Constructive plate boundaries 1) Convection currents in the mantle move the plates 2) 2 plates of oceanic crust diverge/pull apart e.g. North American and Eurasian plates. 3)
The rising magma forces the ends of the plate to push up and buckle.
This creates tensional cracks on the underside of the plates. 4) Magma is squeezed into the gap between the two plates and is cooled by the ocean to form new, solidified rock/basalt. 5) Rising magma forces its way through the tensional cracks and forms sub-marine shield volcanoes on the ocean floor. 6)
With repeated eruptions over millions of years they can grow until
they break the surface of the ocean and become volcanic islands e.g.
Iceland.
