Story: Canterbury region
Page 2 – Geology
The Southern Alps dominate even distant views in Canterbury. Their rocks, which are mainly greywacke, were laid down between 230 and 170 million years ago. The Alpine Fault runs west of the main divide (the summit line of the alps).
Alps are thrust up as the Pacific tectonic plate undercuts the Australian Plate. Ranges uplifted between 140 and 120 million years ago were subsequently worn down. Another period of mountain-building, which formed the present-day mountains, began about 26 million years ago and is continuing. Total uplift has been about 20,000 metres, but erosion has kept the highest peaks north of the Rangitātā River below 3,000 metres.
In the past 2 million years there have been at least five major periods of glaciation. The last ended between 18,000 and 14,000 years ago. Since then, minor glacier advances have left the moraines (ridges of boulders and debris) that are a feature of Canterbury’s mountain landscapes. Since about 1890 the glaciers have been in fluctuating retreat.
The glaciers gouged out wide valleys that have been partly filled with gravels. They also enlarged some depressions into basins between mountains. Lake Coleridge and Lake Sumner are in valleys excavated by glaciers. Smaller lakes lie in the hollows of old moraines.
Banks Peninsula was born when eruptions of basalt began between 15 and 12 million years ago, about 50 kilometres east of the mainland. Over 6 million years these eruptions formed two large, overlapping volcanoes, which later eroded to less than half their original height.
About 1 million years ago alluvial fans, forming east of the rapidly eroding Southern Alps, linked this volcanic mass to the South Island. At times of intense glaciation, the sea was 130 metres below its present level and the coastline 40 kilometres further east. What had been an island was periodically landlocked. After the last peak of glaciation 18,000 years ago, this land link remained. Today’s harbours and bays are valleys that were drowned when the sea level last rose.
Movement on faults in Canterbury has caused earthquakes:
- The north Canterbury earthquake of 1888, centred on Amuri, damaged many buildings and caused the top of the spire on Christchurch Cathedral to collapse.
- A 1901 earthquake centred near Cheviot caused minor damage.
- The Arthur’s Pass earthquake of 1929 caused significant rockslides in the mountains.
- On 4 September 2010 Canterbury was affected by a magnitude 7.1 earthquake, centred about 37 kilometres west of Christchurch, near the town of Darfield. There was no loss of life, but considerable damage to buildings in the city.
- A magnitude 6.3 earthquake occurred on 22 February 2011, with its epicentre near Lyttelton. It caused almost 200 deaths and major damage to Christchurch, with thousands of people made homeless. Around a third of buildings in the central city were expected to be demolished as a result of the quake.
Movement on the Alpine Fault is expected to cause major earthquakes affecting Canterbury in future.
Diamonds are forever?
In the early 1880s, Alford Forest was the scene of a ‘diamond rush’, when J. S. M. Jacobsen announced he had discovered diamonds there. Although geologists said his finds were only small crystals of quartz, people poured into the area. But the geologists were right, and the planned town was never built.
Canterbury does not have many mineral resources. There is some coal, which was sporadically mined mainly at Mt Somers and in the Malvern Hills. Plentiful limestone is quarried for agricultural purposes and in the 19th century was burnt to make mortar. In the 1880s, Mt Somers limestone was exported to Australia.
An outcrop of red-tinted, recrystallised limestone on the Waiau River supplied Hanmer ‘marble’ for many years, while stone from Halswell was used for buildings in Christchurch. Silica sand was mined at Mt Somers for glass making. Clay from Malvern Hills was used in brick and pottery works. On the Port Hills, loess (deposits of a fine, wind-blown sediment) was used to make bricks in Christchurch. The region has abundant gravel and sand for construction and road-making.