(a) Explain 'convergent plate boundary' with suitable examples. Add a note about the characteristics of earthquakes at the…

(a) Convergent plate boundary

A convergent boundary occurs where lithospheric plates collide, causing one plate to be forced beneath another or crust to shorten. In oceanic–continental convergence, dense oceanic lithosphere subducts beneath lighter continental crust, producing a volcanic arc and mountain belt, as in the Andes where the Nazca Plate subducts beneath South America. In oceanic–oceanic convergence, subduction forms an island arc, as in the Mariana and Aleutian arcs. In continental–continental convergence, neither plate readily subducts; crust thickens, folds and faults, as in the Himalaya where India collides with Asia. The subducting slab bends into the mantle, defining a Wadati-Benioff zone of inclined seismicity.

Earthquakes at such boundaries are frequent and are the largest on Earth: shallow megathrust events on the plate interface (Chile 1960, Mw 9.5, Sumatra-Andaman 2004, Mw 9.1), intermediate events within the bending slab, and deep-focus events to about 700 km inside the cold slab. In continental collision zones seismicity is shallow and diffuse, with thrust events on the Main Himalayan Thrust such as Gorkha (Nepal) 2015, Mw 7.8. Subduction can also generate tsunamis through seafloor displacement.

(b) Raster and vector data

Raster data represent the Earth as a grid of cells, each holding a value such as elevation, temperature, or spectral reflectance. It is well suited to continuous phenomena and remote-sensing imagery, where every pixel is sampled. Its advantages are simple storage, easy overlay, and rapid processing of large areas; disadvantages include loss of precision at boundaries, larger files for high resolution, and scale-dependent detail. Vector data represent discrete features as points, lines, and polygons, with coordinates and topology defining adjacency, connectivity, and containment. They are ideal for roads, rivers, administrative boundaries, and mapped geology. Advantages include compact storage for sparse features, exact geometry, and efficient network analysis; disadvantages include complex data capture, difficulty representing continuous fields, and topology maintenance. In GIS, raster supports image classification and surface modelling (for example ResourceSat LISS-IV scenes or CartoDEM on ISRO’s Bhuvan portal), while vector supports cadastral mapping and spatial querying (for example GSI’s digital geological map layers). Raster is better for continuous surface modelling; vector for precise boundary mapping.

(c) Drainage patterns

Drainage pattern is the arrangement of streams controlled by structure, lithology, relief, climate, base level, and time. Dendritic patterns, like branches of a tree, develop on uniform, homogeneous rock with little structural control; the Ganga plain shows a dendritic network. Trellis patterns occur where parallel resistant ridges are cut by cross-valleys, commonly in folded regions; the strike valleys of the Siwalik and Lesser Himalayan ranges are an Indian example. Radial patterns radiate from a central high, such as a volcanic dome or structural uplift; the Amarkantak plateau, source of the Narmada, Son and Johila, is the Indian example. Rectangular patterns develop along intersecting faults or joints, giving right-angle bends, as in many faulted crystalline terrains. Parallel patterns occur on uniform slopes or tilted strata, with streams flowing side by side, as on the Western Ghats escarpment. Lithology controls resistance, structure controls orientation, relief and base level control gradient, while climate and vegetation affect runoff and erosion. Base-level fall can rejuvenate streams and modify the pattern.

(d) Drag folds

A drag fold is a small, asymmetric fold formed in a weak layer by shear during development of a larger fold. In a neat sketch, draw a major fold with an incompetent shale layer sandwiched between competent beds. The sketch should show the major hinge, dragged minor hinges, and competent beds. On the limbs, the shale is dragged into small folds whose hinges are inclined toward the direction of movement. Viewed down the plunge, Z-shaped minor folds lie on the left limb of an antiform and S-shaped folds on the right limb, with symmetric M-shapes at the hinge, and the reverse for a synform. This is Pumpelly’s rule: minor folds mirror the geometry of the major fold, so mapping the change from S to M to Z locates the major hinge.

Because drag folds record the sense of shear on the limbs, they help determine the vergence of the major fold, identify overturned or recumbent folds, and infer thrust or nappe transport. They are especially useful where the major fold is eroded or buried, because the minor folds preserve the kinematic history of the deformation, as in mapping the large folds of the Aravalli-Delhi belt.

(e) Unconformities

Structures showing a gap in the stratigraphic sequence are unconformities, surfaces of erosion or non-deposition. They record uplift, erosion, or interrupted sedimentation. An angular unconformity occurs when older beds are tilted or folded, eroded, and overlain by younger horizontal beds; the erosional surface and angular discordance are diagnostic. A disconformity separates parallel strata but represents a hiatus caused by erosion or non-deposition; it may be marked by a paleosol, channel, or fossil gap. A nonconformity separates sedimentary rocks from crystalline or metamorphic basement, as at the Eparchaean Unconformity of Tirupati, where Cuddapah sedimentary rocks rest on Archean crystalline basement. A paraconformity is a subtle conformable contact where deposition was interrupted without obvious erosion, recognised mainly by missing fossils or radiometric ages.

Field criteria include an erosional surface, basal conglomerate, weathered horizon, paleosol, and stratigraphic or fossil gap. Such surfaces are economically important because they can trap hydrocarbons, host mineral deposits, or define aquifer boundaries.