and lahars or mudflows that occur when either the crater lake water is ejected from Ruapehu during an eruption, or snow melt is released by failure of the crater walls such as failure of an ice dam. separate from an eruptive event. In the case of lahars, they run down the mountain, picking up ash, lapilli, blocks and bombs as well as previously eroded rock, forming a thick, dense mud that is capable of picking up car-sized rocks. An old lahar flow is visible from the approach road to Whakapapa ski field. On Christmas Eve 1953 a sudden lahar at night on Mt Ruapehu knocked out the rail bridge at Tangiwai, causing six carriages of the Wellington-Auckland express to plunge into the Whangaehu River, killing 151 people in New Zealand's worst railway disaster. The percentage of silica is the deciding factor in the thickness or viscosity of the magma and this increases to the north and east of the Tongariro Volcanic Centre where there is a landscape of calderas and volcanic domes making up the Taupō caldera complex (Central Taupō Volcanic Zone) and Okataina caldera complex (Okataina Volcanic Centre). These produced much of the ignimbrite sheet deposits of the plateau. Ignimbrite is formed from very viscous rhyolite magma which is rich in silicon, potassium, and sodium. It in an explosive caldera forming event is the gas-rich material ejected from the ground at high speed forming a giant white hot fountain up to 55 kilometres high. When the ignimbrite falls it flows outwards at high speed (600–900 km/h) and can travel many kilometres, even going up and over hills and mountains before it cools, often trapping air inside. Especially around Lake Taupō the eroded ignimbrite includes pieces of quickly cooled pumice, a lightweight rock that floats on water. The top, unwelded layer of the ignimbrite sheet is relatively soft and easily eroded by streams and rivers, which transport the pumice to the lake. The lower, welded layers of the ignimbrite sheets are more dense, though still porous. These lower compressed layers often show the glint of heated silicon specks. North of the plateau this stone, called Hinuera stone (after a quarry of origin), is produced for use for building cladding. An historic eruption at the north west edge of the plateau at Mangakino about 1 million years ago produced ignimbrite that away in Auckland is up to thick. The last massive