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All the landforms on planet Earth are formed through a combination
of tectonic forces that push the land up, erosional forces that work to tear it down,
and rocks of varying character and resistance that impact the location of erosion.
To review, the term erosion means the physical removal of rock fragments from
one location and transport of them to another location. The breaking down of
the rock chemically or physically is called weathering. The dropping of the rock into a
new location is called deposition. In the next few video tutorials,
we will focus on various agents of erosion such as gravity, running water, waves, glaciers,
and humans. For this video tutorial, we discuss the erosional agent of gravity.
Gravity is a force that pulls material downhill towards the center of the Earth.
Any object sitting loosely on Earth’s surface will feel the pull of gravity and, unimpeded,
will simply roll or side downhill. The heavier the object, the greater the force of gravity.
Friction is the force that sticks loose material to a surface and needs to be overcome by the
force of gravity before objects can move. Friction depends on the materials involved
and the angle of the slope. Rough material has more frictional stickiness. Smooth material,
less so. When my hands are smooth, they slide by each other easily. If I cover them in rough
gloves, it gets harder to slide them past each other. When I bend my hands, or let my fingers interfere,
it’s harder to slide them past each other. All of these are ways to increase the friction and
keep objects from moving downhill. The weight of an object pushing against a surface will also
contribute to friction. When I push my hands together tightly, it’s harder to slide them.
This box, which is sitting on a hillside, is pulled toward the center of the Earth by
the force of gravity, which is its weight. If the box is not moving,
the component of its weight that moves along the surface of the hill is matched by an equal
but opposite force of friction. Notice what happens to the force of its weight along the
surface as the hillside steepens, the arrow gets longer, meaning this surface component
of the force is getting greater. Once we get the hillside steep enough that the weight
component along the surface is greater than the frictional force, the object will move downhill.
We call downhill movements of weathered rock material on Earth’s surface mass movement,
and we can classify the movement into a variety
of types based on the type of material and the way it moves.
The slowest type of mass movement is called creep and refers to the
top soil or sediment slowly moving downhill on a day-to-day basis as
it alternately expands and contracts through daily heating and cooling cycles. That motion,
while happening over distances of less than a millimeter, will overcome small frictional
forces and allow a very small downhill movement each daily cycle. Cumulatively over many years,
we see the top soil or sediment layer creeping its way downhill. Evidence
that creep is occurring is visible when we see trees or fences on a slope bending downhill.
The fastest type of mass movement happens during a rock fall, which
happens when cracks form on rock cliffs and eventually break off chunks of rock,
which fall and collect at the base of the cliff in a pile known as a talus slope. These cracks
can form through a number of processes, such as frost wedging and exfoliation.
For more information on the physical weathering processes, watch the video tutorial on Weathering.
If a large number of rocks fall at the same time they can trap and ride atop a layer of air,
which greatly reduces friction and allows them to move at speeds over
200 miles an hour. We call these movements rock avalanches, and because of their great speed,
they can travel great distances, even up and over ridges. Rock falls and rock
avalanches pose a considerable threat to communities that live in the mountains.
When a layer of rocks slides downhill along a planar surface, usually a rock bedding contact,
fault, or foliation plane, it’s called a slide. Slides can be triggered when
water runs down along the bedding planes, reducing friction between them or when the
base of the rock bed above the plane is excavated and thus is no longer supported.
When material on the hillside moves downhill along a curved surface,
it’s called a slump – think about what happens when you slump down in a chair.
Water can increase the likelihood of all these types of mass movement because it
both reduces friction and adds weight. The more weight, the greater the force
of gravity on the hillside. A little bit of water causes small particles to stick together,
much like when you build sand castles on a beach. Too much, however, and it pushes them
apart. When water content increases enough, it can cause the surface material to liquefy and
flow. These type of mass movements where water content is high are called flows.
Given enough time, all hillsides will eventually fail and be eroded. What
makes a particular hillside more likely than others to fail? What are some things you can
look at for example if you’re looking to buy property and want to know its
stability? First look at the steepness of the hillside. The gentler the slope,
the better. Then look at the rocks and material that make up the slope. The more solid the rock,
the better. Loose, unconsolidated sediments or soils will be most likely to slump or slide. Then
look at the geologic structure of the hillside. If there are bedding, foliation, or fault planes,
are they parallel to the hillside? If so, they’re more likely to fail than those that
are perpendicular to the hillside. Is there a way for water that penetrates the hillside to easily
flow out? Does it sit atop any ponds? Are there any springs? Water adds weight, so the more easily
water can hit the surface and safely run off, the better. Finally, look at the vegetation. Plants
with roots can help keep soils more compacted and connected, especially if the roots are deep.
What are some triggers that could start mass movement on a susceptible hillside? When the
base of a rock bed is excavated, it loses its support. Excavation can happen during building
of highways or roads or houses, or when rivers flood. Weight added to the top of a hillside
will increase the force pushing downhill. Weight is added when houses are built on the top of the
hill or when excavated sediment is piled up on the top of the hillside. Heavy rains or water
leaks from reservoirs or homes at the top of a hillside can reduce friction and add
weight to the hillside. A common trigger for mass movement in California is what happens
when heavy rains first appear in an area that experienced a fire in the previous year and had
its vegetation removed. Finally, a large shaking of the ground, like in an earthquake or explosion
or through heavy foot traffic crossing a hillside, can trigger a mass movement.
In the state of California, mass movements cause the greatest financial and human
impact of any natural disaster. It’s a good idea to pay attention
to the stability of hillsides when building structures, communities, and hiking trails.
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