INTERWEAVING HISTORIES: HOW VOLCANIC VALLEY FORMATION IMPACTS MODERN RESTORATION

Volcanic processes are commonly seen as constructional, leaving cones, lavas, summits, and ridges. As these landscapes erode, their formational histories influence the resulting topography, erosional processes, and groundwater pathways. Banks Peninsula on the East Coast of New Zealand is a heavily eroded Miocene volcanic landscape. Radially incised valleys lie around the eroded flanks of the volcanic complexes that formed the landscape; yet, little in-depth research has been conducted on the specific erosional development of these inner harbor valley systems. This study investigates varied erosional processes and localized springs in Rāpaki Valley, a geomorphic outlier amongst other valleys of Whakaraupō Lyttelton Harbour due to its unusually straight orientation.

The mapping of lava sequences, as well as the correlation and projection of eroded dip surfaces, provide evidence that the current valley orientations are parallel to the strike of the exposed lava flows. Valley orientation segments change with strike changes within lava flow sequences, reflective of the original eruptive packages of the volcanic complex. This underlying control of lava flow dip is further reflected in the location of meteoric, lava-hosted perched springs. Mapping of associated lava flow sequences enabled the identification of recharge zones. Subsurface flow is also influenced by sub-vertical volcanic dykes, which may interrupt gravitational flow, thereby redirecting groundwater, and/or becoming storage zones.

GIS-aided geomorphic mapping identified a close relationship between erosional processes and slope angle. Utilizing these relationships, restoration recommendations can be made to reduce the impact of erosion, sediment loss, and rockfall runout. Further restoration planting is recommended within the recharge zones of springs to promote the interception and infiltration of rainwater into the groundwater system. This study highlights the value of understanding the history of landscape evolution, valley formation and slope processes and how this knowledge can inform restoration efforts.