Mass Wasting 1

Summary

This lecture explores mass wasting, also known as mass movement or landslides, and how it occurs when material moves downslope. Factors like slope angle, gravity, and material weight play a key role in triggering mass wasting events. The speaker shares a personal experience hiking Mount Lassen, where a dangerous slope covered in boulders demonstrates the 'angle of repose.' The lecture covers different types of mass movement, including rockfall, mudflows, and creep, and how they vary in speed and moisture requirements. Real-life examples show how these events affect both natural and urban environments.

Takeaways

• 😀 Mass wasting (also called mass movement or slope failure) refers to the downslope movement of materials due to gravity, typically over short distances.
• 😀 Mass wasting is different from long-distance transport like rivers moving debris from mountains to oceans, which covers much larger distances.
• 😀 The angle of the slope plays a significant role in determining the type and amount of mass wasting that occurs.
• 😀 Gravity pulls material downwards, and the weight of the material on the slope influences the type of mass wasting event.
• 😀 A personal story about a field trip to Mount Lassen illustrates how steep slopes and loose boulders can cause hazardous conditions during mass wasting events.
• 😀 The 'angle of repose' is the steepest angle at which materials like rocks remain stable on a slope due to a balance of gravity and friction.
• 😀 Adding extra weight, such as a person, water, or additional material, can trigger a mass wasting event, especially if the material is at the angle of repose.
• 😀 Different types of mass movement are classified based on speed and the amount of water involved: rockfall (fast, dry), mudflow (fast, wet), and creep (slow, dry).
• 😀 Rockfall occurs on steep slopes and doesn’t require water to trigger the movement. It is common in areas where mountains are blasted for highways, such as the 14 freeway in California.
• 😀 Creep is a slow form of mass wasting where gravity pulls the upper layer of soil downslope over time, leading to the tilting of structures like fences and utility poles.
• 😀 In urban areas, creep can cause long-term damage to infrastructure, such as leaning fences and damaged electricity poles, especially in areas like Santa Barbara, California.

Q & A

What is the main concept behind mass wasting?

Mass wasting, also known as mass movement or slope failure, refers to the downslope movement of material, such as rocks, soil, or debris, under the influence of gravity. This movement typically occurs over a relatively short distance, from higher to lower elevations.

Why is the term 'mass wasting' used instead of 'landslide'?

While 'landslide' is a common term, 'mass wasting' or 'mass movement' is used because it encompasses a broader range of downslope movements, not just those that involve large, visible landslides. It also includes smaller, slower movements like soil creep.

What are the main factors that determine when mass wasting occurs?

The main factors that influence mass wasting are the angle of the slope, the weight of the material on the slope, and gravity. The steeper the slope and the heavier the material, the more likely mass wasting is to occur.

How does the angle of repose affect mass wasting?

The angle of repose is the steepest angle at which a material, like rock or soil, can remain stable without sliding. If the angle of the slope exceeds this angle, the material is more likely to move, especially when additional factors like weight or moisture are added.

Can you provide an example of a real-life experience related to mass wasting?

The speaker shares a personal story about hiking up Mount Lassen in Northern California, where they encountered a rocky, unstable slope. The weight of a person stepping on a boulder caused a chain reaction, triggering movement in the surrounding rocks—an example of a mass wasting event.

What is rockfall and how is it triggered?

Rockfall is a fast type of mass wasting where rocks or boulders fall from steep slopes. It is usually triggered by gravity and doesn't require moisture. Rockfalls are common along highways where mountains have been cut through, and safety measures like rockfall netting are used to prevent accidents.

What is soil creep, and how does it affect infrastructure?

Soil creep is the slowest type of mass wasting, where soil moves gradually downslope due to gravity. It doesn't require moisture. Over time, it can cause damage to infrastructure like fences, roads, and utility poles, as the slope’s movement slowly displaces these structures.

Why do steep slopes at Mount Lassen make mass wasting events more likely?

Steep slopes, like those at Mount Lassen, create an environment where material can become unstable and prone to movement, especially when external factors like weight, water, or additional material are added. The boulders on the slope, in particular, were positioned at or near the angle of repose, making them vulnerable to movement.

What role does water play in mass wasting events?

Water can play a crucial role in triggering mass wasting events by adding weight and reducing friction. For example, mudflows, a type of fast-moving mass wasting, require a lot of moisture to occur. Water can lubricate the slope, making it easier for material to slide downslope.

How are mass wasting events classified?

Mass wasting events are classified based on the speed of the movement and the amount of moisture involved. For example, rockfall is fast and dry, while mudflows are fast and wet, and soil creep is slow and dry.