[4] The north peak of Huascarán from which the avalanche originated had been considered unstable since 1962, when a smaller collapse wiped out several villages of the Callejón de Huaylas valley near Yungay.

The 1970 earthquake destabilized a glacier and snowmass which surged rapidly downhill, becoming a mudflow as it accumulated large volumes of loose dirt, rock and surface water.

The death toll was made worse due to the earthquake having occurred on a Sunday, as thousands more people had congregated in Yungay for market when the mudflow struck and leveled the city.

The slide then entered the Río Santa and caused extensive damage as it flowed all the way downstream to the Pacific Ocean, a distance of 160 kilometres (100 mi), mostly through a narrow canyon.

Following the disaster, the Peruvian government conducted relief efforts and planned to move the provincial capital from Yungay to a safer location at Tingua.

"[1]: 26 Due to its rugged terrain, high elevation and location in an active seismic zone, the Callejón de Huaylas, or upper Río Santa valley has historically been vulnerable to large landslides, mudslides and avalanches.

"[6] The name of nearby Ranrahirca village means "pile of thrown stones" in Quechua, suggesting that debris avalanches have been observed in the valley since ancient times.

In the late 1960s, Yungay's permanent population was around 4,000[9] or 5,000,[10] which swelled greatly on market and church days when thousands of visitors arrived from the surrounding villages.

[9] With the notable exception of the massive stone cathedral at the center of town, the city consisted predominantly of two or three-story adobe and wood structures with heavy tile roofs, dating from the Spanish colonial period.

[11] In the years leading up to 1970, Yungay – sometimes nicknamed "Pearl of the Huaylas Corridor"[12] – had become one of the most popular tourist destinations in Peru due to its picturesque location, architecture, and proximity to Huascarán and the Llanganuco lakes.

Due to the steep slope of the underlying rock and lubrication by glacial melt, there was nothing to stop the ice from sliding downhill at high speed.

American climbers David Bernays and Charles Sawyer observed, in the aftermath of this slide, "a massive vertical slab of rock being undermined by Glacier 511"[15] posing the threat of a much larger collapse.

[15][3]: 150  Prior to that, Bernays had attended a public meeting in Huaraz where he called the Expreso coverage "exaggerated", though he did not deny the possibility that a large enough avalanche could affect Yungay.

[5] The rock fell some 600 m (2,000 ft) vertically[5]: 302  before landing on Glacier 511 and sliding nearly 3 km (1.9 mi) down its surface, accumulating a large volume of firn snow as it went.

With area soils saturated near the end of the rainy season,[18] and the large amount of snow and ice scraped off the glacier surface,[5] the avalanche quickly liquefied into a fluid, fast-moving mudflow.

[5] According to a U.S. Geological Survey report published the same year, the mudflow may have achieved its unusually high speed due to "air-cushioned flow", a mixture of snow, ice and entrapped air that allowed the bulk of the material to essentially float over the ground.

[1] The initial acceleration of the mass down the low-friction glacial surface was also a major factor, catapulting the material downhill at a much higher speed than if it had slid over bare rock or earth.

Part of Huascarán was falling... You could see a giant wave of gray mud, about 60 metres (200 ft) high, about to hit the left hand side of the city.

Material traveled as much as 83 m (272 ft) vertically up the opposite bank, destroying part of Matacoto village, before reversing and falling back into the river with a thunderous roar.

"[5]: 297  Within thirty minutes the landslide dam overflowed, and debris and mud flowed downstream for some 100 km (62 mi) at speeds up to 35 km/h (22 mph), causing damage all along the river banks.

[1]: 12  At Huallanca below Cañon del Pato, the river reportedly dried up for an hour after the earthquake, due to its temporary blockage upstream, then abruptly rose 20 m (66 ft) from its bed.

[5] The number of deaths from the 1970 avalanche has been difficult to quantify; due to the sheer size of the devastated area and the depth to which the towns were buried, thousands of missing bodies were never found.

Only about 400 people survived in Yungay proper, 300 of which were children, who had been attending a circus in the local stadium; reportedly a clown led them to higher ground just in time before the mudflow struck.

[5]: 297  Much of the mud that initially buried the city drained away within a few days of the disaster, leaving a plain of larger rocks which remains today in much the same state.

Although this location was much safer from landslides than old Yungay, it was geographically constrained – "aid personnel speculated that if the capital of the province were relocated there, it would soon outgrow the safe area and spill over into dangerous zones.

President Juan Velasco Alvarado saw the disaster as "a unique opportunity" to carry out his populist agenda, which aimed to erase socioeconomic divides in rural provinces such as Yungay.

[9] The social composition of the town was completely changed, especially since a large number of old Yungay's survivors ended up moving to coastal cities, to be replaced by migrants from rural areas.

[20] Visitors can still see a portion of the cathedral wall, the burial mound and statue of Christ, the four palm trees in the central plaza, and the wreckage of a bus.