Volcanoes of Auckland

Auckland Volcanic Field.

Magma beneath Auckland comes from partially melted mantle rock at about 80 kilometres depth. This magma rises to the surface in small batches which usually erupt in a different place each time to form a new volcano. During its ascent to the surface, the chemistry of the primitive magma may evolve by the growth within it of clinopyroxene and olivine minerals that crystallise at higher temperatures and preferentially remove some elements from the remaining molten mix. The background in this diagram is the coloured seismic shear-wave velocity profile kindly provided by geophysicist Nick Horspool.

Mt Victoria

Crater Hill

Mt Eden

Schematic cross-section of the plumbing and eruption style that resulted in the outpouring of lava flows from the base of an Auckland scoria cone.

This tree was partly pushed over and lost its top and branches during early blasts of volcanic ash and gas from nearby Maungataketake Volcano. It was subsequently buried by layers of volcanic ash. In recent decades coastal erosion of the low cliffs of tuff (hardened ash) has uncovered this in situ tree trunk near the steps down to the beach at the end of Renton Rd, Ihumatao.

Inside Wiri Lava Cave with its classic Gothic arch cross-section and surge benches on the walls. The ropey pahoehoe floor is the remnants of lava that did not manage to drain completely out of the lava flow feeder tube. PHOTO: ALASTAIR JAMIESON, 2010.

Cross-section showing lava cave formation: Most lava caves form inside lava flows. The outside of the flow cools and solidifies, forming a crust of basalt around an internal tube filled with flowing hot lava. When the supply of lava stops, the molten liquid drains out leaving an empty tube or cave. ILLUSTRATIONS: GEOFF COX.