Extracts from Cooke-Ravian Volume of Volcanological Papers (editor, R. W. Johnson), Geological Survey Of Papua New Guinea Memoir 10, 115-123 (1981) including a description of the first eruption of Ritter Island Mar. 24, 1700.

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ERUPTIVE HISTORY OF THE VOLCANO AT RITTER ISLAND

by R.J.S. Cooke (Compiled by R. W. Johnson)

Ritter Island was a 780 rn high,, steepsided, conical volcano when first observed by Europeans, in 1700. It had a surnmit crater that was in violent eruption and may have been disharging nuées ardentes. Other eruptions were reported during rare observations in the eighteenth and nineteenth centuries. The cone collapsed on 18 March 1888; there was apparent]y litttle or no preceding or accompanying eruptive actvity, and the collapse caused a 12-15 m tsunami that devastated neighbouring coasts. About 2 square km of rock was engulfed, and only a small remnant of the original island was left. Brief eruptions in 1972 and 1974 are the first known since the 1888 subsidence, and may have accoompanied additional minor episodes of collapse.

I N T R O D U CT I O N

Before European settement in the isands of present day Papua New Guinea during the 1870s and 1880's the best known volcano in the area was arguably Ritter Island 23 km west of New Britain (Fig. l). This uninhabited island (known as Volcano Island until the late 1880s) had been observed from passing ships since l700. Certainly more detailed descriptions of its eruptions had been published than were available for the other active Papua New Guinea volcanoes then recognised. The island was largely destroyed on 13 March 1888 by a catastrophic event that caused destructive tsunamis on neighbouring coasts and resulted in considerable loss of life. No eruptions, nor indeed any other types of activity, were reported after 1888 until minor eruptions in 1972 and 1974. This lack of recent activity,coup]ed with the remoteness of Ritter, has led to the volcano being little known in Papua New Guinea at the present day. The known eruptive history of Ritter Island is compiled in this paper. It is a more comprehensive account than previous ones given by Hammer (1907), Sapper (1917, 1927), and Fisher (1939, 1957).

T O P O G R A P H Y A N D S T R U C T U R E

Before being destroyed in 1888, Ritter Island was reportedly a very regular cone about 400 toises high and 600 toises in diameter - that is about 780 m high and 1170 m across - containing a summit crater

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(Dumont D'Urville,1832). An average angle of more than 50 degrees at the base of the cone is suggested by these figures. This seems exaggeratedly steep, but Dumont D'Urville was an experienced naval surveyor whose judgement should be reliable, and in fact he commented on the island's 'remarkable steepness' (page 538, translated from the French). G.E.G. Schleinitz (quoted in Anonymous, 1888a) also remarked on the excessive height of the island relative to its base. Several illustrations of the pre-1888 island are known (Fig. 2), and all are consistent with the existence of an unusually steep-sided cone. The present-day island (Fig. 3) re- aining after the 1888 even is 1.9 km long, 200-300 m wide, about 140 m high, and represents only a small part of the eastern margin of the original island (note that the length is much greater than the width of the island estimated by Dumont D'Urville). This remnant is arcuate in plan, and has a steep scarp concave to the west, resembling a crater or caldera wall. Its eastern side is part of the original flank of the cone, and enters the water steeply, as in the illustrations in Figure 2. A small islet is present off the southern tip of the main arcuate island. The Russian research vessel Galich carried out a bathymetric survey around the island in December 1974. Results are not yet published, but the existence of a submerged caldera west of the island was concerned (G. Tomolov, personal communication, l976). The caldera has an average diameter of about 2.5 km, and a depth of more than 300 m below sea level. The volume lost in 1888 seems to have been about 2 square km.

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RECORDED ACTIVITY 1700

William Dampier was the European discoverer of Ritter Island, and he observed the first, and possibly the strongest, known eruption (Dampier, 1939) In bright moonlight, on the evening of 24 March 1700, 'At 10 a Clock I saw a great Fire bearing North-West

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by West blazing up in a Pillar, sornetimes very high for 3 or 4 Minutes then falling quite down for an equal Space Of Time; sometimes hardly visible, till it blazed up again ... I got up and view'd it for about half an Hour, and knew it to be a burning-Hill by its Intervals. . . In the Morning I found that the Fire we had seen the Night before, was a burning Island; and steer'd for it' (page 214) . 'March the 25th 1700, in the Evening we came within 3 Leagues of this Burning-hill, being at the same Time 2 Leagues from the Main. The Island all Night vomited Fire and Smoak very amazing]y; and at every Belch we Heard a dreadful Noise like Thunder, and saw a Flame of Fire after it, the most terrifying that evcr I saw. The Intervals between the Belches, were about half a Minute; some more, others less; Neither were these Pulses or Eruptions alike; for some were but faint Convulsions in Comparison of the more vigorous; yet cven the weakest vented a great deal of Fire; but the largest made a roaring Noise, and sent up a large Flame 20 or 30 Yards high; and then might be seen a great Stream of Fire running down to the Foot of the Is]and, even to the Shore. From thc Furrows made by the descending Fire, we could in thc I)ay Time see great Smoaks arise, which probab]y were made by the sulphureous Matter thrown out of the Funnel at the Top, which tumbling down to the Bottom, and there lying in a Heap, burn'd till either consumed or extinguished; and as long as it burn'd and kept its Heat, so long the Smoak ascended from it; which we perceived to increase or decrease, according to the Quanty of Matter discharged from the Funnel. But the next Night, being shot to the Westward of the Burning-Island, and the Funne] of it ]ying on the South-sidc, we could not discern the Fire there, as we did the Smoak in the Day when we were to the Southward of it' ( pages 214-215) Dampier's map, and his sketches of the volcano and adjacent islands (Fig . 2A), allow positive identification of Ritter Island. This first description of activity is still the longcst and most detailed eye-witness account available of Ritter Island.

References cited in extracts above:

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