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Geomorphology - Surficial Processes and Landscapes

Part II - Mass movement (aka mass wasting)

    When you look out across a verdant landscape, you probably see grass and trees, hills and valleys. When a geologist looks at a landscape, s/he sees cause and effect -- process and product. That is, I see landslide scars, erosional remnants of once great mountains, fault scarps, episodic tectonic uplift, slope recession, and countless other products of the balance between the processes of degradation and aggradation. If you will recall, aggradation is the building up of the Earth's surface, degradation is what wears it down.
    Likewise when you look at a stream valley you might envision a channel, bank-full, eroding away the channel perimeter and slowly widening its pathway across the terrain. On the contrary, that valley, just like the stream channel indicated in the accompanying figure, owes its breadth as much to slope failure as to active erosion by the flow of the stream. Don't get me wrong, erosion by flowing water is critical to the ensuing slope failure, and the stream is very effective at channel downcutting and lateral cutting (channel migration), but the actual act of widening the channel typically occurs by the process known as mass wasting. It is the combined action of gravity and hydraulic action -- not erosion alone --  producing failure of the banks, widening the stream channel, and producing the dramatic landform we call a stream valley. However, without bank undercutting by the hydraulic action of the stream, the slope failure wouldn't be possible. In this situation, therefore, hydraulic action (the force of moving water) and mass wasting work hand-in-hand. To illustrate the difference, in the accompanying figure, I have shown the volume of earth material, hypothetically, that direct stream erosion (hydraulic action) would remove versus the volume that might be mass wasted. Of course, this would be different for every stream, but this is one possible (but highly simplified) representation.

    It may not be intuitively obvious, but stream valleys big and small are our planet's most common landform. Accordingly, mass wasting, or mass movement, is amongst the most important earth processes shaping the land.

    Geologists define mass wasting as downslope movement of earth material as a unit under the influence of gravity. Of course, the latter part goes without saying -- everything is under the influence of gravity -- but it signifies that the mass (the unit of earth material) moves when the resistance of friction, which tends to keep it in place, no longer exceeds the force of gravity, which tends to pull it downslope.

    All types of mass movement involve sliding, falling, flowing, or heaving of earth material. Common experience sufficiently explains the first three; the fourth -- heave -- merits further discussion.

Heave is imperceptibly slow movement of earth materials -- usually soil -- downslope in a near-surface, moist, uncompacted layer. In this layer, soil materials undergo a daily cycle of expansion and contraction, due either to wetting (expansion) and drying (contraction) or freezing (expansion) and thawing (contraction).
Expansion occurs perpendicular to the slope surface, but contraction, which is under the influence of gravity, is strictly vertical -- i.e. towards the Earth's center. Consequently, the particles of soil slowly, imperceptibly, move downslope, more-or-less as a unit, producing what geologists call soil creep. There are many easily recognized phenomena that result from this process. These include leaning walls and tombstones, pistol-butt trees, and a variety of other odd features. These will be discussed later. 

Now that you are familiar with the four or five types of movement involved in mass wasting, I can discuss the defining characteristics of the resulting processes.

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