Thomas Delano sends us news from San Francisco, already on the way to Central America.
Let us return to the third stage of his world tour of active volcanoes: the volcanic archipelago of Vanuatu in the Pacific, where it passed through the island of Tanna, and the Yasur, by the island of Ambrym, with the lakes Lava of Benbow and Marum, finally by the island of Ambae and the acid crater lake Voui.
Yasur, an almost circular pyroclastic stratoconum surmounted by a summit crater 400 meters wide, is contained in the small caldeira Yenkahe. This young volcano is in Strombolian to vulcanian activity since its first observation by Captain Cook in 1774.
The Ambrym volcano island is made up of a large volcano rising to 1,800 meters above the surrounding seabed. Its main cone is truncated with a caldera 12 km in diameter, dated around 2,000 years. The post-caldeira volcanic activity developed at Marum, 1270 m. and the Benbow, 1160 m.
The last major eruptions occurred in 1986 and 1988-89. The lava lake, which is variable in volume but present almost permanently in the Mbuelesu-Marum crater, periodically flushes out in the form of basaltic flows on the caldera floor (which can overflow the caldera during major eruptions as in 1913-1914), while that of the Benbow is drained outside the caldera by the axial fissures of the western part of the island. The magma is also ejected as ashes, lapilli and slag, which partly fall in the caldera, partly on the NW flank of the island. Extra caldera flows often emanate from areas of west and east axial cracks, and strong explosive manifestations frequently occur at the western and eastern ends of the island (as well as at sea, in their extension) through interaction between sea water and magma.
During the last two centuries, Ambrym had at least eight major eruptions with extra caldera flows and sometimes severe destruction occurred as in 1820, 1894, 1913 and 1929. For 50 years, no extra caldera lava flow is produced. In normal times, the impact of the activity of the volcano is limited to the caldera, the flank of the island located at the NW of the cones and the coast between Craig Cove and Ranon. However, when the wind is turning, other flanks and coast may be affected. During a major eruption, ash falls can affect the entire island, except perhaps its northern end.
The island of Aoba (1496 m) corresponds to the emergent part of the voluminous volcanoes of the archipelago (3900 m high from the ocean floor and about 2500 km³ in volume, making it an all-inclusive volcano - exceptional). Two concentric calderas crown it, the innermost one including three lakes, of which the lake Voui (2,1 km in diameter) installed in the present crater of the volcano.
Recent volcanic activity includes the formation, about 4 centuries ago, of cones surrounding the explosive craters of Voui and Manaro Ngoro. It also includes the emission of basalt flows from N'dui N'dui from cracks on the flanks about 300 years ago. Lahars (mudslides carrying blocks and trunks of trees that occur when large deposits of ash, unstable, are picked up by the torrential rains usually accompanying eruptions) probably destroyed villages on the SE side of the river, Island, some 120 years ago, causing several victims. Numerous deposits of lahars, thick and probably not older than 100-300 years, have been observed on all the coasts of the central part of the island. An eruption would have occurred in 1914 with ash emission and descent of lahars (12 deaths). Finally, a small cone of ash formed in 1966 inside the caldera.
Three bubbling zones with enormous bubbles (10 m in diameter) and a general russeting of the shore forest were observed at Lake Voui on 13 July 1991 by a Vanair pilot. It was the first time he had observed such a phenomenon, which could only be recent, as the forest was still intact in May of the same year. On July 24, 1991, aerial reconnaissance showed only three areas of whitish waters in the middle of the lake and a vegetation burned by acid gases to the lip of the crater, 120 m above the water. An abnormally strong release of SO2 between May and July 1991 is probably the cause of the phenomena observed.
- photos & video Thomas Delano
- Vanuatu Geohazards Observatory - link
- Global volcanism Program - Yasur - Ambrym - Ambae / Aoba