Hamish is a Geologist and Emeritus Scientist at GNS Science with a continuing involvement with Geology and an interest in History. Born in Christchurch and brought up and educated in Dunedin with a scholarship to Cambridge he has had a distinguished career with GNS which included being seconded to Te Papa where he wrote the concept for the Awesome Forces exhibition (1998-2019), one of the highlights of the museum. Hamish now spends time at GNS and leads educational trips.
Hamish explained the wider scene: how oceanic crust on the Pacific plate is slowly being subducted from E to W under continental crust of the Australian plate at the rate of 5mm a year. The plate boundary is entirely east of the North Island but swings to the west and cuts through the South Island along the Hope and Alpine faults from Kaikoura to Inchbonnie to Milford Sound. In the deep south from Milford Sound southwards, direction of subduction switches so that oceanic crust on the Australian plate is being subducted from W to E under continental crust on the Pacific plate. Between Milford Sound and Amberley (roughly speaking), there is no subduction. Instead, there is collision between continental crust on the Pacific plate and continental crust on the Australian plate, giving rise to the Southern Alps. Subduction is a process that involves the sinking of denser oceanic crust (dominated by basalt) beneath less-dense, more buoyant continental crust (dominated by granite).
Plate collision has resulted in Wellington Harbour: it has formed as a fault angle depression within a dynamic plate boundary setting on the eastern edge of the Australian plate.
Wellington Harbour is shallow and flat-bottomed, and its floor is dominated by silt, rather than sand or mud. Fortunately, it is self-flushing and does not need to be dredged.
When the Wellington Fault moves the E side (harbour) moves sideways to the SW by 3-4m and the W side (western hills) moves vertically 1-2 m. It last moved around AD1500 and it moves about every 2,000 years….but this is based on less than 10 known fault movement ‘events’.
The Wairarapa Fault last moved on Jan 23 1855 and was greater than 8.2 on the Richter scale. It moved 13 m sideways and 6.5m vertically. The shaking lasted one minute and caused considerable damage but because the settlers, forewarned by the 1848 lesser quake, had built their houses out of wood there were no deaths, except for "Baron" Alzdorf who had insisted on building his hotel out of stone and paid the price for his insistence. A geological history of plate movements can be seen at Cape Turakirae where a remarkable flight of four raised storm-beach beach formations can be seen each seen at 6.5m vertical intervals. More than 20 fault movement events have been recognised on the Wairarapa Fault indicating a repeat period of 2000 years.
Hamish also paid tribute to Gladys West a black American who, with three others, developed the GPS system which allows such precise measurement. ‘Lithospheric sag’ whereby the crust is locally drawn down as a function of subduction, explains the formation of Cook Strait, the drowning of the Marlborough Sounds and the Wanganui Bight
20,000 years ago, during the last ice age, sea level was 125m below present level and there was no Cook Strait: it was a gulf and there was continuous land between what is now North Island and South Island. Average surface temperature was just 5 degrees below present. So, a 1 degree change in temperature results in a 25m change in sea level.
Other interesting items Hamish mentioned included:
New Zealand was given its name by the Dutch cartographer Joan Blaeu (not Abel Tasman) in 1648.
*Zealandia is used as the name of the hidden continental crust from which New Zealand protrudes.
*The sea level reached its present height about six thousand years ago. Before that Wellington Harbour did not exist as such.
*Uplift of the Remutakas is surprisingly young: they were below sea level less than 2 million years ago.
*The "extra water" causing the sea level to rise will come from the melting of the ice sheets of Greenland and Antarctica and changes in the shape of the sea floor.
Hamish also leads study trips to various places. His next trip to the Chatham Islands currently has two vacancies. If you are interested in the trips, or would like further information about Wellington geology his email is h.campbell@gns.cri.nz