Abstract

The Skagi peninsula of Northwestern Iceland is the location of the Snæfellsnes-Húnaflói paleo-rift axis, active from ~3.3-16 mya, resulting in a regional flexure zone. The regional flexure is disrupted by the Hjallin lens, a ~150 m thick, ~3 km long extrusive body. Originally mapped as an intrusion, the lens exhibits well-developed columnar jointing, a flow-banded chilled basal contact, and an underlying sedimentary layer containing organics. Field relationships suggest the lens in a syn- (or post-) subsidence extrusion emplaced into a local bowl-shaped depression. Despite its thickness, the Hjallin Lens is nearly uniform in composition and mineralogy. This study was completed with the goal of exploring systematic compositional and textural changes, and to answer the question, how can a thick extrusive body cool quickly enough to form a fine-grained basalt with uniform mineralogy and composition?

Fe-Ti oxides represent lava conditions at emplacement and were used to evaluate the proposed question. For this study, 18 thin sections were collected, evaluated for textural information, and imaged by microscope and SEM-EDS. FIJI software was be used to determine crystal boundaries and area percentages of Fe-Ti oxides in thin section. This information, alongside individual crystal measurements, was used in ShapeCalc and CSDCorrections1.6 to determine CSD diagrams. These were then used to interpret cooling patterns associated with different emplacement mechanisms. CSD diagrams show a zigzagging pattern between small and large crystal-sizes that are synonymous with sequentially emplaced, thin basalt flows. This, in conjunction with compositional uniformity and the formation’s size, suggest that the Hjallin lens is made of thin, sequentially emplaced, fused flows.

Advisor

Pollock, Meagen

Department

Geology

Disciplines

Geology | Volcanology

Keywords

Hjallin Lens, Volcanology, Emplacement, Crystal Size Distribution (CSDs)

Publication Date

2026

Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis

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