(a) Elucidate the types of glacier and illustrate the erosional and depositional features produced by the valley-glaciers. (20…

(a) Types of glacier and landforms of valley glaciers

A glacier is a perennial mass of ice that flows under its own weight. On form, glaciers are of these types: (i) valley (alpine) glaciers, confined to valleys, like Gangotri, Zemu and Siachen in the Himalaya; (ii) cirque glaciers in bowl-shaped hollows; (iii) piedmont glaciers, where valley glaciers spread out at a mountain front; (iv) ice caps and ice sheets, which bury the land and flow outwards, as in Antarctica and Greenland; and (v) tidewater glaciers that end in the sea.

Erosional features. Ice erodes by plucking, where meltwater freezes in joints and lifts blocks, and by abrasion, where rock debris in the ice scours the bed, leaving striations and polish.

(i) Cirque is an armchair-shaped hollow at the valley head, often holding a tarn.

(ii) Arête is a knife-edge ridge between two cirques, and a horn is a pyramid peak where several cirques meet, like the Matterhorn.

(iii) U-shaped valley has a wide flat floor and steep walls, with truncated spurs and hanging valleys whose streams fall as waterfalls.

(iv) Roche moutonnée is an abraded bedrock hump, smooth on the up-ice side and plucked on the down-ice side.

(v) Fjord is a glacial trough drowned by the sea.

Depositional features. Till is unsorted glacial debris.

(i) Moraines are lateral (along the sides), medial (where tributary glaciers merge), terminal or end (at the snout) and ground moraine.

(ii) Erratics are boulders moved far from their source.

(iii) Eskers are sinuous ridges of sand and gravel left by subglacial streams, and kames are mounds of meltwater deposits.

(iv) Outwash plains and kettles lie beyond the terminus. Drumlins are streamlined till hills, better developed under ice sheets.

A cross-section and a plan of the valley glacier should show these features.

(b) Origin, composition and classification of meteorites

Meteorites are fragments of extraterrestrial bodies that survive passage through the atmosphere. Most come from the asteroid belt between Mars and Jupiter, broken off by collisions. A few come from the Moon and Mars, thrown out by impacts. They formed about 4.56 Ga, so they record the early solar system and are used to date the Earth.

Composition. They consist of silicates (olivine, pyroxene), iron–nickel alloys (kamacite, taenite) and sulphide (troilite). Some carbonaceous types also hold water-bearing minerals and organic compounds.

Classification. It rests on the proportion of metal to silicate and on texture.

(i) Stony meteorites are the commonest group. Chondrites contain chondrules, small spherules of once-molten silicate, and are primitive and undifferentiated; they are carbonaceous, ordinary (H, L, LL) or enstatite types. Achondrites lack chondrules and are igneous rocks from differentiated bodies, including lunar and Martian meteorites.

(ii) Iron meteorites are Fe–Ni alloy. Etching shows the Widmanstätten pattern, from intergrown kamacite and taenite, and they represent the cores of disrupted parent bodies.

(iii) Stony-irons contain both phases: pallasites have olivine crystals in metal, and mesosiderites are breccias of silicate and metal.

They are also described as falls, seen to land, or finds.

(c) Rock cleavage and its types

Rock cleavage is a closely spaced, parallel planar fabric produced by deformation, along which the rock splits. It differs from bedding, which is depositional, and from mineral cleavage, which is an atomic plane in a crystal. Cleavage forms perpendicular to the axis of maximum shortening (σ₁), so in folded rocks it lies nearly parallel to the axial planes and is called axial-plane cleavage. It cuts across the bedding at an angle, except in the fold hinges.

(i) Slaty cleavage is very fine, continuous and perfect, with flakes of mica and chlorite aligned parallel, as in slate.

(ii) Phyllitic cleavage is coarser and gives a silky sheen, in phyllite.

(iii) Schistosity is coarse, continuous and due to visible platy minerals, as in schist.

(iv) Crenulation cleavage is spaced and formed by micro-folding of an earlier fabric.

(v) Fracture (shear) cleavage consists of closely spaced parallel fractures and is common in brittle, less metamorphosed rocks.

Cleavage is continuous where the whole rock is reoriented and spaced where cleavage domains alternate with unaltered microlithons. Cleavage is useful in finding fold geometry: the angle between cleavage and bedding shows the position on the fold limb, and their intersection gives a lineation. Cleavage refracts on passing between beds of different competence.