(a) Give the classification of landslides and discuss the causes of landslide. (20 marks) (b) What is the structure of the Earth…

(a) Classification and causes of landslides

The Cruden and Varnes classification system categorizes mass movements by combining the type of movement—fall, topple, slide (rotational or translational), lateral spread, flow, and complex—with the nature of the material, which comprises bedrock, coarse debris, or fine-grained earth. Failures range geometrically from shallow translational slides, where failure occurs along a shallow planar boundary parallel to the slope, to deep-seated rotational slumps governed by curved shear surfaces. Varnes also classes landslides by velocity, from extremely slow creep (millimetres a year) to extremely rapid rock avalanches (metres a second), and by water content and state of activity.

Landslide initiation results from internal preparatory factors and external triggers. Geological causes include unfavourable joint sets, daylighting foliation or bedding planes, shear zones, and lithological contrasts. Geomorphological drivers involve slope oversteepening through fluvial toe erosion and tectonic uplift. Dynamic triggers include sudden pore-water pressure spikes from intense rainfall and cyclic shear stresses during earthquakes. Anthropogenic interventions—such as unscientific slope cutting for infrastructure, deforestation, and uncontrolled drainage discharge—severely reduce slope factor of safety.

In India, the geodynamically active Himalayas frequently experience deep-seated rockslides and debris flows triggered by cloudbursts and seismicity, as observed during the 2013 Kedarnath disaster, the Kinnaur rockfalls of 2021 in Himachal Pradesh, and the slow ground subsidence of Joshimath in 2023. Conversely, the Western Ghats are prone to shallow regolith failures and debris flows along transport corridors, typified by the Ambenali Ghat failures and the Malin landslide (Pune district, 2014) during monsoon downpours, and the Wayanad debris flows of 2024.

(b) Structure of the Earth and distribution of elements

Earth is compositionally and mechanically heterogeneous, stratified into distinct concentric shells separated by first-order seismic discontinuities.

The crust, granodioritic under continents and basaltic under oceans, passes at the Moho (about 35 km under continents) into a peridotitic upper mantle of olivine and pyroxene, which changes through the 410–660 km transition zone into a lower mantle of bridgmanite and ferropericlase. At 2,890 km the Gutenberg discontinuity marks the liquid Fe–Ni outer core with light elements (S, O, Si), and at 5,150 km the Lehmann discontinuity marks the solid Fe–Ni inner core, which extends to the centre at 6,371 km.

The Earth varies systematically in composition with depth due to gravitational settling during accretion and core differentiation. Goldschmidt’s geochemical classification outlines this distribution: Lithophile elements, possessing strong affinity for oxygen, partitioned into the silicate crust and mantle (Si, Al, Ca, Na, K, Mg as SiO₂, Al₂O₃, CaO, Na₂O, K₂O, MgO), and atmophile elements (H, N, noble gases) concentrated in the atmosphere and hydrosphere. Siderophile elements partitioned into the iron-nickel core (Fe, Ni, Co, Au, platinum-group elements), while chalcophile elements (S, Cu, Zn, Pb) concentrated in sulphide phases.

Consequently, the bulk Earth is dominated by Iron (~32.1%), Oxygen (~30.1%), Silicon (~15.1%), and Magnesium (~13.9%). In contrast, the continental crust is enriched in lighter, incompatible elements following the order: Oxygen (~46.6%) > Silicon (~27.7%) > Aluminium (~8.1%) > Iron (~5.0%) > Calcium > Sodium > Potassium > Magnesium, illustrating progressive magmatic fractionation from a chondritic primitive mantle.

(c) Classification and importance of meteorites

Meteorites are broadly grouped into three primary classes based on their silicate-to-metal ratio:

Stony Meteorites: Comprise Chondrites and Achondrites. Chondrites are primitive, undifferentiated aggregates containing spherical chondrules, sub-classified into Carbonaceous (CI, CM), Ordinary (H, L, LL), and Enstatite (EH, EL) groups across petrologic grades 1 to 6 (denoting degrees of aqueous alteration and thermal metamorphism). Achondrites are differentiated igneous crustal/mantle rocks devoid of chondrules, including HED (howardite-eucrite-diogenite from 4 Vesta), SNC (Martian), and lunar meteorites.

Stony-Iron Meteorites: Contain subequal proportions of silicates and Fe-Ni metal, divided into Pallasites (olivine crystals set in an Fe-Ni matrix) and Mesosiderites (brecciated silicates mixed with metal).

Iron Meteorites: Composed almost entirely of Fe-Ni alloys (kamacite and taenite), classified structurally by their Widmanstätten patterns (Hexahedrites, Octahedrites, Ataxites) and chemically via trace elements (Ga, Ge, Ir), representing disrupted planetary cores exhibiting varied shock metamorphism stages.

The study of meteorites is fundamental to Earth Science. Radiometric dating (e.g., ²⁰⁷Pb-²⁰⁶Pb) of Calcium-Aluminium-rich Inclusions (CAIs) in chondrites provides the definitive age of the solar system at 4.567 billion years, constraining Earth's age of about 4.54 billion years. Chondritic meteorites represent pristine solar nebula condensates, serving as the benchmark for calculating the Bulk Silicate Earth (BSE) composition. Achondrites and iron meteorites offer tangible analogues for core segregation and early crustal evolution. Furthermore, organic-rich carbonaceous chondrites, such as the Murchison meteorite containing extraterrestrial amino acids and nucleobases, provide insights into prebiotic volatile delivery to early Earth, while metal-rich asteroids remain targets for strategic nickel-iron and platinum-group element exploration. Indian falls include the Dhajala H3.8 chondrite (Gujarat, 1976), the Piplia Kalan eucrite (Rajasthan, 1996) and the Mukundpura CM2 carbonaceous chondrite (near Jaipur, 2017), and the Lonar crater in Maharashtra is an impact crater in Deccan basalt.