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The caldera formed during the 1883 eruption has been subjected to extensive bathymetric and gravimetric analysis (Deplus et al., (1995). Journal Volc. Geotherm. Res. Vol.64, p.23-52). A large flat-bottomed 240m deep depression is found to the SW of Anak Krakatau. Earlier measurements by Escher in 1919 had recorded a depth of up to 280m and noted a small 20m high N-S oriented ridge in the centre of the depression. It can be assumed that remobilized materials and eruptates from Anak Krakatau, which is perched on the NE flank of the depression in an area previously recorded as having a depth of over 180m, account for the gradual filling of the depression. The caldera has a diammeter of approx. 4km, yet the depression extends in the form of a narrow graben about 2km to the E and 1km to the SW. This may mark the location of fault lines underlying the complex. Looking at gravimetric data which can distinguish materials of different density (the low density pyroclastic eruptates have little gravitational significance), it was calculated that a funnel shaped hole having a depth of over 1km is located in the bedrock at the center of the depression. The hole has been largely filled by relatively light recent pyroclastic material and older fragments of the pre-1883 edifice.
The caldera formed during the 1883 eruption has been subjected to extensive bathymetric and gravimetric analysis (Deplus et al., (1995). Journal Volc. Geotherm. Res. Vol.64, p.23-52). A large flat-bottomed 240m deep depression is found to the SW of Anak Krakatau. Earlier measurements by Escher in 1919 had recorded a depth of up to 280m and noted a small 20m high N-S oriented ridge in the centre of the depression. It can be assumed that remobilized materials and eruptates from Anak Krakatau, which is perched on the NE flank of the depression in an area previously recorded as having a depth of over 180m, account for the gradual filling of the depression. The caldera has a diammeter of approx. 4km, yet the depression extends in the form of a narrow graben about 2km to the E and 1km to the SW. This may mark the location of fault lines underlying the complex. Looking at gravimetric data which can distinguish materials of different density (the low density pyroclastic eruptates have little gravitational significance), it was calculated that a funnel shaped hole having a depth of over 1km is located in the bedrock at the center of the depression. The hole has been largely filled by relatively light recent pyroclastic material and older fragments of the pre-1883 edifice.
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