Abstract

On June 8, 1959, the Indonesian Airforce (A.U.R.I.) kindly supplied the Institute Technology Bandung a Dakota aircraft piloted by Captain Soekardi. The purpose of the flight was to acquaint the Geology students with several of the volcanoes in West Java. Reconnaissance was made of Papandajan, Gede and Krakatau and the many inactive volcanoes along the same route.

We were surprised to find Anak Krakatau in a phase of moderate eruption, for there had been no reports of activity since September 20-23, 1953, when "violent eruptive activity took place" (Neve, G.A. de, 1956).

The aircraft arrived at Krakatau at 10:20 a.m. and made several circles of the island group during the next thirty minutes. Four cycles of Vulcanian type eruption took place during this interval, each nearly identical in nature. All the activity came from a small cinder cone approximately 200 meters in diameter and 25 meters high which is located in the center of the older and larger (700 meters diameter) crater of Anak Krakatau.

Phase 1: Black ash and cinder jets 30 to 100 meters into the air, sometimes directed as much as 45° degrees from the vertical. Duration: 1 minute.

Phase 2: Ash and gas in turbulent, gray, cauliflower-like clouds rapidly rising to 500 meters. Duration: 1.5 minutes.

Phase 3: Ejecta ceases while ash and gas cloud slowly rises and dissipates at 1,000 to 1,500 meters. Duration: 2 minutes.

Phase 4: Clear & quiet. Duration: 2 to 3 minutes.

This cycle was then repeated by the sudden renewal of the black ash and cinder jets.

Anak Krakatau first appeared in 1927 from the floor of the submerged caldera formed during the catastrophic eruption of 1883. Its history of activity was well documented until 1941 (Newmann van Padang, 1951). The starting date of this latest activity is not known. The small cinder cone in the center of the crater has not been seen before and is inferred to be a construction of this latest eruption. However, the vegetation on the east tip of the island, although only a few meters high, seems to be flourishing despite the new eruption. This suggests that the new ash and cinder deposits have been largely localized within the older, larger crater, and that the activity reported above represents a near maximum of the renewed activity up to June 8, 1959. It is interesting to note that the vegetation was entirely destroyed on Anak Krakatau by eruptions on October 10-11, 1952, when an ash-layer of nearly 3 meters was deposited on the island (Neve, G. A. de, 1956).

The new cinder cone in the center of the older crater has partially displaced the large crater lake formed in 1952-1953. Two remnants of this lake occur between the new cinder cone and the south rim of the larger crater. The south-southeast lake is crescent shaped and about 400 meters long and 200 meters wide. The other lake on the southwest side of the crater is much smaller. Both lakes are red-brown in color and their surfaces are above sea level.

The asymmetry of Anak Krakatau Island is probably due to several factors. First, the waves from the Indian Ocean enter the 1883 Krakatau caldera from the southwest, aligned with the Sunda Straits and not hindered by the remnant islands Rakata, Rakata Ketjil (Lang), and Sertung (Verlaten). Erosion from these waves is therefore concentrated on the southwest side of the loose ejecta forming Anak Krakatau. For most of its history the crater of Anak Krakatau has been a breached crescentic bay. Tilting has probably also contributed to the southwest asymmetry. In William's map (1941, p. 254, after Escher, Stehn, and Berbeek), the maximum subsea depths of the caldera formed in the great eruption of 1883 occur 2 to 4 kilometers south and southwest of Anak Krakatau. These depths have actually increased 60 to 100 meters during the 40 year period after the caldera formation, and Newmann Van Padang (1933) ascribes this to vertical sagging. Vertical sagging southwest of Anak Krakatau would cause rotational tilting of the island in the direction. Other factors which contribute to the shape of Anak Krakatau include the direction of the seasonal winds which distribute the intermittent eruption products, the refraction of waves by the group of islands surrounding the Krakatau caldera, gravitational phenomena such as cinder avalanches, and of course the erosion gullies formed in the ash and cinders by rainfall and runoff.

The renewed activity of Anak Krakatau again points to the importance of establishing a permanent observation post on one of the islands surrounding the Krakatau caldera. Krakatau has a history of at least 2 major episodes of caldera collapse, the second, in 1883, caused tidal waves killing over 36,000 persons. The first collapse occurred in prehistoric times, but may have been even more destructive in its physical violence on fortunately uninhabited coasts. The present volume of material forming Anak Krakatau is a very small fraction of the pre-1883 bulk of Krakatau, but its occasional violent eruptions suggest the energy source which has constructed and destroyed Krakatau twice already is not exhausted. Little or no catastrophic danger exists at Krakatau at this time or even in the near foreseeable future, but the scientific value of closely documenting and analysing the symptoms of such an important patient as Krakatau should more than justify the small costs of an observation post.

If continuous observation is not possible, then scientific expeditions to the island should be conducted at regular intervals. Such an expedition is now long overdue*, especially in view of the renewed activity of Anak Krakatau.