A large gathering of more than 70 Friends were intrigued by tales about the fascinating geology of Wellington and surrounding areas from geologist and palaeontologist Hamish Campbell, an Emeritus Scientist with GNS Science.

Hamish described how the Hikurangi Trough, which starts in the northeastern South Island and extends up past East Cape, is a boundary plate where the Pacific Plate is diving (or ‘subducting’) east to west beneath the Australian Plate. This subduction zone is drawing down a lighter crust below another of heavier density.

The Pacific Plate is just 20 kilometres beneath Wellington. Several major faults run parallel to each other in the region, including the well-known Wellington Fault. Hamish said little is known about this fault. Based on six past events, the fault moves every 1000-2000 years, and last moved 300-500 years ago. Let’s hope this indicates we won’t be affected by this fault uplifting during our lifetimes!

The shape of Wellington Harbour is often perceived to be a crater. It’s not! It’s a fault angled depression and is a reverse fault—where one block of land moves over the top of another one.

An intriguing study about a new continent; ‘Zealandia: Earth’s hidden continent’ was published by Hamish and Nick Mortimer in 2017. Their research showed how Zealandia, like other continents, is large, is relatively high, has continental crust and contains rocks like granite and greywacke.

Take a look at the GNS maps of how Te Riu-a-Māui / Zealandia formed during its almost 100 million years of history

Geologically, everything in view in the Wellington environment is greywacke. This rock makes up the main ranges of New Zealand formed about 110-120 million years ago when tectonic plate movements uplifted sediments to form new land. Austrian explorer, Ferdinand Hochstetter, introduced the word ‘greywacke’ (which in German basically means ‘grey muck’).

Our greywacke is loaded with zircon crystals, which enables the rate of uplift to be measured with thermal techniques. The greywacke has come up four kilometres in the last eight million years, a massive uplift rate.

It’s fascinating to see active faults around Wellington. If you’re flying from Napier to Wellington and the sun is in the right position, you may see the line of the Wairarapa fault running along the eastern edge of the Remutaka Range. The huge 8.2 earthquake in 1855 produced the biggest lateral movement on a land-based fault in known history.

Walking along the beach at Owhiro Bay Quarry, there are outstanding examples of fault lines—long cracks cutting across layers of greywacke and deep marine sediment bedding layers. There’s a fault bending the rocks every 50 metres.

Hamish’s enlightening revelations about our curious geological heritage has encouraged us to take note of the features of the environment we live in. And wonder more about what lies beneath our feet.

Loralee Hyde
Member