down the valley. It surmounted a ridge and overwhelmed the city of Yungay, killing about 19,000 inhabitants. The effects of the subsequent flow of debris down the Rio Santa were felt far downstream and included damage to a major hydroelectric power plant which was put out of operation for many months. The Yungay disaster was not an unprecedented occurrence. A similar, smaller failure killed several thousand people at Ranrahirca, an adjoining town, in the 1960’s. Moreover, the presence of old debris avalanche deposits in the valley indicates that there have been several similar occur- rences in the past. The present re-located site of Yungay is on such an old deposit and for this reason the hazard to present inhabitants still remains, 6.2.2 Loose surface materials on steep slopes Steep slopes of coherent bedrock often have a surface covering of weathered material or soil a few metres thick. This material is often wet or .saturat,ed‘ by rain or snow melt and the contact with underlying firm material forms a surface of low shear strength. Earthquakes can cause this layer to fail and descend rapidly, destroying farms on, the slope and villages in the valley below. Such failures have resulted in heavy casualties in many areas of the world particu- larly in mountainous regions in tropical and temperate climates. The situation is aggravated where slash and burn agricultunil practice has destroyed natural vegetation cover, There may also be loose deposits on steep slopes not derived from the underlying rock but by deposition from the air, such as volcanic pumice or loose tind-blown silt. If such deposits are’ deep, a particularly dangerous hazard arises, Slope failures during the 1976 Guatemala earth- quake were practically confined to areas covered by dense layers of pumice, In some regions 
**Analysis**
The text primarily discusses natural disasters, specifically landslides and their impact on human settlements and infrastructure. The focus is on the geological and environmental factors that contribute to these disasters, rather than economic, social, or cultural aspects. The text also touches on the long-term effects of these disasters on the environment and human populations.

**Category**
Natural resources and environment, Science and technology