(a) Discuss the causes of various seismic discontinuities in the upper mantle. (20 marks) (b) Discuss the composition, source…

(a) Causes of seismic discontinuities in the upper mantle

A seismic discontinuity is a depth at which P and S wave velocities change sharply or rapidly. In the upper mantle it arises from a change of composition, of physical state, or of mineral phase at the pressure and temperature of that depth.

(i) Moho (5–70 km): the boundary between crust and mantle, where P-wave velocity rises from about 6.8–7.2 to about 8 km/s. It is chiefly compositional, from mafic or felsic crustal rock to peridotite.

(ii) Lithosphere–asthenosphere boundary and low-velocity zone (about 80–200 km): shear velocity falls because the mantle is near its solidus and holds a little partial melt and dissolved water, and is weak. It is a change of state and not of phase.

(iii) Lehmann discontinuity (about 220 km): observed only in some places. Its cause is debated, and it is explained by a change in anisotropy, by the base of the lithosphere or by a small phase change.

(iv) 410 km discontinuity: olivine (α) changes to the denser wadsleyite (β-spinel), at about 13–14 GPa. The transition is exothermic, so it occurs at shallower depth in cold slabs.

(v) 520 km: wadsleyite changes to ringwoodite (γ-spinel), a weaker jump, with garnet changes.

(vi) 660 km discontinuity: ringwoodite breaks down to bridgmanite (Mg-silicate perovskite) and ferropericlase, at about 23 GPa. The reaction is endothermic, so it deepens in cold slabs, and this resists the downward movement of a slab. Some slabs stagnate here, others sink into the lower mantle.

The zone between 410 and 660 km is the mantle transition zone. Density rises across each discontinuity, and the transitions have been reproduced in high-pressure laboratory experiments on olivine.

(b) Fly ash: composition, source, types, hazards and uses

Fly ash is the fine powder carried away in flue gas when pulverised coal burns in a thermal power plant and caught by electrostatic precipitators. Coarser residue falls as bottom ash, and the ash sent to ponds with water is pond ash.

Composition. Mostly glassy spheres (cenospheres are hollow) of SiO₂ (about 40–60%), Al₂O₃ (20–35%), Fe₂O₃ and CaO, with some unburnt carbon and traces of As, Pb, Hg, Cd and Cr. Indian coal is high-ash, and the ash is low in lime.

Types. Under ASTM C618, Class F (low-calcium, from bituminous coal; SiO₂ + Al₂O₃ + Fe₂O₃ at least 70%) is pozzolanic, and Class C (high-calcium, from sub-bituminous coal and lignite, with at least 50%) is also self-cementing. Most Indian ash is Class F.

Environmental hazards. Fine dust (PM₂.₅, PM₁₀) causing lung disease, leaching of heavy metals into ground and surface water, land occupied by ash ponds, and the breach of ash dykes, which floods the surroundings.

Uses. Portland pozzolana cement and concrete, bricks and blocks, embankments and road sub-base, filling of mine voids, soil amendment in agriculture, and lightweight aggregate. Use of fly ash in bricks and construction near thermal plants is promoted by the environment ministry's notifications, and India now uses most of the ash it makes.

(c) Hazards of an active volcanic terrain, during and after an eruption

During eruption.

(i) Pyroclastic flows and surges, hot avalanches of gas and ash moving at high speed, the most lethal, as at Mont Pelée in 1902.

(ii) Lava flows, which destroy property but seldom kill.

(iii) Tephra fall of ash, lapilli and bombs, which collapses roofs, damages crops and stops aircraft.

(iv) Gases (CO₂, SO₂, H₂S, HF) harm people and animals.

(v) Lahars (volcanic mudflows) when hot material melts snow, and tsunamis when an island volcano collapses, as at Krakatau in 1883.

(vi) Edifice collapse and earthquakes, as in the lateral blast of Mount St Helens in 1980.

After eruption.

(i) Secondary lahars and floods after rain reworks loose ash, for years.

(ii) Climatic cooling from stratospheric sulphate aerosol, as at Tambora in 1815 and Pinatubo in 1991.

(iii) Acid rain, ash-fall health problems such as silicosis, and loss of farmland, water and livelihood, with famine and displacement.

Mitigation is by monitoring, through seismometers, ground deformation (GPS and InSAR), and gas emission, by hazard maps, exclusion zones and evacuation plans. In India, Barren Island is the only active volcano.