THE TWO STEPS OF HIMALAYAN GROWTH

had formed by that time.

The Himalayan chain developed first as an Andean-type Geologists have found grains of a mineral called zir- margin as oceanic crust sank beneath southern Asia. At

con in a sandstone in western Australia. Although the that time,the geology of southern Asia was similar to the

sandstone is younger,the zircon gives radiometric dates present geology of the Andes. Only later,after subduc-

of 4.2 billion years. Zircon commonly forms in granite. tion had consumed all the oceanic crust between the two

Geologists infer that the very old zircon initially formed continents,did India and Asia collide. The two-step na-

in granite,which later weathered and released the zircon ture of the process is common to all continent–continent

grains as sand. Eventually,the zircon became part of the collisions because an ocean basin separating two conti-

younger sedimentary rock. Thus,these zircon grains sug- nents must first be consumed by subduction before the

gest that granitic rocks existed 4.2 billion years ago. continents can collide.

Geologists have also found granitic rocks nearly as old The Himalayan chain is only one example of a moun-

as the Acasta gneiss and the Australian zircon grains in tain chain built by a collision between two continents.

Greenland and Labrador.

The Appalachian Mountains formed when eastern North According to one model,the earliest continental America collided with Europe,Africa,and South

crust formed by partial melting in oceanic subduction America between 470 and 250 million years ago. The

zones. Recall that island arcs form today by a similar European Alps formed during repeated collisions be-

mechanism. Thus,the first continents probably consisted tween northern Africa and southern Europe beginning

of small granitic or andesitic blobs,like island arcs or about 30 million years ago. The Urals,which separate

microcontinents,sitting in a vast sea of basaltic crust. Europe from Asia,formed by a similar process about

Modeling suggests that about 40 percent of the pres- 250 million years ago.

ent continental crust had formed by 3.8 billion years ago, and 50 percent had formed by 2.5 billion years ago. Thus,continental crust accumulated rapidly early in

䊳 12.7 THE ORIGIN OF CONTINENTS

Earth history and more slowly after the end of Archean time. Geologists cannot calculate the rate of formation of

Most geologists agree that the Earth formed by accretion continental crust precisely because they are not certain of planetesimals,about 4.6 billion years ago. However,

how much continental crust is recycled back into the little evidence remains to trace our planet’s earliest his-

mantle at subduction zones.

tory. Some geologists argue that the entire Earth melted Most new continental crust now forms in subduction and was covered by an extensive magma ocean. Others

zones; a small amount forms over mantle plumes. Does zones; a small amount forms over mantle plumes. Does

Sediment

Andesite

Asthenosphere Lithosphere

Folded sedimentary and metamorphic rocks

New granite

Figure 12–30 According to one model, the modern continents formed as island arcs su- tured together. During the suturing, sediments eroded from the original islands were com- pressed, folded, and uplifted. Some were subjected to so much heat and pressure that they metamorphosed. New granite formed from partial melting of the crust at the subduction zone.

the evolution of continental crust early in Archean time However,other calculations and scant paleomag- suggest that modern-style plate tectonics had begun that

netic evidence suggest that Archean plates moved at early? Again,conflicting models have been proposed.

about the same speed as modern plates—between 1 and Some geologists stress that the early Archean mantle was

16 centimeters per year. If Archean plates moved as 200 to 400 degrees hotter than today’s mantle. The high

slowly as modern plates do,how did the volume of temperature should have caused rapid convection in the

Archean continental crust grow so rapidly? Another mantle and fast plate movements involving many small

model suggests that early growth of continental crust, tectonic plates. In support of this model,some geologists

and perhaps even of oceanic crust,occurred mainly by point out that most Archean rocks are folded and

“plume tectonics”—production of both basaltic and sheared—a style of deformation that forms at modern

granitic magma over rising mantle plumes (Fig. 12–31). convergent plate boundaries. They infer from this rea-

“Horizontal tectonics” became the dominant process only soning that horizontal plate movement has dominated

in late Archean time,after the mantle had cooled and tectonic activity from the beginning of Archean time to

convection slowed. Modern plate tectonics is dominated the present (Fig. 12–30).

by horizontal plate movements.

218 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

Figure 12–31 Another hypothesis contends that early continental crust formed over rising mantle plumes, in a process called “plume tectonics.”

Granite

Partial melting of upper mantle produces granitic magma

Lithosphere

Mantle plume spreads out at base of lithosphere

Rising mantle plume

Asthenosphere

THE TWO STEPS OF HIMALAYAN GROWTH