Aoraki / Mount Cook

Deep under New Zealand, two giant pieces of Earth's crust push into each other.

The push lifts the mountains higher and higher. Ice and rain wear them down. That is how the sharp peaks were made.

New Zealand sits on the boundary between two of Earth's plates, the Indo-Australian Plate and the Pacific Plate. UNESCO explains that they meet along a great crack called the Alpine Fault, one of only three places where a major plate boundary runs across land.

As the two plates collide, they push up the Southern Alps, which rise to nearly 4,000 meters only about 30 kilometers from the sea. At the same time, glaciers and rivers wear the mountains down. During the Ice Ages, giant glaciers carved deep U-shaped valleys, and glaciers are still carving today.

UNESCO describes the Alpine Fault as the contact zone between the Indo-Australian and Pacific plates, one of only three segments of the world's major plate boundaries on land. Collision along it builds the Southern Alps / Kā Tiritiri o te Moana, which rise to nearly 4,000 m within about 30 km of the Tasman Sea. Pleistocene glaciers carved U-shaped valleys, hanging valleys, cirques and fiords across the region, and moraines and outwash gravels record former piedmont and valley glaciers.

The summit itself is changing. The University of Otago's AORAKI2013 project recounts the history of its measured height: 3,764 m from 1881 surveys, the official figure until a massive rock avalanche fell from the east face on 14 December 1991; a new official height of 3,754 m from photogrammetry afterwards; and, as the summit ridge kept eroding, a 2013 survey by photogrammetry and GNSS that found 3,724 m, 30 m below the 1991 figure and 40 m below the height before the avalanche.

Big rivers of ice called glaciers fill the valleys.

The biggest one is the Tasman Glacier. It is very long and very thick. Rocks cover its lower end.

Five big glacier valleys shape the park: the Godley, Murchison, Tasman, Hooker and Mueller. The Tasman Glacier is New Zealand's largest and longest glacier, the Department of Conservation says. It is 23 kilometers long and up to 3 kilometers wide.

Near its upper part, the ice is about 600 meters thick. Lower down, the glacier is covered in rocks and gravel, so it can look more like a stony valley than ice. You can see it from the main highway into the park.

DOC names the Godley, Murchison, Tasman, Hooker and Mueller as the five major glacial valley systems that shaped the park. The Tasman Glacier, New Zealand's largest and longest, is 23 km long, up to 3 km wide and covers 101 km²; though debris-covered in its lower reaches, its ice is about 600 m deep near the Hochstetter Icefall.

The Otago surveyors' AORAKI2013 work grew out of a study measuring the volume of ice lost by the Tasman Glacier between 1986 and 2008, which built a 3D model of the area from aerial photographs. UNESCO highlights the wider region's glaciers, including the Franz Josef and Fox glaciers, which descend into temperate rainforest.

Climbers tried for many years to reach the top.

In 1894, three New Zealanders finally did it. Today, scientists still climb up to measure how tall the mountain is.

The first attempt to climb the mountain came in 1882, when an Irish climber, W. S. Green, and two Swiss guides got within 20 meters of the top. The summit was first reached in 1894 by three New Zealanders: Tom Fyfe, Jack Clarke and George Graham. Today the park is known as one of the finest mountaineering areas in the world.

In 2013 surveyors climbed high on the mountain carrying satellite receivers to measure its height. As agreed with Ngāi Tahu, they did not step onto the very summit, but measured from the top of the ice cap close by.

DOC's history of the park records the first attempt in 1882 by the Rev. W. S. Green with two Swiss guides, who came within 20 m of the top, and the first ascent in 1894 by Tom Fyfe, Jack Clarke and George Graham. The park is now regarded as one of the finest mountaineering areas in the world, with high-level huts equipped with radios.

DOC also records the Hermitage hotel's local hydroelectric scheme of 1925, built by hand on steep terrain, until the national grid arrived in 1961. On 23 November 2013, two teams of two climbers carrying GNSS receivers measured from the top of the ice cap, about 45 m north of the true summit and 5 m below it; as agreed with Ngāi Tahu and the three Papatipu Rūnanga with shared interests in Aoraki, they did not step on the summit.

Families can walk in the valleys and look up at the mountain.

The climb to the top is only for expert mountaineers. Stay on the track and take all your trash home.

Most visitors enjoy the park from its valley walks, such as the Hooker Valley Track, which can be very busy in summer. The climb to the summit is serious mountaineering, over glaciers and steep rock and ice.

The Department of Conservation warns that avalanches can happen in the park in any season, and that most of the park is complex avalanche terrain. Check the New Zealand Avalanche Advisory and the weather before setting out. Rubbish bins are limited, so take your trash with you, and flying recreational drones is not allowed anywhere in the park.

DOC warns that avalanches can occur in any season and that most of the park is complex avalanche terrain, with very little simple terrain. It recommends checking the New Zealand Avalanche Advisory, which gives daily assessments by elevation and aspect, together with the park's weather forecast; the Avalanche Terrain Exposure Scale rates the terrain itself. Winter access (June-September) may be limited by snow on the road.

Visitor pressure is managed too: parking at White Horse Hill can be limited from November to March, and the Hooker Valley Track can be crowded. Recreational drones are banned, and visitors must carry out their rubbish, since waste in alpine areas may never break down and is unacceptable to Ngāi Tahu; where there is no toilet, a pack-it-out method is required.