Lava Cave Features

 

Lava caves in the Auckland Volcanic Field (AVF) exhibit a fascinating array of geological features. They often possess smooth and undulating walls, shaped by the flow of molten lava during their formation. The interiors can include intricate patterns and textures, such as lava shelves, terraces, and benches, formed as the lava gradually receded or solidified at different rates. Lava stalactites and stalagmites may also form from dripping lava or mineral deposits left behind by water seepage. Floors can vary from rough and rocky to smooth and glassy, depending on the lava’s composition and flow dynamics. Lava tubes, a common type of lava cave, may feature skylights or collapse pits where the roof has caved in, allowing natural light to filter into the underground passages. Additionally, secondary minerals like gypsum, calcite, and various silicates may adorn the cave walls.

Lava stalactites

Lava stalactites, also known as “lavatites” or “lavacicles” are formed through the rapid cooling and solidification of dripping or flowing lava. As molten lava drips from the ceiling of a lava tube or cave, it cools and solidifies in mid-air, gradually forming an elongated, icicle-like structure hanging downwards. These formations (in the AVF) can range in size from a couple of centimeters all the way up to 15 centimetres long. Lava stalactites exhibit various shapes, including the common cylindrical or tubular form known as lava straws, and more complex shapes such as shark tooth stalactites. The color of lava stalactites can also vary, depending on the mineral composition of the lava and the rate of cooling, ranging from shades of black, brown, and red to even translucent forms. Lava stalactites are typically fragile and delicate, making them susceptible to breakage.

Lava stalactites and lava straws in Lavatite and Mira Caves

 

Lava stalagmites

Lava stalagmites, also known as “lavamites” are formed by the accumulation of molten lava dripping from the ceiling and solidifying upon impact with the floor. As the dripping lava cools and hardens, it gradually builds up a cone-shaped or mound-like structure, rising upwards from the ground. These formations can vary in size, ranging from small mounds to towering columns several meters high. The morphology of lava stalagmites can be diverse, with some exhibiting smooth surfaces while others display rough, jagged textures. The color of lava stalagmites, like lava stalactites, depends on the mineral composition of the lava and the cooling rate, ranging from dark hues to vibrant reds and oranges.

Lava stalagmites in Kitenui Cave

 

Skylights

Skylights in lava caves are natural openings formed by the collapse of sections of the cave’s roof. These openings can range in size from small holes to large gaps, allowing sunlight and fresh air to penetrate the otherwise dark subterranean environment. Skylights can occur in various types of lava caves, including lava tubes, pit craters, and lava molds. Their formation is often attributed to structural instability within the solidified lava, seismic activity, or erosion processes. Skylights play a crucial role in the ecology of lava caves, providing light and ventilation that support unique ecosystems, including specialized plant and animal life adapted to these dimly lit environments. They also serve as important access points for exploration.

 

Lava shelves and benches

Lava shelves and benches, are horizontal protrusions that form along the walls of lava tubes and caves. They are created when flowing lava partially solidifies against the walls of the cave, leaving behind a ledge-like feature as the remaining molten lava continues to flow. Lava shelves and benches can vary in size, ranging from small protrusions to expansive platforms extending along the cave walls. Their formation is often influenced by fluctuations in the lava level, resulting in multiple shelves at different heights within a single cave.

Lava Bench in Kermies Cave

Lava Balls

Lava balls are spherical or roughly rounded masses that range in size from a few centimeters to several meters in diameter. Lava balls are typically formed on the surface of ‘a’a lava flows, which are characterised by their rough, fragmented surfaces. The formation process involves the accumulation of molten lava around a core of already solidified lava. As the lava ball rolls along the surface of the flow, it gathers more molten lava, gradually increasing in size. The rolling motion often imparts a distinctive spiral pattern to the internal structure of the lava ball. Lava balls typically continue their rolling motion until they encounter an obstruction, such as a narrowing in the lava flow path, or become adhered to the ceiling of a lava tube during a sudden surge in the lava’s flow rate.

