Geology Reference
In-Depth Information
Adding water to the mix
More than anything else, water causes mass wasting events. This fact may
seem counterintuitive if you have ever built a sand castle or made mud pies. In
both cases, adding water makes the sand or mud particles stick together, allowing
you to build a taller tower or make a stickier pie. But keep in mind that to achieve
the desired effect, you have to add
just the right amount
of water. Too much water,
and your sand castle slides away or your mud pie becomes mud soup. The same
rule applies to sediments on a much larger scale. A little bit of water increases
stickiness, but too much water leads to mass wasting.
One way water triggers mass wasting is by increasing the weight of the material in-
volved. Adding water to soil or sediments (usually through rainfall) increases their
weight, which increases the pull of gravity.
Water can also trigger mass wasting by saturating sediments.
Saturation
oc-
curs when water fills all the space between the particles of sand or rock (called
pore space
) so that the particles can no longer stick together. When they're satur-
ated, sediment particles no longer touch each other. Therefore, they have no fric-
tion, and they can slip right past one another. Without friction to fight the pull of
gravity, sediments move downslope.
Heavy rainfall is the most common way for sediments on a stable slope to become satur-
ated and unstable, resulting in movement downslope, or mass wasting.
Water can trigger mass wasting in multiple ways, but the water itself is not ac-
tually moving the material.
Gravity
is responsible for moving materials in mass
wasting.
Changing the slope angle
