Evolution of Complex Plinian Eruptions: the Late Quarternary Laacher See Case History

SW-NE directions of the major fallout lobes probably reflect the main tropopause wind vector, identical to present-day dominant wind directions. The Laacher See area was inhabited prior to the eruption, judging from archaeological finds, but the absence of both human and animal skeletons beneath the pumice layers suggests that long-lasting tremors and other premonitory activity around the volcano may have driven humans and animals away from the area.

In the proximal facies, flow deposits (ash flow and surge) are channelized in six fans radiating from the Laacher See basin, while fallout deposits dominate on higher ground in between and in the medial and distal facies.

We distinguish seven major stages during the course of the eruption: (A) initial hydroclastic stage; (B) lower plinian fallout stage; (C) main ash flow stage; (D) upper plinian fallout stage intercalated with a second ash flow phase; (E) upper main hydroclastic stage, intercalated with late ash flows; (F) terminal phase of line-grained phreatomagmatic ash layers; (G) immediate stage of reworking of Laacher See Tephra (LST). A shorter phase of hydroclastic activity occurred just prior to the main ash flow stage and minor magma-water interaction is recognizable at other times during the eruption, most significantly from the southern vent in the Laacher See basin where the eruption started.

Primary environmental factors such as crustal lithology, stratigraphy and structure, as well as aquifer location and water supply rate, and induced environmental factors such as repeated vent widening and constriction, downward erosion and lateral migration of the vent, and crater/conduit instability and collapse were superimposed on the dynamic evolution of the gas-pyroclast mixture, resulting in a complex interplay of alternating internal (magmatic) and external processes.

The eruptive products are strongly compositionally zoned from phenocryst-poor, highly differentiated phonolite erupted initially (magma temperature c. 800^o C) to very crystal-rich (up to > 40 vol.-%) and hotter (c. 880^o C) tephritic phonolite mixed with basanite erupted last, with two small compositional gaps about 1/3 and 2/3 through the eruption.

The main plinian activity of Laacher See Volcano probably lasted < 1 day, judging from the duration of similar plinian historic eruptions, mass eruption rate estimates and other types of evidence. Slow deposition of fine-grained ash during the very terminal phreatomagmatic phase, however, was interrupted by at least one phase of gully erosion, and may have lasted for months.

The eruption released > 1 x 10⁸ metric tons of SO₂ and its sulphuric aerosols, deposited in the stratosphere for years (?), may have caused a surface temperature drop of more than 0.5° C in the northern hemisphere.

INTRODUCTION

The eruption of Laacher See Volcano (Germany) 11,000 a BP is one of the major Late Quaternary to historic plinian eruptions of highly evolved magma in central and southern Europe, other examples being the 'Minoan' eruption of Thera, 3500 a BP, and the AD 79 eruption of Vesuvius. Like the two younger eruptions, the Laacher See eruption was probably witnessed by man. The three eruptions were of quite different magnitude and magma composition, having vented 5.2 km³ phonolite (Laacher See), 39 km³ rhyodacite (Minoan: Sigurdsson and Carey 1990) and 3.5 km³ phonolite (Vesuvius: Barberi et al. 1981). Compositional zonation is most pronounced in the Laacher See Tephra. It resulted from the progressive emptying and explosive degassing of a phonolitic to basanitic magma column strongly zoned with respect to volatile concentrations and physical properties. Also, Laacher-See-type eruptions are more complex than model plinian eruptions, because of a complex interplay of alternating internal (magmatic) and external processes governed by primary and induced environmental factors.

The following text is a summary of part of our previous work with emphasis on topics studied at present, especially the opening phase of the eruption, and a more detailed assessment of the Laacher See magma column's compositional zonation. Papers published include Schmincke (1970; 1977a, b; 1981; 1988; 1991), Schmincke et al. (1973; 1983; 1988), Bogaard and Schmincke (1984; 1985), Fisher et al. (1983), Wörner and Schmincke (1984a, b), Wörner et al. (1983), Freundt and Schmincke (1985a, b; 1986), Schumacher and Schmincke (1990, 1991), Tait (1988), and Tait et al. (1989).

