At the place marked with the letter A we have a divergent plate boundary. On this type of boundary, the plates are moving away from one another. As they move away, they leave a gap behind them, and the crust is becoming thinner and thinner with lots of cracks in it. Because of that, the high pressures from the mantle manage to push upwards the magma with ease. The magma reaches the seafloor and it is pushing the crust upwards until it breaks it. As the magma pushes the crust upwards,underwater volcanoes are formed, and when the crust is broken by the magma, the magma flows get on the seafloor, cool of very quickly in the water, and create new crust.