Mass movements, erosion, and deposition

Q: What does a factor of safety less than one mean in slope stability?

Driving forces exceed resisting forces A factor of safety below one indicates possible slope failure. Exam tip: focus on balance of forces.

Q: Why does a slope weaken when pore water pressure increases?

Because effective friction and shear resistance decrease Pore water pressure reduces contact force between grains and destabilizes slopes. Exam tip: write water pressure as a landslide factor.

Q: When is rockslide risk higher on a slope with parallel bedding?

When beds dip in the direction of slope Beds dipping with the slope can act as slip planes. Exam tip: relate structure with slope direction.

Q: What main feature distinguishes rotational slide from translational slide?

Concave curved slip surface Rotational slide moves along a curved concave slip surface. Exam tip: identify the curved slip plane.

Q: On what basis is the subtle difference between earthflow and mudflow understood?

Water content and fineness of material Mudflow may contain more water and finer material. Exam tip: observe viscosity and water content.

Q: What type of process is lahar in volcanic regions?

Rapid debris flow of volcanic ash and water Lahar is a dangerous flow of water and volcanic material. Exam tip: treat it as a special debris flow.

Q: What is the steep rupture zone at the head of a landslide called?

Main scarp The main scarp marks the major rupture at the source area of a landslide. Exam tip: use it as an identification feature.

Q: Why can landslides suddenly increase in a mountainous region after an earthquake?

Because shaking reduces the slope's resisting strength An earthquake can suddenly increase landslide activity because its shaking disturbs the balance of a slope. Vibrations may fracture bedrock, loosen soil and rock, and reduce the strength that holds material against gravity. Shaking can also open cracks through which water later enters, making the slope heavier and weaker. On steep or already unstable mountains, this trigger can cause material to move rapidly downslope. Rainfall, weathering, and human activities can also contribute to landslides, but the question asks specifically about the immediate effect after an earthquake. Rain does not permanently stop, river deposits do not instantly become solid rock, and sea waves do not normally reach mountain slopes. Therefore, option D is correct.

Q: How does river undercutting at the foot of a slope increase mass movement?

It removes basal support and destabilizes the slope A slope is held partly in place by support at its foot. When a river erodes or cuts into the base of a slope, that support is removed. The upper material then has less resistance to gravity, so cracks, slips, rockfalls, or landslides become more likely. The risk can increase further when the material is saturated with water, because water adds weight and may reduce friction between particles. Option A is correct because it identifies the direct process: basal undercutting removes support and destabilizes the slope. It does not make the slope extremely strong, remove the weight of particles, or stop cloud formation. The important link is between river erosion at the slope’s foot and mass movement caused by the loss of stability above it.

01What does a factor of safety less than one mean in slope stability?

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Answer and explanation

Correct answer: A. Driving forces exceed resisting forces

A factor of safety below one indicates possible slope failure. Exam tip: focus on balance of forces.

Practice questions

01What does a factor of safety less than one mean in slope stability?ढाल स्थायित्व में सुरक्षा गुणांक का मान एक से कम होने का अर्थ क्या है?

02Why does a slope weaken when pore water pressure increases?छिद्र जल दाब बढ़ने पर ढाल क्यों कमजोर हो जाती है?

03When is rockslide risk higher on a slope with parallel bedding?समांतर परतों वाली ढाल में शैलस्खलन का जोखिम कब अधिक होता है?

04What main feature distinguishes rotational slide from translational slide?घूर्णी स्खलन को अनुवादी स्खलन से अलग करने वाला प्रमुख लक्षण क्या है?

05On what basis is the subtle difference between earthflow and mudflow understood?मिट्टी प्रवाह और कीचड़ प्रवाह में सूक्ष्म अंतर किस आधार पर समझा जाता है?

06What type of process is lahar in volcanic regions?ज्वालामुखीय क्षेत्रों में लहार किस प्रकार की प्रक्रिया है?

07What is the steep rupture zone at the head of a landslide called?भूस्खलन के शीर्ष पर बनने वाला खड़ी टूटन वाला भाग क्या कहलाता है?