Lava Ball wedged halfway up in Kermies Cave

Tube-in-tube

Lava tube-on-tube caves are occur when two or more lava tubes intersect or overlap vertically. This phenomenon typically arises in areas with multiple phases of volcanic activity, where new lava flows inundate existing lava tubes or flow over pre-existing terrain containing older lava tubes. The resulting configurations can be complex, with younger tubes forming above, below, or even within the passages of older tubes. An excellent tube-in-tube example exists in Alcyone Cave where the entrance is divided into an upper and lower tier.

Lava Falls

Lava falls within caves are solidified remnants of once-molten lava that flowed over steep inclines or ledges within the cave system. These formations are created when lava, during an active eruption, cascades down a slope or cliff within the cave. As the lava cools and solidifies, it preserves the dynamic movement and flow patterns, creating a visually striking feature. Lava falls can vary in size and shape, ranging from small, delicate flows to massive, multi-tiered formations resembling frozen waterfalls. The color of the lava fall can also differ, depending on the mineral composition and cooling rate of the lava.

Lava fall cascading down Northern & Mira Caves

 

Lava Bridges

Lava bridges occur when a section of the roof of a lava tube collapses, leaving behind intact portions that bridge the gap between the collapsed areas. These bridges can vary in size, ranging from small, delicate structures to massive formations spanning several meters. Lava bridges often create dramatic and visually striking features within a cave. Lave bridges exist at the entrances of Kermies and Capella Caves.

Lava bridge at the entrance of Kermies Cave

Lava Rolls

Lava rolls (also known as scrolls) can occur where a thin layer of still-malleable lava adheres to the established cave walls. As the supporting liquid lava beneath this layer flows away or cools, the unsupported “skin” of lava can sag and deform under its own weight, forming the characteristic rolled or folded appearance. Lava rolls often resemble a swiss roll. Auckland’s best example of a lava roll can be found in Pollux cave and is almost 13 metres long.

Pollux Cave’s near 13-metre long lava roll.

 

Wind stripes

Wind stripes appear as smooth, polished grooves or striations, often running parallel to the prevailing wind direction. These formations are created by the erosive action of wind and wind-blown particles, such as sand or ash, when the rock is still molten.

Flow Ledges & Kerbing

Lava cave flow ledges, also known as flow lines or benches, are horizontal shelves or terraces that form along the walls of lava tubes and caves. These formations are created during the active flow of lava within the cave. As the molten lava level fluctuates, it can leave behind remnants of solidified lava adhering to the walls at different heights. These remnants gradually build up over time, forming ledges or benches that mark the previous levels of lava flow. Lava cave flow ledges can vary in size, ranging from small protrusions to expansive platforms extending along the cave walls.

Flow ledges within Saddleback Cave

Wall Linings

Wall linings in lava caves are thin layers of solidified lava that adhere to the interior surfaces of the cave. These linings form as subsequent lava flows pass through pre-existing lava tubes or channels. The molten lava comes into contact with the cooler cave walls, causing a portion of it to solidify and adhere to the existing surface. Over time, multiple layers of lava can accumulate, creating a layered or laminated appearance on the cave walls.

The Mira Cave contains exceptional wall linings of global significance, representing the maximum extent of a secondary lava flow. These linings, measuring between 3-5cm thick, are relatively smooth and uniform in shape, yet highly delicate. Some sections have unfortunately collapsed due to their fragility.

 

Cave Floors

Pahoehoe– These floors are characterised by their smooth, ropy, or billowy surfaces, resembling solidified ripples or waves of molten lava. Pahoehoe lava, known for its low viscosity and fluid nature, cools and solidifies slowly, allowing intricate patterns to develop as the lava flow gradually comes to rest. These patterns can include ropey textures, lava toes, and tumuli (inflated lava rises).

