Final answer:
A debris avalanche is typically caused by magma intrusion, a significant earthquake, or fault movements under a volcano. Earthquakes, volcanic eruptions, and various other factors such as erosion and water saturation contribute to these events. The correct answer is option D.
Step-by-step explanation:
A debris avalanche is usually caused by magma intrusion, a strong earthquake, or the movements of faults beneath the cone-shaped structure of a volcano. There are several contributing factors to landslides, which include debris avalanches:
- Erosion by rivers, glaciers, or ocean waves creating over-steepened slopes.
- Saturation of rock and soil slopes from snowmelt or heavy rains weakening them.
- Earthquakes creating stresses that cause weak slopes to fail, especially those with magnitude 4.0 and higher.
- Volcanic eruptions producing ash deposits, heavy rain, and debris flows.
- Excess weight from the accumulation of rain or snow, or from man-made structures, adding stress to slopes.
Additionally, volcanic eruptions such as the 1980 explosion of Mt. St. Helens can cause massive landslides and debris flows, fundamentally altering the landscape and resulting in significant ecological and geological changes.