Geoscience Seminar - Professor Hans-Ulrich Schmincke, Helmholtz Center for Ocean Research, Kiel, Germany

Titel: Environmental coupling and environmental impact: Internal vs external forcing during the 12 900 BP Laacher See Eruption. Authors: Professor Hans-Ulrich Schmincke, Dr. Mari Sumita & Dr. Cornelia Park.

13.03.2018 | Ann Eg Mølhave

Dato tor 17 maj
Tid 14:15 15:15
Sted Geoscience's auditorie, 1671-137

Abstract:

The 12 900 BP Alleroed Laacher See eruption (LSE), the largest Plinian eruption in Central Europe during the past >200 000 years, is a textbook example of the systematic and stepwise emptying of a magma reservoir(s). The eruption also illustrates the complex interplay of external and internal forcing throughout the entire multiphase eruption.

LSE occurred in 6 main stages: (1) Opening stage, (2) first major Plinian stage, (3) Transitional stage, (4) second major Plinian stage, (5) main late phreatomagmatic stage, (6) temporally extended waning stage sandwiching the climate impact. The main depositional fan axis is oriented SW-NE parallel to the present-day dominant jetstream, minor late fans being directed south- and westwards. Characteristic for LSE ist the subdivision of all stages into several distinct pulses or phases. The volume of dominantly Devonian xenoliths is unusually high. The volume of pyroclastic flows is low but with strong local effects. The pronounced compositional zoning ranges from early, almost aphyric, extremely evolved to very phenocryst-rich mafic phonolite.

Magma-water interactions occurred from the initial vent-opening through the transitional to the late high-energy phreatomagmatic phases and the extended fine-grained waning stages that may have lasted several months. The proximal, transitional (separating the first stage early and intermediate stage Plinian phases) and high-energy late magma-water interactions took place dominantly in the conduit system (groundwater) that repeatedly collapsed. Rain showers during the multiphase transitional stage repeatedly flushed fine-grained ash out of the ash-dominated low eruption clouds. The systematic and stepwise emptying of the compositionally zoned magma reservoir resulted in multiple pronounced changes in the eruptive mechanisms resulting in several transport mechanisms (fallout, pyroclastic flows, surges, etc.), in part widely diverging transport directions and many interruptions of eruptive activity.

Damming upstream at Coblence, the bottleneck entrance to the Neuwied Tectonic Basin (NTB), took place from the beginng of the eruption to the late middle stage. Dams formed exclusively during phases of fallout and were breached during breaks inbetween Plinian bursts. Five major dambreaches released enormous, highly energetic floods that eroded, filled, eroded, filled numerous active and abandoned channels north and south of the main Rhine channel over a wide flood plain area, vastly increasing the water surface. This is reflected in a wide range of spectacular high energy sedimentary structures, the Rhine finally transforming into a slowly moving slush.

About halfway through the eruption, a major pyroclastic flow-fed dam formed 8 km downstream of Andernach across the Rhine. The gradual growth of the lake upstream of the dam finally reached Coblence, terminating the Coblence dams.

 

The final lake eventually extended ca. 140 km through the famous Rhine canyon as far south as Mainz. At ist peak, the lake had a maximum areal extent of ca. 300 km2 and a maximum volume of ca. 2.6 km3. Two stages of dam collapse caused flooding downstream for >50 km probably as far as the Netherlands.

Large volumes of SO2 emitted from the extremely sulfur-rich phonolite magma was injected into the stratosphere via high eruption columns during Plinian phases. They were transformed into sulfate aerosol veils that spread in the northern hemisphere, significantly impacting the climate. Sustained and voluminuous widespread precipitation triggered by the radiation changes not only carved deeply incised channels on the eastern outer slopes of the eruptive center but also accellerated influx of detritus into numerous lakes in northern Germany. These events occurred prior to the very end of the eruption that is represented by a final, accretionary lapilli-bearing fine-grained tephra veil.

Some 200 years later the primary and reworked LSE deposits became strongly overprinted regionally by the cold Younger Dryas events.

Seminar, Institut for Geoscience, Medarbejdere