Identifying multiple eruption phases from a compound tephra blanket: an example of the AD1256 Al-Madinah eruption, Saudi Arabia

Complex eruption episodes commonly produce several phases of tephrafall and/or concurrent falls from multiple vents. Phases of eruption are challenging to reconstruct from the geological record, especially where there is a lack of distinct physical or chemical variations during an eruption episode. A statistical method is proposed for identifying the most likely combination of multiple fall lobes for composite tephradeposits, using a new high-resolution tephrafall map from the basaltic AD1256 Harrat Al-Madinah fissure eruption in Saudi Arabia. This dominantly effusive eruptionepisode lasted 52 days periodically producing tephrafrom several vents along the fissure. Most tephrawas produced from high Hawaiian fountainsand dispersed under differing wind conditions. The widest-dispersed tephraoccurred under phases of the highest fountains, at least 500 m high and probably closer to 1000 m. These high fountainsproduced pyroclasts with a broad range of vesicularity. Similar total versus lobe-specific grain size determinations showed little systematic variation of maximum fountain-height phases. Individual tephralobe properties (vesicle form, density, particle shape and particle-size distribution) in different sectors around the volcano varied only subtly. From the statistical distribution of spot fall-thickness measurements, a semi-empirical tephrafallout model, modified to account for weathering, wind remobilisation and settling, was fitted using maximum likelihood estimation. A range of likely eruption-event scenarios were evaluated, concluding that the AD1256 eruption most likely comprised three separate fall-producing eruptions from its northern vent under differing wind conditions. The first of these occurred concurrently with high- fountainingevents from two other major vents southward along the fissure, producing overlapping fall lobes. Applying this method to other similar compound tephradeposits will help elucidate more realistic eruption scenarios and event reconstructionsfrom the geological record.