PRE-ERUPTION

       Previous volcanic activity:      Laacher See Volcano is the youngest (11,000 a BP) of four evolved phonolitic centres within the East Eifel Volcanic Field (EEVF), which is dominated by mafic scoria cones and lava flows (Fig. 1). Preliminary results of an ongoing tephrochronological survey indicate that its earliest eruptions occurred c. 700,000 years ago. About 215,000 ± 4,000 years BP, the erupted magmas changed from nephelinitic, leucititic and plagioclase-free phonolitic to basanitic, tephritic and plagioclase-phonolitic compositions (Fig. 1).

       Regional setting of eruptive centre and pre-eruption topography:      The LSEC (Laacher See Eruptive Centre) is situated on the higher north-western shoulder of the Tertiary-Quaternary Neuwied tectonic basin, on top of the uplifted Rhenish Shield, and is underlain by Devonian sedimentary rocks. The top of the Devonian basement in the vicinity of the LSEC is at 290-300 m a.s.l., dropping eastward to c. 50 m a.s.l. at the Rhine River, the 200 m high escarpment of the eastern fault being c. 20 km east of the eruptive centre (Fig. 1).

The LSEC is located in a pre-existing basin (maar and/or valley) surrounded by scoria cones with steep radial valleys in the north, and more gentle radial valleys in the south and east leading into west-east oriented tangential palaeovalleys (main tributary drainage pattern). Low passes between scoria cones strongly channelized horizontal transport systems during the eruption (surges, ash flows).

       Time of eruption, season of the year and premonitory activity:      ¹⁴C age determinations on near-vent charcoal inclusions of ash flows and on organic sediments bracketing distal ash layers consistently yield eruption ages around 11,000 years BP, but analytical errors are typically in the range of ± 100 years. Before the eruption, the area around the LSEC and the Neuwied basin to the east was dotted with numerous scoria cones and had a low density birch tree and grass land vegetation, with minor cherry and shrub forest. Tree-ring growth stages, leaves on torn-off branches and seed maturation stages indicate that the eruption took place during the early/middle summer (June/ July?). The two main directions of fallout fans indicate stable SW-NE high-altitude (tropopause level) and north-south tropospheric lower-altitude wind directions.

The Laacher See area was inhabited at the time of eruption, as shown by remnants of a Magdalenien settlement in strata immediately beneath Laacher See Tephra c. 9 km north-east of the eruptive centre, archaeological finds including rock tools, gems made of animal teeth, wooden beads and snail-shells, as well as elaborate carved paintings of animals and simple contour sketches of women, mostly on slates (Fig. 2: Bosinski 1981). No animal or human skeletons, however, have been found in the area so far, even though (a) hundreds of pumice quarries have been mined (and investigated) in the Neuwied basin - which measures some 20 km in diameter - over the decades, leading to almost quantitative removal of Laacher See Tephra over an area c. 200 km², and (b) the Neuwied basin, transected by the Rhine River, represented an optimal habitat in the area for c. 700,000 years, as shown by abundant evidence for human activity (mainly scoria cone crater habitat). The eruption area itself - the pre-existing Laacher See basin - was probably a very attractive dwelling site: a forested semi-closed basin, about 3 km in diameter possibly containing a lake and stream. The central part of this basin, however, was completely destroyed during the eruption.

Longer-lasting tremors and gas emissions may have led to an exodus, with people and animals possibly having fled to the banks of the Rhine River which must have become repeatedly clogged during the course of the eruption, the skeletons possibly having been reworked and washed down-river subsequently. This scenario, however, is purely conjectural.

ERUPTION

Laacher See Volcano erupted a minimum volume of 5.2 km³ highly evolved  to mafic phonolite magma (DRE - dense rock equivalent), resulting in c. 16 km³ of tephra, including c. 0.7 km³ xenoliths (DRE). Some 88% of the bulk tephra is contained in fallout, c. 10% in ash flow, and < 2% in surge deposits. The entire tephra sequence is subdivided into three members (Fig. 3): Lower (LLST), Middle (MLST), and Upper (ULST) Laacher See Tephra, which differ from each other in lithology, chemical composition, eruptive and depositional mechanisms, and areal extent.

       Facies:      Four main types of depositional areas can be distinguished from each other:

   (a) Fallout lobes governed by wind vectors and height of eruption column; ash clouds from lower columns - resulting from hydroclastic eruptions - were transported to the south, alternating with high plinian eruption columns and wind transport to the north-east (Nickenich facies) (Fig. 4).

   (b1) Radial proximal flow deposit fans (five regional fans) governed by low passes between older scoria cones surrounding Laacher See basin. They contain the bulk of the diverse types of surge deposits and the more sluggish pyroclastic flow deposits. The main fan - the Mendig fan - extends for about 6 km south-east of the crater area away from the lowest pass (Fig. 5).  

