CONTINENT–CONTINENT COLLISION

CONTINENT–CONTINENT COLLISION

Antarctica

By about 55 million years ago,subduction had consumed

(d) 40 million years ago

all of the oceanic lithosphere between India and Asia (Fig. 12–28c). Then the two continents collided. Because

Figure 12–27 (a) Gondwanaland and Laurasia formed both are continental crust,neither could sink deeply into

shortly after 200 million years ago as a result of the early the mantle. Igneous activity then ceased because sub-

breakup of Pangea. Notice that India was initially part of duction had stopped. The collision did not stop the north- Gondwanaland. (b) About 120 million years ago, India broke off from Gondwanaland and began drifting northward. (c) By

ward movement of India,but it did slow it down to about 80 million years ago, India was isolated from other continents

5 centimeters per year. and was approaching the equator. (d) By 40 million years ago, Continued northward movement of India was

it had moved 4000 to 5000 kilometers northward and col-

(a) Continental

Sediments and sedimentary

Oceanic

India crust

rocks on continental rise

crust

Tibet

and slope

Lithosphere

Forearc Volcano (b)

Rising magma

Zone of

Asthenosphere

melting

Volcanism ceases when (c)

Folds and thrust faults

in sedimentary rocks

India begins to underthrust

(d) Initial thrust

Figure 12–28 These cross-sectional views show the Indian trench and subduction complex had formed. Volcanoes and Asian plates before and during the collision between India

erupted, and granite plutons formed in the region now called and Asia. (a) Shortly before 120 million years ago, India, south-

Tibet. (c) By 40 million years ago, India had collided with Tibet. ern Asia, and the intervening ocean basin were parts of the

The leading edge of India was underthrust beneath southern same lithospheric plate. (In this figure, the amount of oceanic

Tibet. (d) Continued underthrusting and collision between the crust between Indian and Asian continental crust is abbreviated

two continents has crushed Tibet and created the high to fit the diagram on the page.) (b) When India began moving

Himalayas by folding and thrust faulting the sedimentary rocks. northward, the plate broke and subduction began at the

India continues to underthrust and crush Tibet today. southern margin of Asia. By 80 million years ago, an oceanic

The Himalayan Mountain Chain: A Collision Between Continents 215

began to underthrust beneath Tibet. As a result,the formed when the range was an Andean margin. Rocks of thickness of continental crust in the region doubled.

all types were metamorphosed by the tremendous stresses Thick piles of sediment that had accumulated on India’s

and heat generated during the mountain building process. northern continental shelf were scraped from harder base-

The underthrusting of India beneath Tibet and the ment rock as India slid beneath Tibet. These sediments

squashing of Tibet have greatly thickened continental were pushed into folds and thrust faults (Fig. 12–28d).

crust and lithosphere under the Himalayas and the Some of the deeper thrusts extend downward into the

Tibetan Plateau to the north. Consequently,the region basement rocks.

floats isostatically at high elevation. Even the valleys lie The second way in which India continued moving

at elevations of 3000 to 4000 meters,and the Tibetan northward was by crushing Tibet and wedging China out

Plateau has an average elevation of 4000 to 5000 meters. of the way along huge strike–slip faults. India has pushed

One reason the Himalayas contain all of the Earth’s high- southern Tibet 1500 to 2000 kilometers northward since

est peaks is simply that the entire plateau lies at such a the beginning of the collision. These compressional

high elevation. From the valley floor to the summit, forces have created major mountain ranges and basins

Mount Everest is actually smaller than Alaska’s Denali north of the Himalayas.

(Mount McKinley),North America’s highest peak. Mount Everest rises about 30 meters from base to summit,whereas Denali rises about 4200 meters. The

THE HIMALAYAS TODAY difference in elevation of the two peaks lies in the fact

Today,the Himalayas contain igneous,sedimentary,and that the base of Mount Everest is at about 50 meters, metamorphic rocks (Fig. 12–29). Many of the sedimen-

but Denali’s base is at 2000 meters. tary rocks contain fossils of shallow-dwelling marine

Comparisons of older surveys with newer ones show organisms that lived in the shallow sea of the Indian con-

that the tops of some Himalayan peaks are now rising tinental shelf. Plutonic and volcanic Himalayan rocks

rapidly—perhaps as fast as 1 centimeter per year. If this

Figure 12–29 Wildly folded sedimentary rocks on the Nuptse–Lhotse Wall from an

216 CHAPTER 12 G E O L O G I C S T RU C T U R E S , M O U N TA I N R A N G E S , A N D C O N T I N E N T S

rate were to continue,Mount Everest would double its contend that it was largely,but not completely,molten. height in about 1 million years,a short time compared

In either scenario,the Earth was hot and active about 4.5 with many other geologic events. However,normal fault-

billion years ago. Magma rose to the surface and then ing throughout the range is evidence that the mountains

cooled to form the earliest crust. From the evidence of a are oozing outward at the same time that they are rising.

few traces of old ocean crust combined with calculations If the newly formed,steep mound of honey discussed

of the temperature and composition of the earliest upper earlier were covered with a layer of brittle chocolate

mantle,geologists surmise that the first crust was com- frosting,the frosting would crack and slip apart in nor-

posed of a type of ultramafic rock called komatiite. mal faults as the honey spread outward. The upper few

Komatiite is the volcanic equivalent of peridotite—the kilometers of rocks of a rapidly rising mountain chain

rock that now makes up the upper mantle. (Recall from such as the Himalayas are like the frosting,and normal

Chapter 4 that ultramafic rocks have even higher mag- faulting is common in such regions. As blocks of rock

nesium and iron concentrations than basalt,which is a slide off the mountains,they compress adjacent rock near

mafic rock.)

the margins of the chain. In this way,normal faults in one According to one hypothesis,heat-driven convection region frequently form thrust faults and folds in a nearby

currents in a hot,active mantle initiated plate movement region. But,at the same time,tectonic forces resulting

in this early crust. Dense komatiites dove into the man- from the continent–continent collision continue to push

tle in subduction zones,where partial melting of the up- the mountains upward. No one knows when India will

permost mantle created basaltic magma. As a result,the stop its northward movement or how high the mountains

oceanic crust gradually became basaltic. will become. However,we are certain that when the rapid

When did the earliest continental crust form? The uplift ends,the destructive forces––normal faulting and

3.96-billion-year-old Acasta gneiss in Canada’s North- erosion––will lower the lofty peaks to form rolling hills.

west Territories is the Earth’s oldest known rock. It is metamorphosed granitic rock,similar to modern conti- nental crust,and implies that at least some granitic crust