Pay attention to the terms and expressions in bold

MOUNTAIN BUILDING (R.P – 8.2.7, 8.2.8, 8.2.9)

Large regions of the Earth consist of mountains. Most occur in rows called ranges. Parallel ranges and intervening plateaus form chains such as the Andes and North American Cordillera. Related mountain chains and ranges make up mountain systems, notably the Tethyan (Alpine-Himalayan) and Circum-Pacific systems.

Orogenesis, or mountain building, occurs along mobile belts – places where colliding lithospheric plates disrupt the continental crust. Such mountain-building belts are known as orogens and orogenic belts are belts of fold mountains – mountains created by crustal deformation and uplift. Geologically recent orogenic belts mostly rim continents. But ancient orogenic belts (the Ural Mountains for example) can occur deep inside a continent where lithospheric plates were welded together long ago.

Fig. 46. Mountain building

Mountain building is a complex process. Deep troughs of accumulated offshore sediment, volcanic rocks, bits of oceanic crust, and scraps of foreign continents can all be swept against one continent and welded on as mountain ranges. Most of mountainous western North America consists of more than 50 suspect terranes – mighty slabs of alien rock that independently rotated and migrated north along the western edge of North America.

Here are three major mountain-building processes.

1. Oceanic plate subduction below another oceanic plate – this process created the Aleutian Islands and other mountainous island arcs.

Fig. 47. Ocean-ocean subduction zone

2. Oceanic plate subduction beneath a continent– involving island-arc collision, this process helped produce the Andes.

Fig. 48. Ocean-continent boundary

3. Double continent collision – the way in which the Alps and Himalayas formed.

Fig. 49. Continent-continent collision

Describe the mountain building process, using the following information and the diagrams in R.P 8.2.7, 8.2.8, 8.2.9

Island-arc orogeny

a. subducted oceanic crust

b. low outer island-arc of sediments squeezed by subducting oceanic crust

c. inner island-arc of mountainous volcanoes produced by the “bobbing up“ of light, subducted, melted oceanic crust and sediments.

Cordilleran belt orogeny

a. island-arc (c) and continent (d) with offshore sediments (b) advance on two plates, one subducting below the other.

b. collision squeezes and rucks up sediments (b) between island-arc volcanoes (c) and continent (d), producing a сordilleran mountain chain such as the Andes.

c/d. the subduction zone is replaced by a new one.

Colliding continents

a. continents (d) advance on separate plates

b. collision rucks up marginal sediments (b) and the ocean shrinks.

c. the oceanic crust between is subducted and the two continents collide, forming mountain ranges like the Alps / the Himalayas

(Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

EXERCISES