Complete the text using the correct form of the words in brackets. 

A snow avalanche begins when an ___________ (1.STABLE) mass of snow breaks away from a slope. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. The moving mass picks up even more snow as it rushes downhill. A large, fully ___________ (2.DEVELOP) avalanche can weigh as much as a million tons. It can travel faster than 320 kilometers per hour (200 miles per hour).

Avalanches occur as layers in a snowpack slide off. A snowpack is simply layers of snow that build up in an area, such as the side of a mountain. In winter, repeated snowfalls build a snowpack dozens of meters thick. The layers vary in ___________ (3.THICK) and texture.

The bonds between the layers of a snowpack may be weak. ________ (4.MELT) snow that refreezes may cause a slick coating of ice to form on the surface of a layer. A new snowfall may not stick to this slippery layer, and it may slide off. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added ___________ (5.WEIGH) or vibration can easily send the top layers of a snowpack hurtling downhill.