   (b2) More mobile pyroclastic flows extended radially on the outer slopes of the crater (proximal flow facies) and followed tangential river canyons up to 10 km away from the main crater area (medial and distal flow facies).

   (c) Low density ground-hugging ash clouds spread largely independently of local topographic irregularities and formed fine-grained ash layers with distribution patterns intermediate between fallout lobes and flow fans, extending up to the eastern boundary of Neuwied tectonic basin, c. 20 km east of Laacher See Voleano.

   (d) Intra-crater facies comprising surge and fallout deposits with dominantly erosional contacts, and co-ignimbrite lithic breccias on the steep inner slopes of the crater (scoria cone ring), and coarse MLST fallout (collar collapse) deposits on the gentle slopes of the south-western crater basin.

       Eruptive mechanisms:      Different types of fallout, flow and surge deposits were formed during both hydroclastic (phreatomagmatic, vuleanian) and pyroclastic (e.g. plinian) eruptive processes:

   1. During the initial vulcanian phase, wet surge and fallout layers were deposited in the proximal facies, grading into more widespread mud-cloud deposits in the medial facies (basal LLST).

   2. Major medial and distal plinian fallout deposits (white pumice) of the LLST phase are represented proximally by collar collapse deposits and surge deposits with abundant ballistic blocks.

   3. Lateral migration of the main vent started during an intermediate vulcanian phase that generated near-vent ballistic, surge and fallout deposits, grading into medial and distal ash cloud and sub-plinian fallout deposits (MLST-A).

   4. During the main phase of crater collapse and migration (main ash flow interval), ash flow and overbank ash cloud deposits alternate with plinian fallout deposits (MLST-B).

   5. Phase MLST-C comprises plinian fallout deposits, as well as intermediate ash flows and overbank deposits in the proximal and medial facies.

   6. The main upper vulcanian phase consists chiefly of high flow regime: (a) breccia flows and (b) surges with dunes, antidunes, and chute-and-pool deposits, apart from (c) massive viscous flow deposits (largely proximal facies), and (d) distal fallout deposits (ULST).

       Initial phreatomagmatic stage:      Prior to the eruption, a highly evolved, low-density magma column may have risen buoyantly. Initial explosions may have resulted from interaction between the rising magma - or an advancing hot gas phase - and shallow groundwater of aquifers located in a major thrust and older maar diatreme, possibly facilitated and accelerated by fractures in the magma chamber roof region that resulted from initial updoming of the rising magma column.

The eruption started with pressure waves expanding chiefly through passes between older scoria cones surrounding the Laacher See basin. The pressure waves defoliated trees which were uprooted and twisted-off up to c. 2.5 km from the vent. Bending of trees is recognizable as far as c. 3.5 km from vent; beyond they are generally vertical and rooted (Fig. 6). During the following initial phreatomagmatic phase (< 0.1 vol.-% of total tephra volume erupted), two sets of massive surge/fallout ash layers were deposited near the vent containing accretionary lapilli in the more distal facies. Condensation of water vapour resulted in deposition of 'mud' layers and abundant vesicle tuffs (Fig. 7).

The initial surges produced a continuous set of thin, green to light brown, fine-grained, mud-like ash layers. All units are texturally similar, showing normal grading with larger xenoliths (maximum grain size < 2-3 cm) concentrated at the base and, in many cases, formation of small erosional channels at the contact to underlying strata. The poorly sorted surge deposits contain four main components:

   (a) Juvenile clasts: dense, angular glass shards in the matrix show that little or no vesiculation had taken place, evidence that vaporization of external water played a major role during the early eruptive history.

   (b) Xenoliths: Lower Devonian slates fragmented to grain sizes ranging from silt-size to 3 cm; pebbles. vein quartz, and clasts of unconsolidated Tertiary clay, and fragments of basanitic to tephritic scoria and dense lava probably torn from an older scoria cone in the pre-Laacher See volcanotectonic basin and from lava flows that erupted from Wingertsberg volcano and made their way into the Laacher See basin. Rare larger clasts - mostly scoria - are interpreted as local pick-ups from scoria cones surrounding Laacher See basin. Almost all xenoliths are armoured by a thin ash layer, some are wrapped by ash and plant remains.

