A plate tectonic constructive (divergent) margin is marked by a ridge with a rift valley running down the middle of it. It is also associated with shallow focus earthquakes, high heat flow and vulcanicity. Where a constructive plate margin occurs on land, the land masses on either side of it are moving apart at a rate which can be measured directly, as in Iceland (usually at a rate of a few centimetres per year). Try our latest Earthlearningidea, 'A 'mantle plume' in a beaker'. Ask your students to try to visualise what might be happening out of sight, below the plate margin. Then explain that this activity tries to model some of the processes. Remind them that the mantle is essentially solid, but that we shall be using a viscous liquid, in order to speed things up to fit the time scale of the lesson!
There are many other plate tectonics activities on our website - just search the Keyword Index.