‘A’a – These floors are characterised by their rough and jagged surfaces. They form from the cooling and solidification of ‘a’a lava flows, which are known for their high viscosity and tendency to break apart into angular fragments as they move. The resulting surface is often covered in loose, sharp-edged clinkers, making traversing these floors challenging. Within caves, ‘a’a lava fragments are usually fused to the cave floor.

Block – These floors are characterised by their jumbled and chaotic appearance, composed of large, angular blocks of solidified lava. These blocks are typically formed during the final stages of a lava flow when the remaining molten lava cools and contracts, fracturing into irregularly shaped fragments.

Clinker – These floors are characterised by their loose, fragmented, and irregular surfaces composed of broken lava fragments known as clinkers. These clinkers are typically formed during the cooling and solidification of ‘a’a lava flows, which are known for their high viscosity and tendency to break apart as they move.

Scoria Flows – These floors are composed of scoria, a highly vesicular volcanic rock formed by the rapid cooling and gas release of lava. Within the confines of a lava cave, scoria flows can exhibit a range of textures and morphologies. In some cases, the scoria may appear as a jumbled mass of loose fragments, while in others, it may have coalesced into a more solidified and coherent flow.

Mineral Deposits & Films

Lava caves, while primarily composed of solidified lava, can also host a variety of secondary mineral deposits and films. These deposits often form after the initial cave formation, as mineral-rich water seeps through the porous lava rock. The most common minerals found in lava caves include gypsum, calcite, and various types of opal. Gypsum often appears as white or translucent crystals, sometimes forming intricate patterns or delicate formations. Calcite can create a range of speleothems, including stalactites, stalagmites, and flowstone, adding to the cave’s aesthetic appeal. Opal, a hydrated form of silica, can be found in a variety of colors and forms, from milky white to vibrant reds and oranges.

Biofilms are also found within the AVF forming thin layers or coatings on the surfaces of rocks, walls, and ceilings. These biofilms consist of complex communities of bacteria and typically derive energy from inorganic sources like iron, sulfur, or manganese, a process known as chemolithotrophy.

Biofilm and crystals found within the Mira Cave

 

More information on mineral deposits found within the Auckland Volcanic Field

Trees & Roots

Tree roots are a testament to the resilience and adaptability of plant life. As trees grow on the surface above lava tubes and caves, their roots seek moisture and nutrients. In their search, these roots may penetrate the porous lava rock, extending downwards into the cave environment. These roots often appear as thin, delicate tendrils hanging from the ceiling or growing along the walls and floor. While seemingly intrusive, these roots play an important ecological role within lava caves. They provide a source of organic matter and nutrients for cave-dwelling organisms. Additionally, they can contribute to the structural integrity of the cave by anchoring loose rock and soil. Conversely, they can also contribute to structural instability with many examples within the AVF of tree roots cracking and dislodging cave ceiling rock.

In some unique situations, trees manage to grow within lava caves. Such growth is facilitated by the porous nature of lava rock, allowing tree roots to penetrate the cave floor and extend downwards into the subterranean cavities.

Rangitoto Island’s lava caves provide a unique environment for the growth of Griselinia and Pohutukawa tree roots. As these native trees thrive on the island’s volcanic landscape, their roots often penetrate the porous lava rock, extending downwards into the cave systems. The Griselinia, with its robust root system, can create intricate networks within the caves, seeking out moisture and nutrients. Pohutukawa roots, known for their tenacity, are also capable of venturing into the depths of the lava tubes, anchoring the trees and adding to the biodiversity of the subterranean environment.

A Pohutukawa tree growing inside Pollux Cave

 

Cave Fungi

Specifically adapted fungi is found growing in Auckland caves in complete darkness. Unlike surface fungi that rely on sunlight for photosynthesis, cave fungi derive energy from various sources, including the breakdown of organic matter. It is likely they have associations with plant roots, aiding in nutrient uptake.