Mass Wasting
If
a rock slides off of a mountain and no one is around to hear it, does it make a
sound? Well, I don't know about the noise this activity would create, but I do
know that mountains erode and that rocks and debris can slide and fall down
mountain slopes in massive amounts. In this lesson, you will learn about a
process called mass wasting and the factors that cause this movement of
material.
Mass wasting, which is sometimes called mass movement or slope movement,
is defined as the large movement of rock, soil and debris downward due to the
force of gravity. In other words, the earth's outer crust is being 'wasted'
away on a 'massive' scale and falling to lower elevations.
Mass
wasting is a type of erosion and it is capable of making big changes to the
side of a mountain. These changes can happen suddenly, as in one minute the
rock is there and the next it is gone, or it can happen more slowly over time.
You might think of this process as a landslide, and this term is sometimes used
interchangeably with mass wasting. However, the term landslide is a bit
limiting and does not allow for a description of the many different triggers
and types of erosion that can happen on this large of a scale.
Causes of Mass Wasting
Now,
we mentioned that mass wasting is mainly due to gravity. So we see that
mountains have an ongoing tug-of-war with gravity. Gravity is constantly trying
to pull rock and debris down the slope of a mountain. At the same time, the
resistive forces of the mountain, including the cohesive strength and internal
friction between the materials, referred to as the mountain's shear strength,
constantly pulls back against gravity.
The
shear strength works to maintain the slope's stability and keep the materials
in place. This is a lot like a mountain climber gripping onto the side of a
mountain and resisting gravity. The climber uses his grip strength to resist
gravity, like the mountain uses its shear strength.
With
this understanding, we see that the causes of mass wasting occur when
gravitational force overcomes the resistive forces of the mountain. And, since
gravitational pull is always constant, then we see that mass wasting occurs
when something changes the mountain's ability to resist gravity.
For
instance, an increased slope steepness increases mass wasting simply because
the gravitational force acting on a steep slope is greater than the force
acting on a gentle slope. Increasing the steepness of a slope is one way man
can increase mass wasting. For example, if a road crew cuts away a slope to
make room for a new road but makes the angle of the slope too steep, the slope
will be prone to mass wasting, and you will want to cross your fingers when you
drive past this steep slope so no rocks or debris fall on your car!
Increased
water is another factor that plays an important role in mass wasting. Water can
wash away small particles that help keep the mountainside intact. This is
similar to what happens when a wave comes ashore and washes away a sandcastle.
The abundant water breaks apart the small sand particles and destroys the
structural stability of the castle you spent the afternoon building.
If
an area has decreased vegetation, it will be more prone to mass wasting.
Vegetation stabilizes soil particles on the surface and anchors soil under the
surface through its root system. This is much like comparing two sand dunes on
a beach. If one sand dune has grasses growing on it, it will resist the erosion
of water and wind better than a sand dune without vegetation.
Another
factor that plays a role in mass wasting is earthquakes. The violent shaking
that occurs in a region where an earthquake takes place has the ability to
break off sections of mountains or hills, causing them to slide down the slope.
What Is Soil Erosion?
When things erode, they wear away due to some force acting on
them. Just look at any coastline and you will notice how the constant pounding
force from wind and waves causes erosion of the rocky structures, leaving
behind all kinds of interesting cliffs, caves and structures. Soil is not
immune to erosion, and like rocks along a coastline, soil can erode due to the
effects of forces, such as water, wind and farming practices. In this lesson,
we will learn about soil erosion and the factors that cause it.
Soil is naturally created when small pieces of weathered
rocks and minerals mix with organic materials from decaying plants and animals.
Soil creation is a slow process, taking many years. However, the soil that is
created is constantly subjected to natural and manmade forces that disrupt it.
Soil erosion is defined as the wearing away of topsoil. Topsoil is the
top layer of soil and is the most fertile because it contains the most organic,
nutrient-rich materials. Therefore, this is the layer that farmers want to
protect for growing their crops and ranchers want to protect for growing
grasses for their cattle to graze on.
Water Erosion and Surface Water Runoff
One of the main causes of soil erosion is water erosion,
which is the loss of topsoil due to water. Raindrops fall directly on topsoil.
The impact of the raindrops loosens the material bonding it together, allowing
small fragments to detach. If the rainfall continues, water gathers on the
ground, causing water flow on the land surface, known as surface water
runoff. This runoff carries the detached soil materials away and deposits
them elsewhere.
There are some conditions that can accentuate surface water
runoff and therefore soil erosion. For example, if the land is sloped, there is
a greater potential for soil erosion due to the simple fact that gravity pulls
the water and soil materials down the slope. Also, water will have an easier
time running across the surface, carrying topsoil with it, if the ground is
already saturated due to heavy rains or the soil lacks vegetation to keep the
soil in place.
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