The correct answers are:
- A crack forms in oceanic crust;
- New material pushes older rock away from the ridge;
The spreading of the seafloor occurs at the places where there's divergent plate boundary. In 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 much thinner. Because the crust is much thinner, the magma from the mantle is able to easily push upwards through the cracks created by the high pressures from bellow. As the magma reaches the seafloor it cools off very quickly, thus creating new crust. Because there's constant magma flows coming from bellow, the new material is pushing the older crust away, which is why the youngest crust is found at the mid-ocean ridges, while the oldest at the subduction zones.