Eruption history of Rangitoto volcano

Abstract

Investigation of a 150-m-deep core drilled into Rangitoto volcano, Auckland, reveals a volcanic history extending farther back in time than previously known. The oldest lava flow has been dated by radiocarbon ages on wood and shell from enclosing estuarine sediments at about 6000 years old. Most of Rangitoto is made of a pile of lava flows constructed between 650 and 550 years ago. Traditionally it was thought that Rangitoto volcano had formed in one or two brief episodes about 550 years ago. However, the rocks in the drill core suggests this was a late phase of activity that post-dates most of the volcano’s construction. The prolonged and perhaps episodic activity is consistent with the studies of microscopic ash from Rangitoto preserved in sediments at Lake Pupuke. The composition of magma erupted from Rangitoto has changed with time due to processes of melting in the mantle. However, there no evidence of interaction between the rising magmas and Earth’s crust. This implies the magmas ascend fast and there would be limited warning times in any future eruptions. However, this new study means hazard scenarios for regions traditionally classified as ‘monogenetic’ need to encompass the possibility of prolonged episodes of activity and reawakening of volcanoes, a significant implication where infrastructure is built on such regions.