Iceland’s sizzling secret – an unusual sedimentary rock?

Sedimentary rocks are rare in Iceland's predominantly magmatic landscape. We analyse a sedimentary rock from NW Iceland that exhibits remarkable behaviour: it releases large amounts of gas while immediately decomposing upon contact with water. The sample is brown, fine-grained, and has a low density. At high microscopic magnification, a planar texture is visible. Previous analysis concluded that the rock is x-ray amorphous and does not contain swelling clay minerals that would explain its behaviour.

New XRD analysis reveals that the material consists to 70 wt. % of illite and to 20 % wt. of illite-smecite mixed layer. In addition, it contains goethite, quartz, and, as confirmed by infrared (FTIR) analysis, organic material. These results are complemented and supported by Inductively coupled plasma mass spectrometry and scanning electron microscope analysis.

µCT measurements show no significant pore volume above 1 µm. Most likely, the gas is not only trapped in pore space between the clay particles, but also in the interlayers of the interstratified phases (illite-smectite). One of two independent analyses of the gas released during decomposition shows significantly elevated CO₂ concentrations compared with air and we identified organic molecules that hint to oil and plants.

To understand if the extraordinarily rapid decomposition can be solely attributed to the clay content, we investigate further using a wide range of analytical methods, e.g., porosimetry measurements and surface area determination by gas adsorption. We also aim to gain inside into the geothermal history and organic petrography of the rock by applying a Rock-Eval pyrolysis.