Mastering the Mysterious – research project on Ascension’s “blebs”

My name is Annabelle and I am a master’s by research student at Durham University studying an unusual glassy pyroclastic rock (some say they resemble chocolate brownies/cookies, however I can assure you, they are not). To geologists they immediately stand out as they are super weird and don’t resemble what usually is erupted out of a volcano! I was eager to undertake this project due to the unique morphology and intricate detail of the samples, as unlocking the secrets of their formation will help us to understand vent processes in fissure eruptions – one of the most common types of volcanic eruptions.

Recent fieldwork to Ascension Island uncovered these mysterious glassy pyroclasts which were co-erupted with scoria during a small-volume basaltic eruption. They are comprised of dense glass bombs that vary in size from milimeter spherical droplets, to blobs 10’s of centimeters across. They occur in low abundances in scoria-dominated ramparts (elongate walls that are found adjacent to fissure).

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Above are photographs taken by Rich Brown during the 2018 field season 1) Tongue-like pyroclast that flowed on impact. 2) Broken glass filaments with thin glass bridging between them (a favourite as it resembles the Two Towers from The Lord of the Rings). 3) Larger glassy pyroclast that has undergone considerable post-impact flow. D) Glass droplet frozen onto the side of a scoria clast on the fissure edge.

The unusual shapes and features of these pyroclasts pose numerous questions about their formation and origin.

Some key questions are;

  1. Are they a product of stagnant lava that has drained back into the vent?
  2. How was it entrained into the lava fountain and erupted alongside normal scoria?
  3. Were they erupted at a normal temperature or superheated somehow?
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Photographs of hand samples of the pyroclasts. A) The base of one of the pyroclasts which contains scoria and lithic clasts (clasts of lava from an older eruption). B) Cut section showing how the glass has flowed internally and interacted with the scoria as it landed. C) Cuboid glassy clast with near parallel sides (shaped like a chocolate – but again not edible!). D) Spherical glass drop stuck to the exterior of the scoria clast, these appear to be quite a common feature across the samples and in the field photographs. E) A smaller feature of photograph A, there are delicate thin filaments bridging a narrow crack in the exterior of the sample. F) Bubbles in the glass which follow the edge of scoria clasts.

I am fortunate to be investigating these hypotheses under the supervision of a large group; Rich Brown, Kate Dobson, Fabian Wadsworth and Katy Chamberlain. Rich (my lead supervisor) and I are carrying out a comprehensive 2D and 3D textural and physical analysis of the samples to gain a better understanding of their origin. I have also been working with Fabian using numerical techniques to constrain an approximate temperature of the glassy drops when they were erupted. Determining whether they were erupted unusually hot may be a reason why they look so weird.

Additionally, I have been working with Kate on a machine which fires X-rays onto either small samples or cores that have been drilled out of the pyroclasts. This creates 2D pictures and 3D renders of the samples to gain a better understanding of the textures and interactions between the glass and scoria. In the coming months I will be working alongside Katy to investigate the geochemistry of the erupting magma, this will be completed by using an electron microprobe for major element chemistry. Preliminary qualitative chemical analysis work indicates that the glass is a more evolved composition that the co-erupted scoria.

I am very excited to gain more insight into the formation of these extremely weird rocks over the course of my masters and I hope to be back and able to explain further about their origin so make sure to keep an eye out for updates!

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