Story: Wairarapa region
Page 3 – Land and climate
Wairarapa has three main landscapes: the western mountainous zone, the central lowlands and rivers, and the eastern uplands.
Western mountainous zone
The Rimutaka and Tararua ranges are part of the North Island axial range and stretch from the south Wairarapa coast to the Manawatū Gorge. They are formed from blocks of greywacke and argillite rock. The landscape is geologically young, as shown by 1–2-million-year-old marine and alluvial deposits on the top and western slopes. West of Masterton the mountains are more than 1,500 metres high, falling to 400 metres near the Manawatū Gorge.
Central lowlands and rivers
The lowlands comprise three basins:
- The Pahīatua basin extends north-east from Mt Bruce into southern Hawke’s Bay.
- The Masterton basin reaches from Mt Bruce to Carterton.
- The Wairarapa basin stretches from Carterton to Palliser Bay.
All were created over the last million years out of alluvial gravels carried by rivers from the Tararua Range. The rivers cut valleys and gorges to the basins, where they formed large fans, terraces and flood plains. The main rivers are the north-flowing Mangatainoka and Mangaone and the south-flowing Ruamāhanga, Waingawa, Waiohine and Tauherenīkau. The Mangatainoka and Mangaone rivers flow into the Manawatū River. The others drain into Lake Wairarapa or Lake Ōnoke, on the edge of Palliser Bay.
Wairarapa’s eastern uplands – including the Waewaepa and Puketoi ranges – are made of uplifted sandstone, mudstone and limestone. The landscape has broad valleys and steep hill country, rising to 800 metres. Many slopes are unstable. Landslides are common after heavy rain, a problem made worse by deforestation and overstocking. To the south is the Aorangi Range, distinguished by its greywacke basement rock.
Wairarapa is prone to earthquakes. The coast is about 90 kilometres north-west of the Hikurangi Trough, where the Pacific tectonic plate meets the Australian Plate. As the plates collide, the oceanic crusts concertina, splintering into blocks along the Wairarapa and associated fault lines. Some of these have tilted upward to form the Tararua Range and eastern uplands. Wairarapa has a large number of strike-slip faults – faults that move sideways rather than vertically.
New Zealand’s largest recorded earthquake occurred in 1855 along the Wairarapa fault. It measured a magnitude 8.2 and (unusually) ruptured the surface. Evidence in the landscape shows that the fault moved as much as 6.4 metres vertically and 18 metres horizontally. It was the greatest deformation and rupture of land that geologists had ever known.
When the earth moved
In June 1942, a young Tīnui soldier was on leave from Carterton camp and taking a bath at his grandmother’s house. When the first quake struck the water began sloshing violently. His first impulse was to take cover under a door frame, but he feared exposing himself to his grandmother, so clung on in the bath. Back at the camp, he was woken by his bed shaking, and thought his mates were playing a joke – but it was another quake.
In 1942 a chain of earthquakes hit the Wairarapa. The first jolt was on the morning of 24 June, followed by a 7.2 magnitude tremor that evening. Centred near Masterton, the earthquake triggered landslides, damaged buildings, and toppled chimneys as far away as Wellington. An estimated 4,700 Wairarapa chimneys fell down. In early August the earth moved again, demolishing many rebuilt chimneys. Some fell again in a final shake in mid-December.
Sheltered by the Tararua Range, the Wairarapa has a dry, warm climate. It receives between 800 and 1,200 millimetres of rain each year, with western areas wetter than the east. Annual sunshine hours average more than 1,900 for Masterton.
Summer weather is warm, dry and settled. Typical maximum daytime temperatures range between 20 and 28°C, sometimes rising above 30°C. Winters are cool to mild and frosts are common. Typical maximum winter temperatures range from 10 to 15°C.
Westerly winds prevail, and strong north-west föhn winds can occur in spring and summer. The winds gather strength as they come down the Tararua Range, and can reach 170 kilometres per hour at Castlepoint. Wairarapa also receives heavy rain from the south and east, which can cause flooding.