The "HMS Beagle", with on board amateur geologist Charles Darwin, arrived at the volcanic island of St Helena July 8, 1836, where it stayed until afternoon of July 14, afterwards proceeding its journey back to Great Britain.
Since Van Diemens Land Darwin's written notes and observations had become very fragmentary - maybe because of the short stops by the Beagle, maybe due Darwins homesickness after four years on sea . However Darwin dedicated 15 pages to the general geology of St Helena (aside his rants about the laundry service on board) and - what he called - "St Helena Model".
Already on the island of St. Jago a young Darwin had noted geological evidence, suggesting that volcanic island did rise from the sea. Now, after his field experience in South America, Darwin was sure of this idea, as he found again layers formed in the sea exposed along the dry shores of the island:
"The successive sheets are either closely united together, or are separated from each other by beds of scoriaceous rock and of laminate tuff, frequently containing well rounded fragments. The interstices of these beds are filled with gypsum and salt; the gypsum also, sometimes occurring in thin layers. From the large quantity of these two substances, from the presence of rounded pebbles in the tuff, and from the abundant amygdaloids, I cannot doubt that these basal volcanic strata flowed beneath the sea."
At Darwins time the origin of volcanoes was still a geological riddle. One model - proposed by the eminent German geologist Leopold von Buch (1774-1835) - stated that volcanoes form like a bubble on earths crust: first geologic forces push up the ground and form a mountain. The peak of this newborn mountain is however unstable and as the summit collapses - forming a volcanic crater - magma emerges from the underground, causing a volcanic eruption. Lava and ash where therefore secondary features of an active (strato-)volcano and not the primary "construction material" of the entire volcanic mountain. Von Buchs "crater of elevation" hypothesis was very popular at the time and shared by most European geologists. Two French geologists, Armand Dufresnoy (1792-1857) and Lonce Elie de Beaumont (1798-1874), proved even mathematically that slopes of volcanoes were too steep to be formed by solidified lava flows!
Fig.1. Topographic map of Santa Cruz de La Palma (Canary Islands), published by von Buch in 1814 in his book "Description physique des lles Canaries, suivie dne indication des principaux volcans du globe". Von Buch assumed that the radial valleys were fissures caused by the uprising "volcanic bubble" - in fact these valleys are formed by fluvial erosion, but von Buch underestimated in the dry climate of the Canary Islands the power of episodic erosion.
However the "crater of elevation" model proposed very fast rates of elevation and Darwin, based on his previous observations, had already adopted the gradual geology as proposed by geologist Charles Lyell.
Darwin used the observations on St Helena to formulate an intermediate hypothesis (published in 1844 in his book "Geological Observations on the volcanic islands and parts of South America visited during the Voyage of H.M.S. "Beagle") - volcanoes rise by slow, gradual and episodic events, finally erupting as proposed by von Buch. He also suggests that to settle the debate it was necessary to carefully determinate the inclination of lava flows. In 1850 Lyell demonstrated on a lava flow of Mount Etna that the lava solidified in situ on the steep slopes - lava flows were therefore not layers inclined by the rise of the mountain, but repeated deposition of lava flows on the slopes formed the entire volcano:
"...we must abandon the elevation-crater hypothesis: for although one cone of eruption may envelop and bury another cone of eruption, it is impossible for a cone of upheaval to mantle round and overwhelm another cone of upheaval so as to reduce the whole mass to one conical mountain." Lyell in his monographic work ""On the Structure of Lavas which have consolidated on Steep Slopes: with Remarks on the Mode of Origin of Mount Etna and on the Theory of "Craters of Elevation", published in 1858.
In the end a volcanic eruption in the Mediterranean Sea will confirm the model as proposed by Lyell and (with some reluctance) supported by Darwin- in 1831 naturalists, on board of the ship "Etna", experienced the birth of the island of "Ferdinandea", observing that repeated eruptions and deposition of lava and ash were slowly forming a new mountain.
CHANCELLOR, G.R. (1990): Charles Darwin's St Helena Model Notebook. Bull. Br. Mus. Nat. Hist. 18(2): 203-228
HERBERT, S. (2005): Charles Darwin, Geologist. Cornell University Press: 485