   (c) Matrix ash: mostly disintegrated Tertiary clay and material from other surficial rocks, a minor amount of dense glass shards, and a high percentage of phenocrysts (amphibole, pyroxene), mostly gained from erosion and disintegration of xenolithic material in the vent and during transport.

   (d) Organic matter: near-vent, the basal tuff layer is mixed with twigs and leafs and is locally rich in carbonized logs (Fig. 6). Most leaves in the first two very fine-grained flow units are well preserved and intact next to the stalk region, where the leaves have been torn from the twigs. Much ground vegetation is present as well, such as grass and less common herbs. Grass (either rooted or torn off), twigs and even leaves with an elongation ratio of at least 3:1 show sub-parallel orientation, allowing us to reconstruct the flowage pattern of both pressure wave and surges, which do not always follow radial directions from the vent but are deflected around larger topographic barriers.

A second surge-forming event was generated after a lull in explosive activity, when groundwater in the immediate vicinity of the small initial vent was recharged. The second phreatomagmatic explosions might have been even stronger than the initial ones, judging from the higher fragmentation of xenoliths (increased magma/water ratios). The thickness of the final set of surge deposits is more strongly affected by topography, showing that the ground-hugging ash clouds must have been much cooler and thus less mobile due to advanced phreatomagmatic chilling.

The initial vulcanian (phreatomagmatic) phase opened the vent and led to a rapid decompressing of the rising magma column. Many volcanic eruptions start in a similar way. This is not surprising, because the earth's crust, especially in its uppermost highly fractured and porous regions, is rich in water and aquifers. Magmas and/or a preceding hot-gas front ascending into the uppermost crust will thus commonly encounter and interact with groundwater, whose rapid heating and expansion may thus be the actual trigger of the subsequent decompression and explosive degassing of the rising magma columns.

       Plinian fallout stages:      Fallout tephra layers occur throughout the entire sequence and dominate the near-vent Nickenich facies (Fig. 8). Two main plinian convective stages, resulting in widespread fallout deposits, occurred between the lower and middle, and middle and upper hydroclastic stages. The first convective stage (LLST) produced white, aphyric, highly evolved phonolitic pumice lapilli (40 vol.-%). Roughly in the middle of the second convective stage (~ 30 vol.-%), essential clasts rather abruptly change from highly inflated, white, highly evolved pumice (MLST-B) to light greenish-grey, more mafic, and increasingly denser pumice (MLST-C). At the MLST-B/MLST-C boundary, the fragmentation level intersected a major chemical interface (compositional gap in matrix glass compositions) in the magma column, resulting in the coarsest airfall deposits of the sequence (MLST-C1), and the highest mass eruption rate estimated at c. 5 x 10⁸ kg/s.

Fine-grained Laacher See Tephra was deposited in two main fallout fans to the north-east and south (Fig. 4). It occurs as a thin ash layer in peat bogs throughout Central and Northern Europe. To the north-east, where the southern boundary of the late Quaternary Scandinavian ice shield forms the detection limit, Laacher See ash can be traced as far as Denmark and Sweden, up to 1,100 km from the vent. Typical thicknesses along the main E-NE fan axis are 50 m at 1 km distance, 5.5 m at 10 km distance, 0.3 m at 100 km distance, and 0.002 m at 1000 km distance. To the south, the tephra is found up to 600 km away in eastern France, Switzerland and northern Italy.

Directions of plinian fallout fan axes show a significant feature: the dominant transport direction of ash clouds during plinian stages was (south-) eastward for about 10 km from source. Then the direction changed to the north-east, depositing the distal facies. This fallout distribution pattern reflects dominant wind strengths and directions at different altitudes, e.g. high-speed (west-) south westerly winds at high altitude (tropopause level; jet-stream). Fallout from the initial, intermediate and final hydroclastic stages, in contrast, was distributed mainly to the (south-) south-east, reflecting lower eruption columns caused by energy partitioning due to water-magma interaction, and north-westerly winds in the lower atmosphere (Fig. 16). This palaeo-wind pattern is similar to present-day conditions during late spring/early summer, the likely season of the eruption.

