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| Titel |
SEM analysis of tephra collected in rain in Copenhagen during the 2010 eruption of Eyafjallajökull |
| VerfasserIn |
Christian Bender Koch, Thomas Bjerring Kristensen, Merete Bilde |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057414
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| Zusammenfassung |
| Volcanic eruptions are localized sources of reactive, fine-grained materials that spread into
and modify the properties of the atmosphere. The tremendous variability in concentration and
sizes of particles makes assessment of ash behavior a huge technical challenge. Rain
accelerates leaching of the particles from the air column thus providing access to
spatially confined information on some basic properties (particularly size) of the ash
particles.
We here report on the morphology and size of particles attributed to be of volcanic origin in
rain collected in Copenhagen in April 2010 following the eruption of the Eyafjallajökull
volcano. The particles exhibit angular morphology and are extremely variable in
size, typically of dimensions between a few micrometers and approximately 50
micrometers.
Precipitation was collected using a glass funnel and a glass container on top of the HCØ
building complex at Universitetsparken in Copenhagen (55.7°N, 12.6°E). Rain
water was collected: From April 20th 14:30 to April 21st 10:00 local time 410 mL
rain was collected. From April 21st 10:00 to April 21st 19:30 350 mL rain was
collected.
The precipitate was filtered using a 0.45 my cellulose filter (D=4.0 cm) and prepared for SEM
analysis by adhering small parts of the filter onto the supporting Al stub and coating with
carbon by sputtering to minimize charging effects. A FEI Inspect instrument operated
between 5 and 20 kV was used for imaging. Semi quantitative elemental analysis was
obtained using an Oxford energy dispersive x-ray detector.
Admixed with the tephra particles of biological and technical origin were also identified.
The most abundant tephra observed was blocky shards dominated by angular edges and sharp
protrusions. The surfaces of the particles appear mostly fresh and clean but often
agglomerates of rounded particles of much smaller size are found adhering to the surface.
These particles are interpreted as originating from physical fracturing of much larger
particles. The particles appear dense indicating that gas-driven fracturing has been of minor
importance.
We also observed several spheres (~20-30 μm) giving the impression of hollow
structure with abundant (approximately ring shaped) opening to the exterior with
flow lines on the surface. They are preliminary interpreted as remelted cellular
particles.
In addition to that aggregates (with dimensions up to ~20-40 μm) of mostly blocky
micron-sized particles having angular to subangular edges, often showing muschled fractures
were present. They appear to have been welded together either by mechanical impact or late
stage heating from gas phase.
In conclusion this direct investigation shows that tephra particles can be both simple and
composite. The contrasting morphologies will exhibit different fundamental physical
properties for comparable sizes, rendering extraction of size information from scattering
difficult. Our results document the occurrence of particles in the tephra of larger size than
previous results derived from scattering |
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