Geology : Glowworm Caves, Waitomo, New Zealand

Landform Evolution

Layered limestone outcrops showing 5° tilt to SE.

So how do these caves form?

The key ingredients for caves here are limestone, water and time.

Limestone is a sedimentary rock made from calcareous remains of marine organisms. 30 million years ago, much of NZ was under water - a shallow sea. The rock we see today is composed of remains of tiny shell fragments (mainly bryozoans), and a few spectacular deposits of giant oyster shells, giant scallop fossils occur sporadically also. Layers built upon layers and compacted under additional layers of mud and sand. Eventually tectonic forces lifted all this material above sea level to form the elevated landscape we know today.

Much of NZ has a maritime climate, Waitomo on the Western side of the North Island receives 1800-2400mm rainfall per year. But water alone will not dissolve limestone, it needs to be acidified with the help of dissolved Carbon Dioxide gas derived from biological activity in the soil. Water cannot percolate through the dense, pure Waitomo limestones. Instead, water will flow along the matrix of fractures occurring within the limestone. The slightly acidic water slowly hollows out cavities along the fractures, some develop into caves. Only large passages, partly or completely abandoned by the water are suitable for humans to visit.

Limestone gorge upstream of both caves running SE and typical fracture lines running NE. Photo Prof. John Hopkins, Calgary University.

Spellbound's Mangawhitakau/ Oparure block of sedimentary rocks, 6-9 Km South of the Waitomo Caves Village, which overlay Waitomo Limestones are uniformly tilted at a 5 degree dip towards the South East. As a result acient streayways down the dip, over the surface mudstone rocks and has cut valleys running in a SE direction. The Mangawhitikau Stream eventually cut down to the limestone layers below, then, found its way into one of the regular parallel fractures running to the Northeast and formed the glowworm cave. The result - Spellbound's main river gorge runs SE and the cave systems run NE.

Mangawhitikau stream has been diverted several times, each time the river finds a new path along a fracture in the limestone it abandons its old path. Visitors can clearly see an abandoned dry gorge and abandoned dry cave passages (the Cave of the Spirit) during the tour.

Vistors to the Cave of the Spirit will notice high silt banks at the end of the cave walkway. These indicate that in times of extreme flooding the Mangawhitikau Stream has flowed along its former stream bed. The flood would deposit fresh sediments and debris inside the Cave of the Spirit, and it would also push fossil moa remains further into the cave then bury them.

Glossary;

• Karst - A limestone landscape, originally studied by German Scientists in Croatia
• Moa - Extinct family of Ostrich like birds that inhabited NZ
• Bryozoa - Tiny Brachiopod type shell.

Maori Words;

• Mangawhitikau - don't swim across this stream Manga = stream whiti = swim across, kau = don't
• Oparure - village 6km S/E of Waitomo Village
• Mairoa Ash - An airfall deposit from Taranaki Volcanics- upto 100,000 bp
• Tomo - To enter, also used as a noun for vertical cave entrance (pot hole or chimney)