       Transition from LLST to MLST:      A regionally widespread (at least 100 km² areal extent) multiple bed dominated by Devonian xenoliths at the top of the LLST fallout sequence (Fig. 8) is thought to have resulted from a major period of vent failure, accompanied by large-scale collapse of the crater probably at depths of several hundred metres. This event was followed by the second, middle phreatomagmatic phase (< 0.2 vol.-%), which generated fallout and surge deposits with 'mixed' grain size and clast density distributions, rich in large ballistic blocks in the Mendig fan. The eruptive vent started to migrate laterally during this stage towards the north-east, as shown by changes in the type of Devonian ('Siegen facies' sandstones instead of 'Hunsrück facies' slates) and basaltic (basanitic instead of tephritic lava) xenoliths. In the vicinity of Laacher See crater, these volcanotectonic events are reflected by numerous, typically small-scale, faults in the tephra sections, most of which show maximum subsidence during the central phreatomagmatic phase, the resulting rugged topography being smoothed out by subsequent MLST-B ash flow deposits.

     Ash flow stages:      Tree stages of pyroclastic flow formation (10 vol.-% of entire tephra) are distinguished. The first most prominent sequence of ash flows (MLST-B; Fig. 9, 10) formed at the begining of the second convective stage. The second set of ash flows was erupted towards the end of the second convective stage (MLST-C). The alternation of pyroclastic flow and plinian fallout deposits in stratigraphic units MLST-B and MLST-C (Fig. 3, 11) reflects multiple column collapse due to vent widening and recovery of stable convective columns during vent migration, and gradual destabilization of convective columns due to increasing influence of external water and more mafic gas-poor magma composition (MLST-C) towards the third ash flow stage (ULST). Pyroclastic flows of the third, main hydroclastic stage (ULST) are intercalated with base-surge dunes and breccia beds (Fig. 9), and probably formed by bulk collapse of low, overcharged, phreatomagmatic eruption columns.

The passage of pyroclastic flows is recorded by deposits of three main flow regimes preserved in three facies areas: (a) a proximal flow facies with surge deposits (Fig. 12) and lithic breccias originating from the passage of turbulent flows with strong erosion of preceding fallout and flow deposits on the steep slopes of the inner and outer crater flanks. Erosional channels are up to several meters deep. (b) A medial flow facies characterized by laminar flow and minor erosion. Ground layers were deposited in areas with steep valley flanks of high surface roughness. Lithic breccias occur at valley bends and sites with sudden reductions in slope gradients (hydraulic jump). (c) A distal flow facies developed on the far lower slopes and flat valley bottoms, where motion was governed by plug flow.

Longitudinal variations in grain size and partial depositional volumes, and the structure of single flow units indicate a systematic evolution of flow-forming eruption columns, including (a) a continuous decrease in column collapse height, (b) a maximum mass discharge into the flow roughly half way through each eruption, and (c) a decrease in bulk grain size late in the eruption.

       Fine-grained tephra layers:      Proximal fine-grained, thinly-bedded ash layers were deposited in several ways: (a) fine-grained, commonly poorly sorted ash layers ranging from a few mm to several cm in thickness occur throughout the LLST in the Mendig facies. Many are rich in disintegrated Tertiary clay. They are interpreted as very small ash flows and surges (as in the initial stage deposits) and fallout of wet ash (mud). (b) The overbank facies of valley-filling MLST-B ignimbrites was deposited from lateral, diluted, finer-grained flow lobes covering higher ground and interfluves (Fig. 9). Proximal overbank deposits clearly show characteristics of flowage, e.g. reversed size-grading of pumice. Distal deposits are generally rich in accretionary lapilli. (c) Co-ignimbrite and co-surge dust layers chiefly settled as aggregate fallout from ash clouds, as indicated by grain-size distribution and experiments. (d) In the ULST, fine-grained ash layers were chiefly deposited during the final stages as fallout of muddy ash (Fig. 11).

       Terminal stages of the eruption:      Major late-stage phreatomagmatic eruptions (15 vol.-%) were triggered by magma chamber wall collapse at 3-4 km depth and partially open vent conditions. This enabled more viscous, crystal-rich magma to erupt in steam-dominated columns, generating coarse-grained 'cold' base surge and ash flow deposits, as well as widespread (> 600 km) fallout ash. Late-stage influx of external water and magma-water interaction appears to have been triggered primarily by abrupt changes in the internal properties of the erupting magma (i.e. volatile contents). Similar late-stage hydroclastic eruptive phases are typical of many eruptions, especially of those strongly zoned in composition and with steep volatile gradients.

During the final phreatomagmatic explosions (deposits < 0.1 vol.-%), very fine-grained ash-flow, mud-fall and accretionary lapilli deposits (Fig. 11)were emplaced, indicating repeated evacuation and replenishment of a deep-reaching vent/diatreme system, and highly efficient comminution processes.

