Predicting the Slide: Landslide Risk Factors Quiz

Slopes made of loose soil or fractured rock are much less stable than those made of solid, intact bedrock. This unconsolidated material lacks a strong internal bond, making it easier for gravity to pull it downward once the slope's balance is disturbed by water or vibration.

Wildfires destroy vegetation that normally anchors the soil with its roots. Furthermore, fires can create a "hydrophobic" soil layer that prevents water from soaking in, causing rain to run off quickly and sweep away large amounts of debris and soil.

When trees and shrubs are cleared for farming or construction, the natural "rebar" of the root systems is lost. This significantly weakens the soil's resistance to gravity and allows water to saturate the ground more quickly, increasing risk.

Building roads adds weight and often involves cutting into the "toe" of the slope, which removes support. Leaking pipes add extra water to the soil, which increases pore-water pressure and reduces the friction that keeps the slope stable.

As a slope becomes steeper, a larger portion of the force of gravity is directed parallel to the ground. This increases the "driving force" that tries to move material downhill. Slopes exceeding their natural angle of repose are inherently unstable.

Hazard mapping identifies zones with high risk based on slope, soil type, and history of movement. By using these maps, urban planners can ensure that critical infrastructure is built in stable areas, reducing the potential for loss of life and property.

Water reduces the friction between grains of soil. When the ground is saturated, the water effectively "greases" the slide plane, allowing gravity to pull the mass downhill much faster than if the material were dry.

While risk factors define how likely a slide is, triggers are the specific events that start the movement. Shaking from volcanoes or earthquakes can break the frictional bond, while heavy rain provides the lubricant and extra weight needed for a slide to begin.

If the layers of rock (bedding planes) are tilted in the same direction as the surface of the hill, they act like a slide. Water can seep between these layers, making it very easy for the top layer to slip off and crash down the mountain.

While rain is a major factor, landslides can occur in any climate if the slope is unstable. Dry areas can experience slides due to earthquakes or human construction. Gravity and slope stability are universal factors that apply to mountains and hills globally.

Debris flows are particularly dangerous because they move with incredible speed and force, often looking like a river of wet concrete. They can carry large boulders and trees, destroying everything in their path.

Leaning trees (often called "drunken trees") show that the soil is slowly creeping downhill. Cracks in the ground or sudden shifts in house foundations indicate that the underlying support is moving, serving as an important warning sign.

Retaining walls are engineering solutions designed to add extra resisting force to a slope. By physically blocking the movement of soil and providing a heavy counter-weight at the base, these walls help balance the driving force of gravity.

As large amounts of snow melt, the water soaks into the ground over a long period. This can saturate deep layers of soil that a quick rainstorm might not reach, increasing pore pressure and triggering slides in mountainous regions.

Geologists look for "old" landslide scars because once a slope has failed, it is often weakened and prone to moving again. Mapping these historical events allows scientists to identify the most vulnerable parts of a mountain range.