eruption, or transport, caused change in the layer grain size from lapilli bombs to ash in every layer. Magma erupted was crystal rich (20%) with phenocrysts of 2 feldspars (anorthoclase & plagioclase), hornblende (dark needle shaped) and augite (shorter, dark green/black). With 10-20% vesicles, this was not a pumice but indicated a water quenching of the magma as it erupted, rather than magmatic degassing.
There had been regional metamorphism of 3 groups of country rock:- Devonian slates, Devonian sandstones, and low grade phyllite & mica schists.
Contact metamorphism changed the mineralogy of the regionally metamorphosed rock so that it crumbled between finger & thumb, as the rock became 'sandy'. This was caused by the magma chamber at the phyllite level, or at the boundary of the unmetamorphosed Devonian rocks. The country rock, rich in silica, was taken up by the magma, i.e. phonolite magma erupted, reacted with country rock taking up silicate, and became a trachyte.
The phonolite with large crystals of mafic composition indicates it was not highly evolved, so from the deepest level of the magma chamber; the end of the eruption, 215,000 years ago.
The caldera at this location was too large for the Huttenberg Sand Pit deposits so it is probable that these de-posits were from an earlier eruption, but the expected ring dykes have not been found.
Moderate to poorly sorted clasts were present in a well bedded pumice which did not have any cross bedding. Very few crystals but many vesicles, and the rock were whiter than expected. The occasional large holes in the face were from tree branches & trunks which had been dragged along with the flow and now long gone, although they possibly changed to charcoal before disappearing.
Basalt tuff was present but no Devonian rocks. Huttenberg tephra is always found at the base of scoria deposits (lava lumps) and at a second locality in the quarry, the lowest unit was the scoria; horizontally bedded, poorly sorted ash and basalts (ash -lapilli in size) lay above, showing well bedded material with some low angle cross bedding.
This was a surge deposit of magma; mainly basalt containing olivine & clinopyroxene minerals, Devonian sandstone & slates, and pieces of Huttenberg tephra. Evidence indicated the basalt had been water quenched and as tephra pieces present, the Huttenberg eruption was older (215,000 compared to 150,000 ya).
These deposits were parallel to the flank & cone and during the ice age (sometime bet. 215,000 and 150,000 ya) the structure developed in a cold, tundra vegetated permafrost period, with freeze/ thaw to several metres depth. This did not destroy the bedding and, when no ice above, movement downwards of the plastically composed rock, folded the bedding over, creating a very unusual structure. A classic example of 'plastic deformation'.
The locality had several eruption centres and in the rubble we saw clinopyroxene crystals up to 10mm diameter which reflected blue/green in the strong sunlight. The basalt contained 10% clinopyroxene crystals and a little olivine and biotite mica (mg rich). We were looking at a massive structureless basalt which had a 'welded core' as a mixture of basalt bombs from nearby vents welded together in a very hot, explosive eruption and fell back (about 200000 ya).
At the base of the volcano we saw the weathered surface of the older strata, and the volcano plug with baked tuff alongside.
The baked rock was from a previous eruption and the columner jointing was into the face; indicating that cooling was vertical. The plug was in 3 distinctive bands; greenish at the centre indicating hot rock not exposed to air but to volatile vapour & gases, then reddish…. hot and exposed to air, and black on the outside indicating exposure to air when cold.
Last visit of the day but notices stated 'entry prohibited' as this was a popular tourist site. The restriction did not deter as 10 information boards had been erected along a footpath which rose above, and overlooked, the quarry. Unfortunately, wording was in German but the drawings were colourful and understandable.
The eruption occurred about 200,000 ya and the clear, sunlit rock face showed evidence of 6/7 eruptions, with the volcano to the west. It was of basalitic composition with an ash fall on top.
As with the other days, this was another hot, dusty, unshaded day, but tremendously interesting and informative, which made the OU Summer School on the 'hard rock' - course pale into insignificance. Ex-students wished they had brought along their S339 books, as this day really brought them to life.
Dave Williams (Branch Organiser -Yorkshire)
Eifel Field Trip - Day 4
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.
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.
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.
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.
(Photos by Kirsty Crocket)
Eiffel Field Trip - Day 6
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.
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 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.
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’.