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 restrictive 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 restrictive 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, 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.
Slumps and
Because there are different factors that cause mass wasting, not
all wasting happens in the same way. In some cases, we may see a large portion
of the slope fail and slide or roll downhill. In other cases, large collections
of debris may flow, like a river, down the side of a mountain mixed with water
or air
.
One of the types of mass wasting that is an example of the slope
failing is a slump. This is the sliding of coherent rock material
along a curved surface. It's almost as if the section of the mountain or
hillside slumps its shoulders and shrinks in height. This is easy to image
happening if you consider that a slump often results due to an undercutting of
the mountain's base. For example, water might erode the base of a cliff.
Without the base, the outer sections of the mountain slumps down as a unit or
multiple units.
A rock slide is another example of mass wasting
of a slope. A rock slide is the sliding of rock material down a mountain. It is
similar to a slump, but a rock slide does not move along a curved surface like a
slump. Therefore, in a rock slide, we see rocks sliding down a pre-existing
surface, such as an underlying layer of rock. In a way, it is as if the rocks
are moving down a slide on a playground. As a result, we will see a collection
of fallen rocks at the base of a rock slide.
Debris Flow and Earth
flow
Other types of mass wasting involve the downward movement of unconsolidated materials. These are collectively referred to as flows because the materials flow, like a fluid, down a slope, much like cement flows out of a cement truck
No comments:
Post a Comment