Describe the landforms associated with constructive plate margins and explain their formation

At a constructive margin two plates separate or diverge. There are two types of constructive margins. Firstly, where plates move apart in oceanic areas they produce mid-oceanic ridges. Secondly, where they move apart in continental areas they produce rift valleys. 

Mid-oceanic ridges: the rising convection currents in the asthenosphere push up the crust and cause rifting or cracking. This forces the two plates to diverge creating a central rift. New crust is created as the plates diverge because magma rises to the surface along this fissure from the asthenosphere. The lava which erupts is low viscosity basaltic lava so creates gentle sided shield volcanoes as it can flow a long way before it cools. Occasionally the submarine volcanoes may grow above sea level to form volcanic islands. Iceland is the largest example on the Mid-Atlantic Ridge. As the sea floor spreads so the age of the island increases away from the central rift. The central rift contains transform faults. They form at right angle to the plate margin. The parts of the spreading  plates on either side of the transform faults may be moving at different rates leading to friction and ultimately to shallow earthquakes, originating near the surface. 

Rift valleys: occur at constructive margins in continental areas. The heating and up-doming of the crust leads to fracturing and rifting. As the sides of the rift move apart, central sections drop down to form rift valleys. Active volcanoes are surface evidence of volcanic activity beneath the rift valley. The largest rift valley is the Great East Africa Rift Valley which extends 4000km long and is 50km wide. The rift valley might widen further allowing the sea to inundate it, possibly cutting off part of East Africa.