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

1. 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.

2. 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.

3. 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)

2.EXERCISES

2.1 Find the synonyms to the following words

1. collide	a. dry up
2. disrupt	b. join
3. weld	c. compress
4. sweep (against)	d. appear
5. squeeze	e. pile up
6. bob (up)	f. destroy
7. ruck (up)	g. move
8. shrink	h. clash

2.2 Match the definitions with the correct term.

1. ranges	a. mighty slabs of alien rock
2. mountain systems	b. mountain building
3. orogenesis	c. mountains created by crustal deformation and uplift
4. orogen	d. chains and ranges
5. terranes	e. mountains occurring in rows

2.3 Complete the following short description of mountain building. Scan the text once more.

Orogenesis is the term used to describe (1) ______________. Orogenesis takes place where (2) _______________ plates disrupt the continental crust. Such mountain belts are known as (3) ______________. They are mountain belts created by (4) _____________ and (5) __________. Geologically orogenic belts usually (6) ________ continents. Mountain building is a very complex process. Troughs of (7) ________________ offshore sediments, volcanic rocks, are swept (8) _____________ one continent and then (9) __________ on as mountain (10) ____________.