Ruapehu, 9,175 ft, the highest mountain in the North Island, is an active volcano lying at the southern end of the Rotorua-Taupo volcanic district in Tongariro National Park. It appears to have had a long active history, and mud-flow debris from Ruapehu was deposited in each of the last three Pleistocene glaciations. Immediately to the north of Ruapehu, forming part of the same coalescing volcanic mass and aligned along a line striking NNE, are the major volcanoes Ngauruhoe and Tongariro. Other volcanic peaks of lesser altitude are numerous. The broad irregular summit area of Ruapehu includes the prominent points of Paretetaitonga, 9,025 ft in the west; Te Heuheu, 9,040 ft in the north; Cathedral Rocks, 8,150 ft in the north-east; Mitre Peak, 8,450 ft in the south-east; Girdlestone Peak, 8,715 ft in the south; and Ruapehu, 9,175 ft in the south-west. Below and between Paretetaitonga and Ruapehu is a crater lake at 8,375 ft occupying the present crater. The high points mentioned are not separate volcanoes but peaks on an irregular crater rim, with East Crater, West Crater, Girdlestone Crater, and Crater Lake as younger and smaller craters contained within the larger. The rocks of which Ruapehu is built are andesites, mainly labradorite and labradorite pyroxenite andesites, with much glass in the ground mass. The andesites are appreciably more basic and of darker colour than those of the Egmont chain of volcanoes.

Although Ruapehu has frequently erupted steam and ash, and Crater Lake is normally warm and sometimes boiling, the only lava eruption during European times occurred in May and June 1945. During this period ash eruptions reached such a degree of violence that ash falls were reported as far away as Wellington, 145 miles distant.

The largest river rising on Ruapehu is the Whangaehu, but it is not of any considerable size, even when it reaches the coast near Turakina to the south of Wanganui. The Whangaehu drains the eastern side of Crater Lake by slow percolation through and beneath an ice dam. From time to time the dam is breached, and the river's importance is considerable in that it has carried several lahars or mud flows from Mount Ruapehu. Lahars have been recorded on 13 February 1861, 1 May 1889, 10 March 1895, in 1903, on 22 January 1925, and 24 December 1953, wrecking a railway bridge immediately prior to the arrival of the Wellington-Auckland Limited express. The train plunged into the river with the loss of 151 lives. This has been called the Tangiwai Disaster. A flood-warning system has now been erected on the river to prevent further occurrences.

Although from about 3,000 ft and upwards Ruapehu is composed predominantly of lavas and ash, the material composing the surrounding countryside is a mass of unsorted andesitic debris ranging from large boulders many feet across to fine mud. The surface of this material is generally smooth except in the north-west, where there are a large number of conical hills. This debris constitutes the ring plain of Ruapehu and was deposited during the Pleistocene glaciations by gigantic lahars from the mountain. The conical mounds were formed by collection of debris around abnormally large blocks when they settled from the lahars.

Today Mount Ruapehu is probably one of New Zealand's most popular mountain resorts, and tourist facilities range from huts to the excellent tourist hotel, the “Chateau”. Ski tows have been installed, and in season many thousands of visitors are often on the mountain at one time.