       Xenoliths:      A progressive downward migration of the eruption focus (fragmentation level) during evacuation is indicated by a gradual change of xenoliths in the stratigraphy (Fig. 13) from (a) surficial Quaternary phonolitic tuffs, basalt tuff, scoria and lava flows and Tertiary clay and conglomerate deposits (altogether less than c. 100 m thick in the area), through (b) Devonian slates and sandstones to (c) regional metamorphic phyllites and schists and quartzites underlying the Devonian sediments at 4 to 5 km depth. Frequent episodes of crater widening and northward migration superimposed on this gradual downward coring are reflected in recurring layers rich in xenolithic fragments of near-surface rocks, the best example being the MLST-A layers preceding the main (first) interval of ash flow formation (Fig. 13).

Dominantly ballistically emplaced basaltic xenoliths, some exceeding 2 m in diameter and 20 metric tons in mass, occur at Wingertsberg c. 1 km south-east of the presumed main initial (LLST) crater, and at Suessenborn c. 2 km north-east of the presumed MLST-B vent (Fig. 7, 14). The noticeable rounding of many of these blocks is interpreted as abrasion (and subsequent thermal spalling) in the vent following crater collapse during repeated episodes of crater erosion and widen-spalling. Much of the energy needed to finally eject these huge blocks may be due to both magmatic and phreatomagmatic overpressure building up beneath the crater fill of collapsed lava flows and, at later stages, Devonian basement, temporarily blocking the vent.

       Compositional zonation:      The reconstruction of chemical and physical gradients and layers in the magma column is based on volume determinations of individual stratigraphic intervals, each with strongly diverging transport axes and distribution patterns due to alternating pyroclastic and hydrodasric eruption mechanisms and surge, flow and fallout transport systems. Prior to the eruption, the Laacher See magma column with an erupted volume of c. 5.2 km³ (DRE) was strongly zoned with, from top to bottom, a temperature gradient from 800^o C to 880^o C, water contents from > 4 wt.-% to < 2 wt.-%, and phenocryst contents from < 1 vol.-% to > 40 vol.-%, resulting in viscosities from roughly 10³ Pas to 10⁵ Pas. The magma column consisted of at least 8 structural and compositional units, including, from top to bottom (Fig. 15):

   (1) an extremely evolved cupola of systematically zoned, volatile-rich, aphyric magma (LLST; c. 1.7 km³),

   (2) a thin layer or interface (top of LLST) with steeply decreasing bulk rock volatile (e.g. fluorine) and incompatible trace element concentrations, analytical scatter and syn-eruptive magma mixing probably camouflaging a ftrst compositional gap for most elements except, for example, Zr (gap = 430 ppm) and Nb (gap =60 ppm),

   (3) a second main body of systematically zoned, phenocryst-poor, highly to moderately evolved phonolite magma (MLST-A to MLST-B; c. 1.7 km³),

   (4) a second thin layer or interface (MLST-B/MLST-C boundary) at which the magma composition changes from evolved to mafic phonolite (white vs. light grey pumice). A compositional gap at this level is clearly indicated by the major element composition of matrix glass (Fig. 15), but apart from a minor (170 ppm) gap in Zr only poorly documented by whole rock data, probably again due to limited magma mixing along the interface,

   (5) a layer of systematically zoned, phenocryst-poor, mafic phonolite magma (c. 0.9 km³; MLST-C),

   (6) a layer of crystal-rich, mafic phonolite magma with abundant crystal cumulates (c. 0.8 km³; ULST),

   (7) a third thin layer (or interface) at which mafic phonolite magma and crystal cumulates are mingled with more mafic, parental basanitic-tephritic magma (top of ULST), and

   (8) an underlying, non-erupted, mafic ('parental') magma body of basanitic-tephritic composition (20 to 50 km³ according to model calculations), from which the above units are derived by fractional crystallization, diffusive diffusion-convection and magma mixing processes.

