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
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
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.
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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) __________ .
READING COMPREHENSION
INFORMATIVE READING: How continents evolve You will read a passage, followed by questions about it. For each question, you need to select the one best answer A, B, C, or D to each question. Answer all the questions following the passage on the basis of what is stated/implied in the passage (R.P – 8.2.2, 8.2.4, 8.2.5, 8.2.6 )
Close study of the rocks of continents reveals ancient cores with progressively younger rocks tacked on to their rims. Each core, or craton, originated as a microcontinent, possibly like this. Two converging, cooling,
horizontal currents in the mantle tugged on a tract of thin, early crustal rock then sank.
5This squashed and thickened that patch of crust. Its base bulged down and melted, releasing light material that punched up through the crust above. Such rock resorting could have formed the first small slabs of continental crust. Later, sea-floor spreading swept island arcs and sediments against microcontinents as mobile belts – belts of deformed and buckled rock. Accretion of this kind formed full-blown continents. 10About 5% of today’s continental crust had formed by 3.5 billion years ago, half by 2.5 billion years ago, most by 0.5 billion years ago. Once formed, continents are not immutable – they can be reworked, but not destroyed. Coalescing produced the supercontinent Pangea about 300 million years ago. Rifting later broke it up. Earth’s crust splits open above “hot spots” – fixed plumes of molten rock rising in the mantle.
15A plume formed the volcanic Hawaiian Islands by punching through the thin oceanic Pacific Plate passing over it. Plumes raise domes in the thick, rigid continental crust. A dome is liable to split in three as cracks grow outward from its top. Where three cracks widen, oceanic rock wells up into the spreading gaps.
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20The continent is split apart and a triple junction then separates three lithospheric plates. If spreading happens only in two cracks, two plates form. The third crack becomes an abandoned trough or rift. Nigera’s Benue Trough and Ethiopia’s Afar Depression are two such so-called aulacogens. (Lambert “The Field Guide to Geology” 1988, Cambridge University Press)
1. What is the main purpose of the passage?
(A) to show how first slabs formed the continental crust
(B) to show how continents evolved
(C) to show how the Earth’s crust split
(D) to show how mobile belts deformed
2. Where in the passage does the author state how the continental crust was
formed?
(A) Lines 3-7
(B) Lines 10-14
(C) Lines 7-11
(D) Lines 18-22
3. It can be inferred from the passage that the phrase “hot spots” refers to:
(A) ash flow
(B) lava flow
(C) cooled magma
(D) molten rocks
4. According to the passage, today’s continental crust was formed
(A) 2.5 billion years ago
(B) 0.5 billion years ago
(C) 3.5 billion years ago
(D) 300 billion years ago
5. According to the passage, full-blown continents were formed by
(A) accretion
(B) depression
(C) subduction
(D) separation
6. The author of the passage implies that the formed continents are
(A) stable
(B) progressive
(C) spreading
(D) immutable
7. What was the necessary conditions for the formation of Pangea?
(A) resorting
(B) subducting
(C) coalescing
(D) converging
8. According to the passage, plumes
(A) raise domes in the continental crust
(B) push molten rocks in the mantle
(C) split the continent apart
(D) deform the rocks
LISTENING COMPREHENSION
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