Lava Cave Types

 

The primary lava type found in the Auckland Volcanic Field (AVF) is basalt, a low-viscosity lava that flows readily and cools relatively quickly. The fluid nature of basaltic lava often leads to the formation of long, sinuous lava tubes. These tubes typically exhibit smooth, ropy surfaces known as pahoehoe, formed by the wrinkling and folding of the cooling lava crust. While individual lava tubes are common in the AVF, some caves form complex systems with multiple levels, interconnected passages, and even chambers. These systems are often created by multiple lava flows that occur over time, with each new flow adding to or modifying the existing cave structure.

Classic Lava Tubes

Classic lava tubes, the most prevalent type of lava cave in Auckland, are characterised by their elongated, tunnel-like structures. They are formed when the surface of a lava flow cools and solidifies, creating a hardened crust while the molten lava continues to flow beneath. As the lava flow subsides, it leaves behind a hollow conduit, often extending for considerable distances. They are commonly found in volcanic regions with basaltic lava flows (such as the AVF), where the lava has a low viscosity, allowing it to flow and form these tunnel-like structures.

Lava tubes can vary significantly in size and complexity, ranging from single passages to intricate networks with multiple levels and interconnected chambers. Multilateral lava tubes are characterised by the presence of multiple, parallel conduits, often exhibiting branching and rejoining patterns (e.g. The Gemini Caves – Castor & Pollux). In contrast, multilevel lava tubes are formed when separate flows create vertically stacked tube systems, with one tube positioned directly above or below another, sometimes spanning multiple levels (e.g. Wallaby Cave).

Wiri lava cave is Auckland’s longest known classic lava tube at 290 metres long.

Classic lava tubes found in the Auckland Volcanic Field

 

Surface Tubes

Surface tubes, also known as lava runners, are shallow channels formed by highly fluid lava flowing over a hardened surface. These formations are typically small and inaccessible. They are created when flowing lava rapidly cools and solidifies on its outer edges, forming a hardened crust while the still-molten interior continues to drain away. This process effectively turns the lava flow inside out, leaving behind a hollow, open channel on the surface.

The best examples of surface tubes within the AVF can be found on Rangitoto.

A semi-accessible surface tube on Rangitoto

Lava Shafts

Lava shafts are vertical conduits that connect the surface to underlying lava tubes or lava flows. They are typically cylindrical or funnel-shaped and can vary in depth and diameter. These natural passageways form when magma rises to the surface, creating a vent or fissure, and subsequently solidifies, leaving behind a hollow shaft. The process begins with the eruption of viscous lava, which builds up a volcanic cone or shield. As the lava continues to flow, the exterior crust solidifies, while the interior remains molten, creating a pipe-like structure. As the lava drains, the crust collapses, forming a shaft.

The formation of lava shafts is influenced by factors such as magma viscosity, gas content, and the presence of pre-existing weaknesses in the rock. Once formed, lava shafts can vary significantly in size, ranging from narrow fissures to large-diameter conduits capable of transporting vast quantities of magma during volcanic eruptions.

Lava shafts are not common in the AVF. They typically reach a maximum length of 20 metres.  The Ruapotaka Shaft (Maungarei/Mt Wellington) is one of the best examples of an AVF Shaft. There is also a 20 metre deep shaft on Rangitoto which is possible to climb down inside without the aid of ropes.

 

Pit Caves

Pit caves, also known as pit craters, are vertical cavities formed in solidified lava flows. They can form when a section of a lava tube’s roof collapses, creating a vertical pit or shaft. These caves are characterized by their distinctive pit-like entrance and can vary in size and shape, ranging from small openings to large depressions exposing extensive underground passages.

They are very common on Rangitoto and often contain five meter long, circular caves with very wide openings.

Pit crater on Rangitoto

Tree molds

Tree molds are essentially cavities or impressions left behind in solidified lava flows, preserving the shape of trees that were once engulfed by molten lava. The formation of lava tree molds typically occurs when a lava flow encounters a forested area. As the hot lava engulfs the trees, the intense heat causes the wood to burn away, leaving a hollow space within the cooling lava. Over time, as the lava solidifies and the remaining organic material decomposes, a distinct mold of the original tree trunk is preserved in the hardened lava.

Tree mold from Pupuke volcano lava. Takapuna Beach.