       Eruption dynamics:      Like many other plinian tephra deposits, the Laacher See plinian Tephra shows an overall increase in grain size throughout the course of the eruption. Mass eruption rates of about 3 to 4 x 10⁸ kg/s with vent radii of 200 to 250 m are indicated by the initial velocities of m-size ballistic blocks (basalt xenoliths; 300 to 350 m/s) and magmatic gas releases of 2 to 2.5 wt.-% H₂O for most of the second plinian eruptive stage. The maximum mass eruption rate, however, is reflected by the coarsest and densest fall deposits, which were deposited near the end of this stage (MLST-C1), after essential clasts changed from highly vesicular, white pumice (median density = 0.6 to 0.7 g/cm³) to vesicle-poorer, greenish-grey pumice (median density = 0.9 to 1.4 g/cm³: Fig. 16). This is where the fragmentation level in the magma chamber intersected the second compositional interface layer in the zoned magma column, and magma composition changed from highly evolved, volatile-rich to more mafic, hotter, but volatile-poorer and slightly more viscous phonolite magma. At the same time and stratigraphic level, contact metamorphic xenoliths from the magma chamber walls, whose abundance systematically increased throughout MLST-A and -B deposits (Fig. 13), become a significant xenolith component in the tephra deposits (2 to 6 wt.-%), indicating a rather systematic fragmentation of the magma chamber walls, and hence destruction of the 'thermal shield' that so far separated the c. 850^o C magma from aquifers in the surrounding crustal rocks.

The sudden increase in eruption vigour thus cannot be explained by the obvious changes in intemal magma properties alone. Instead, the processes operating during this eruptive phase most probably demonstrate that internal magmatic and external environmental conditions and processes generate a complex, multi-stage feed-back system, involving (1) the fragmentation level of a sustained plinian eruption transecting a compositional boundary in the magma column, (2) the subsequent tapping and unsteady degassing of volatile-poor magma batches, incapable of maintaining overpressure or pressure equilibrium in the magma chamber, (3) spalling of the (contact metamorphic, walls of the temporarily underpressured magma chamber,  (4) access of aquifer water to the still actively degassing magma, and (5) increasing mass eruption rates due to explosively expanding external water vapour (increasing magma temperature; high magma/water ratio; deep-level contact).

The three major phases of vulcanian activity (initial, intermediate, main upper) are thus interpreted as due to interaction of magma and external water at three different levels: (a) During the initial stage, rising magma and/or hot gases interacted with shallow groundwater (possibly also lake water: evidence from plant imprints), and triggered the first plinian eruptive stage. (b) During the intermediate stage, deep-reaching collapse of conduit walls and crater collapse along with lateral vent migration tapped new water reservoirs - probably aquifers along thrust faults and/or water-saturated diatremes of older basaltic maars and scoria cones. When the intermediate vulcanian episode ceased (exhausted water reservoirs ?), pyroclastic eruptions continued with alternating convecting and collapsing plinian eruption columns.  (c) Only during the late stage (see above), following evacuation of c. 4 km³ magma, magma/water interaction occurred at 4-5 km depth (open system conditions?).

LATE AND POST-ERUPTION EFFECTS

       Duration of entire eruption:      With estimated mass eruption rates in the order of 3-5 x 10⁸ kg/sec, the LS plinian eruption(s) probably lasted about 7 to 11 hours. The duration of the intermediate, and especially the final main vulcanian stage(s) is more difficult to assess (1 week?). The ULST shows a fining-upward sequence with plane beds, vesicle tuffs and accretionary lapilli-bearing beds becoming more abundant upwards (laminated division). At least one regional widespread erosional event occurred during which m-deep, steep erosional channels were carved prior to deposition of final accretionary lapilli-bearing olive-buff silts. These silt deposits (< 0.01% of the bulk magma volume) also include horizons with rudimentary soil formation and Aeolian reworking, and may represent several terminal phreatomagmatic, gas-rich eruptions which may have been separated by longer pauses in explosive activity (months?, years?).

       Sulphur release and climatic response:      The Laacher See magma was rich in sulphur, as shown by abundant hauyne phenocrysts, and high sulphur concentrations of essential lapilli (bulk-rock XRF analyses). While bulk rock sulphur contents of highly inflated pumice clasts from plinian eruptive stages (LLST, MLST-B) are invariably low (~ 0.01 wt.-%), those of lapilli from vulcanian and water-influenced plinian stages (e.g. LLST, MLST-A, MLST-C, ULST) range from 0.01 up to 0.2 wt.-%, depending on the vesicularity of the samples (Fig. 17). The data indicate a nearly quantitative sulphur degassing of the magma during plinian stages, but partial retention of sulphur due to quenching during hydroclastic eruptive stages. While some of the data are clearly at the detection limit of the analytical method, and detailed microprobe/ion-probe studies are needed to assess this problem more properly, a rough estimate of the bulk sulphur release can be calculated using the minimum-sulphur (post-degassing) and maximum-sulphur (pre-degassing) envelopes shown in Fig. 17. Given that > 60% of the bulk erupted magma is contained in the plinian deposits, and using degassing efficiencies between 10 and 50% for vulcanian and water-influenced plinian stages, the total mass of SO₂ released by the Laacher See eruption is estimated as 1.15 x 10¹¹ kg. This is probably a minimum estimate, since sulphur contents of glass inclusions in LLST phenocrysts are up to twice as high than the maximum measured on LLST bulk rock samples.

