A Lithofacies Classification of Complex Ash Aggregates from the 536 A.D. Phreatoplinian Eruption of Ilopango, El Salvador.

Abstract

Ash aggregation is a key factor determining the residence time of ash in the
atmosphere. Aggregates range in size from particle clusters <40 μm in diameter to
complex aggregates several centimetres in diameter. Complex aggregates are typically found within ignimbrites and are identified by their sub-spherical shape and multiple continuous and non-continuous internal layers. Here I present the first lithofacies classification scheme for the interior layers of complex ash aggregates. SEM images of thin sections and X-ray computed microtomography (XCT) scans of whole aggregates allows these layers to be analysed in both 2D and 3D. I combine quantitative image analysis techniques with qualitative observations to examine differences in the particle size and structure of layers in complex aggregates from the 536 A.D. Phreatoplinian eruption of Ilopango. Three end-member lithofacies are identified:
1. Porous, clustered fine ash (pcA): a moderately sorted fine ash with a massive structure composed of ash clusters ~20-60 μm in diameter.
2. Massive fine ash (mA): a very poorly sorted fine ash with a low porosity, massive framework containing particles <2 mm and sub-rounded vesicles <150 μm.
3. Massive ultra-fine ash (mFA): a medium to well sorted ultra-fine ash that fines outwards with a tightly packed, massive framework.
Each of these lithofacies can be attributed to a different formation mechanism dictated by the availability of liquid water. A consistent non-repeating lithofacies appearance order is observed, indicating all aggregates share a linear formation sequence. Aggregate growth is dominated by the hierarchical accretion of smaller aggregates under saturated to sub-saturated conditions within an eruption or coignimbrite plume. Ultra-fine grained outer rims are accreted under increasingly dry conditions, requiring the final descent to be into a laterally moving pyroclastic density current.