What Happens at Constructive Plate Boundaries Constructive plate boundaries occur where plates diverge (move apart) and where new earth’s crust is constructed as a result of this divergence. A constructive (divergent) plate boundary occurs in the mid-Atlantic area where the


American plates move away from the Eurasian and African plates. Figure 11 explains what happens.


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Convection currents in the earth’s mantle pull in opposite directions and create tension (stretching) on the earth’s crust. This tension is so great that the crust is torn apart. Numerous cracks called fault lines and fissures appear on the crust. Earthquakes are common as fault lines and fissures are formed.


Magma from the earth’s mantle emerges steadily up through the fissures and helps to push the plates apart. The magma hardens quickly and forms new oceanic crust. The new crust becomes gradually wider as the plates continue to move apart. This process is called sea-floor spreading. It happens at about the same rate as your fingernails grow.


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The rising magma builds up a great mid-ocean ridge called the Mid-Atlantic Ridge. Most of this ridge remains below sea level. But some of it rises so high that it forms islands such as Iceland.


Some magma emerges more violently through individual pipes to form many hundreds of volcanic mountains. Most of these mountains lie beneath the Atlantic.


Did you know? Iceland:


l is made up of two different tectonic plates


l grows a little bit wider each year (Why?)


l contains more than 200 volcanic mountains.


Atlantic Ocean Mid-Atlantic Ridge Faults and fissures 2 Atlantic Ocean 1 1 4 Volcanic mountains


Mid-Atlantic Ridge


10 Iceland 3 principal active volcano


You learned about sea-floor spreading on page 3.


North American Plate


Eurasian Plate


Krafla I c e l a n d Reykjavik


Convection current 8


Magma wells up


Convection current


11 A constructive plate boundary in the Mid-Atlantic. Give the locations of two other constructive plate boundaries. Use Figure 6 to help you.


North American Plate


Eurasian Plate


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