Aerosol clouds derived from the Laacher See eruption may have resided in the stratosphere for several years, and may have had a significant impact on the evolution of Late Glacial climate in the northern hemisphere: the short-term mean surface temperature decrease was probably at least 0.5^o C. The phonolitic LSE thus had an even more severe climatic impact than the silicic Minoan eruption of Santorini, even though the latter produced nearly 10 times the mass of magma (Fig. 18).

Interestingly, fir trees that were preserved in Late Glacial loam deposits at Dättnau/Switzerland, show a dramatic decrease in tree-ring thickness at about 11,020 a BP (Kaiser 1979), which roughly coincides with the likely radiocarbon age of the Laacher See eruption (Fig. 18). More high-precision ¹⁴C age determinations of the LST eruption, however, are needed to allow a more straightforward correlation of the two event records. Whether the eruption of Laacher See Volcano (and other explosive volcanic eruptions) contributed to the stage of climatic deterioration known as the 'Younger Dryas' (c. 10,800 to 10,000 a BP), which followed the 'Bölling/Allerød' warm phase at the end of the last glaciation and coincides with the end of the Magdalenien culture in central Europe, is a matter of current investigation.

       Reworked tephra, filling of the crater basin and present activity:      The late stage of the LSE is well documented owing to a soil cover that is generally up to c. 0.5 m thick. Immediately after the eruption (prior to soil formation), the lowlands in the Neuwied basin and flood plains bordering the Rhine River were covered with up to 10-m-thick reworked fluvial (and lacustrine) volcanoclastic deposits and lahars.

Laacher See crater is figure-8-shaped due to the c. 1 km lateral displacement of the vent during the early MLST-B eruptions, the lake contour (3.3 km²) roughly coinciding with the two partly overlapping main conduits which have a funnel-shaped downward extension, judging from seismic evidence. The basin is surrounded by a ring of older scoria cones, most inter-cone lows being filled with Laacher See tuff-ring deposits (area inside rim = 4.9 km²). In a first post-eruptive phase, coarse volcanoclastic breccias were deposited in this basin, indicating rapid erosion of the inner basin walls prior to sealing by vegetation (Bahrig 1984). This was followed by slower and more fine-grained lake sedimentation, with occasional influx of coarser-grained deltaic sediments stretching inward of major passes. The present state of Laacher See Volcano is characterized by moderately vigorous CO₂-degassing (gas composition all but identical to Lake Nyos (Giggenbach et al., 1990).

       The Laacher See tephra event unit:      Tephra event units (Schmincke and Bogaard 1990) encompass (a) the proximal, medial and distal tephra facies generated by explosive volcanic eruptions during a few days to weeks, (b) co-genetic volcanotectonic events, (c) the reworked tephra facies formed by rapid reworking, commonly within a few weeks to years after a major eruption, (d) acidity and high-conductivity layers in ice cores formed by precipitates of volcanic aerosols, and (e) the climatic, environmental and societal impact of an explosive eruption (Fig. 19). The Laacher See Tephra excellently illustrates this concept.

The eruption is represented by single and/or compositionally zoned fallout ash layers in the distant medial and distal depositional facies, grading into a complex sequence of ballistic, fallout, ash flow, and surge deposits close to the source, all being accumulated in an extremely short time interval. Other, well-documented event records include the (pre-?), syn- and post-eruptive volcano-tectonic activity around the eruptive centre, volcanotectonically induced (?) volcanoclastic deposits (i.e. lahars) and fluvial and lacustrine sediments derived from immediate reworking of the tephra, its repercussion on the biosphere both in the vicinity and hundreds of kilometres away from the vent, as well as its effect on Late Glacial surface temperatures and the history of early man in central Europe.

---------------------------------------------------

--------------------------------------------------------