Following my last post in the Curator of Petrology blog The island that disappeared, we take a closer look at the type of volcanic eruption that created the ephemeral island, the rocks produced by this type of eruption, and meet Empedocles – the submarine volcano that gave birth to Graham Island.
- Samples from the 1831 eruption present in The NHM collection
Type of eruption
The submarine volcano that gave birth to Graham Island, produced an eruption known today as Surtseyan eruption, named after the island of Surtsey off the southern coast of Iceland. This is an explosive type of eruption caused by the violent water-magma interaction.
Volcanologist Carlo Gemmellaro, Professor of Natural History at Catania University, who studied mount Etna, visited the area between the 11th and 14th August 1831. He sketched the volcanic eruptions with incredible accuracy, and the drawings can now be compared to modern submarine volcanic eruptions and show striking similarities.
The picture above (left) of a Surtseyan eruption (1963-1967) , taken from Iceland in Pictures blog, shows characteristic “rooster tail” ejections of volcanic ash and other volcanic products. This can also be clearly seen in Gemellaro’s drawings (right) reported in “Relazione sui fenomeni del nuovo vulcano” (Catania, 1831, Atti Ac. Gioenia Sc. Nat., VIII).
Type of rocks produced
The samples we have in our collection are very precious as they cannot be collected again. They consist mainly of effusive igneous rocks such as Basalt, Obsidian (volcanic glass), volcanic sand, and other products of alteration by heated water – here are some examples:
Obsidian collected from Graham Island (1831)
This natural volcanic glass is the result of rapid cooling of the volcanic lava. Obsidian has been used since the Stone Age to produce sharp blades or arrowheads, and is still used today by some surgeons for scalpel blades.
Rounded vesicles are visible on the surface of this particular specimen. These were formed by gas escaping from the molten magma.
Basalt is a dark-coloured, fine-grained, igneous rock. It usually forms when magma reaches the Earth’s surface and cools quickly, commonly as a lava flow. Vesicles can often be observed, which form due the gas escaping from the magma as it cools down. Due to the rapid cooling no big crystals can be seen with the naked eye. However, under the optical microscope, tiny crystals can be seen such as: Plagioclases, Pyroxenes and Olivines.
Empedocles – the submarine volcano that gave birth to Graham Island
Thanks to geological studies carried out in the area, by INGV (Italy’s National Institute of Geophysics and Volcanology) it is now known that Graham Island or Isola Ferdinandea was part of the underwater volcano Empedocles, 30 km (19 mi) south of Sicily.
The volcanic structure is around 400 meters high, with a base 30 km long and 25 km wide, characterised by three volcanic centres on top of the main structure. It has been described as resembling a cake with three candles on it, and one of the candles (a volcanic centre) was the one that broke the surface in 1831 to give birth to the island that disappeared.
The above image generated by INGV shows the context in which the underwater volcano Empedocles sits off the coast of Sicily.
The volcano rises from the continental shelf, which is the underwater landmass which extends from the continent. This is characterised by the presence of two banks (underwater hills) Nerita and Terribile.
The white arrow in the image above points at the volcanic centre on Empedocles responsible for the 1831 eruption.
Other bloggers have also picked up on the story of Empedocles under the title “The Baby Volcano That Started A 187-Year-Long Territorial Dispute”.
The island lives on as we have historical specimens in our collections and is a nice example of unique historical material that could never be collected again being made available via the museum.