Tectonic Plates Cross Section

Figure 2.03. Schematic cross section of plate tectonics. A simplified cross section of important features of tectonic plates and their interactions by José F. Virgil and Robert I. Tilling (Simkin et al., 2006, This Dynamic Planet). Click on the image to see a larger version with more information.

2.3 Why? Divergent plate boundaries

Why do igneous rocks form at divergent plate boundaries? Figure 2.03 above is a schematic cross section of the earth's surface from This Dynamic Planet. Two types of divergent plate boundaries are shown in Figure 2.03, a mid-ocean ridge and a continental rift. In Figure 2.03, reddish magma drops are drawn beneath the two types of divergent boundaries shown, but no explanation for the magma is evident. What do you believe is the reason for volcanoes at divergent plate boundaries? Choose from the following list and press "Enter":

    Pick one.
    The earth's mantle is a liquid (magma). Fracturing of the crust at rifts allows the molten mantle to rise through the crust to the surface.
    Convection moves the mantle upward beneath divergent boundaries. Pressure is lowered on the rising mantle, causing it to melt and feed volcanoes.
    Sea water seeps into the mantle through the faults at divergent boundaries and causes the mantle to melt and feed volcanoes.
 None of the above.
        Make your choice and press "enter."