08Why can landslides suddenly increase in a mountainous region after an earthquake?भूकंप के बाद पर्वतीय क्षेत्र में भूस्खलन अचानक क्यों बढ़ सकते हैं?

09How does river undercutting at the foot of a slope increase mass movement?नदी द्वारा ढाल के आधार को काटना जन संचलन को कैसे बढ़ाता है?

10When does wave erosion increase slope failure on sea cliffs?समुद्री चट्टानों पर तरंग कटाव से ढाल विफलता कब बढ़ती है?

11What mainly increases cohesion on a slope?ढाल पर संधारण बल मुख्यतः किससे बढ़ता है?

12Why may the same material move faster on a steeper slope?अधिक तीव्र ढाल पर समान पदार्थ तेजी से क्यों खिसक सकता है?

13What is the role of seasonal expansion and contraction in soil creep?मृदा सर्पण में मौसमी फैलाव और सिकुड़न का क्या योगदान है?

14Which condition is necessary for solifluction in periglacial regions?परिहिमानी क्षेत्रों में सोलिफ्लक्शन के लिए कौन सी शर्त आवश्यक है?

15How can a debris avalanche differ from an ordinary debris flow?मलबा हिमस्खलन सामान्य मलबा प्रवाह से कैसे अलग हो सकता है?

16Which combination is most useful in landslide hazard mapping?भूस्खलन जोखिम मानचित्रण में कौन सा संयोजन सबसे उपयोगी है?

17Why is climate important while assessing erosion rate?अपरदन दर का आकलन करते समय जलवायु क्यों महत्वपूर्ण है?

18Why is sheet erosion difficult to identify at an early stage?शीट अपरदन को आरंभिक अवस्था में पहचानना कठिन क्यों होता है?

19What can develop if gully erosion is not controlled?गली अपरदन नियंत्रित न होने पर स्थल पर कौन सा परिणाम बन सकता है?

20What effect will a sudden decrease in river carrying capacity have on deposition?नदी की वहन क्षमता में अचानक कमी का निक्षेपण पर क्या प्रभाव होगा?

21How can headward erosion change a river system?सिरोन्मुख अपरदन नदी तंत्र को कैसे बदल सकता है?

22How does a stronger stream affect another stream in river capture?नदी अपहरण में अधिक शक्तिशाली धारा दूसरी धारा को कैसे प्रभावित करती है?

23Why are river terraces found in a rejuvenated river?कायाकल्पित नदी में नदी तराशे हुए वेदिका क्यों मिलती हैं?

24How are deposition and erosion related in a graded river?संतुलित नदी में निक्षेपण और अपरदन का संबंध कैसा होता है?

25What is the common basic process in both alluvial fan and delta?जलोढ़ पंख और डेल्टा दोनों में समान मूल प्रक्रिया क्या है?

Analytics choices

01What does a factor of safety less than one mean in slope stability?

02Why does a slope weaken when pore water pressure increases?

03When is rockslide risk higher on a slope with parallel bedding?

04What main feature distinguishes rotational slide from translational slide?

05On what basis is the subtle difference between earthflow and mudflow understood?

06What type of process is lahar in volcanic regions?

07What is the steep rupture zone at the head of a landslide called?

08Why can landslides suddenly increase in a mountainous region after an earthquake?

09How does river undercutting at the foot of a slope increase mass movement?

10When does wave erosion increase slope failure on sea cliffs?

11What mainly increases cohesion on a slope?

12Why may the same material move faster on a steeper slope?

13What is the role of seasonal expansion and contraction in soil creep?

14Which condition is necessary for solifluction in periglacial regions?

15How can a debris avalanche differ from an ordinary debris flow?

16Which combination is most useful in landslide hazard mapping?

17Why is climate important while assessing erosion rate?

18Why is sheet erosion difficult to identify at an early stage?

19What can develop if gully erosion is not controlled?

20What effect will a sudden decrease in river carrying capacity have on deposition?

21How can headward erosion change a river system?

22How does a stronger stream affect another stream in river capture?

23Why are river terraces found in a rejuvenated river?

24How are deposition and erosion related in a graded river?

25What is the common basic process in both alluvial fan and delta?