The Role of Water in Mass Movement

Have you ever made a sand castle? How does dry sand compare to damp sand in terms of cohesiveness? How does dry or damp sand compare to very wet sand? You probably know that dry sand does not stick together well. Very wet sand does not stick together either—it just turns into a soggy pile of loose sand. To make a really great sand castle, you need just slightly damp sand—not too dry and not too wet, so that all the particles stick together.

In the same way, water plays a similar role in mass movement. When soil and rock are dry, they lack cohesiveness and slide away from each other easily. Likewise, when they are saturated and very wet, the pressure from the water is greater than the force of cohesion, and the materials flow easily. But damp sediments have greater cohesion and allow for a steeper angle of repose before a slope fails and materials move downhill.

Under what climate conditions would mass movement be most common?
 

Mass movement is especially common after very heavy rains or in the spring after large amounts of snow suddenly melt, making the ground very wet. The very wet soils lose cohesiveness and tumble downhill.