Lengdargráðum

Lengdargráðum is a term used primarily in Icelandic geology to describe a specific kind of stratigraphic gradient observed in volcanic ash deposits. The word combines “lengda” meaning “length” and “gráður,” a variant of “gráður,” meaning “gradient” or “complex.” In practice, lengdargráðum refers to the gradual change in thickness, composition, and grain size of tephra layers over geographic distance from an eruption vent. The concept is important for reconstructing eruption dynamics, including the directionality of pyroclastic flows and ash dispersal patterns.

In the textbook on Icelandic tephrostratigraphy, lengdargráðum is used to categorize deposits that show a systematic decrease in particle size with increasing distance from the source. The gradient is plotted against distance, producing a lengdargráðum diagram. This diagram helps volcanologists distinguish between multiple eruption pulses and infer wind patterns that affected ash deposition. The term is most frequently applied to the 1783–1784 Laki eruption, but also to smaller phreatomagmatic events such as the 1986 Eldfell eruption on Surtsey.

Research on lengdargráðum has extended into the field of paleoclimate, where variations in ash layers record shifts in prevailing meteorological conditions. By comparing lengdargráðum across different stratigraphic sections, scientists can identify regional variations in eruption style. The method has been adopted by several European research institutions and is documented in the proceedings of the International Volcanology Society.

The study of lengdargráðum demonstrates how detailed sedimentological analysis contributes to broader understandings of volcanic hazards. It remains a specialized, though increasingly utilized, tool within the Icelandic and broader volcanological communities.