In this image, we see the part of the oceanic crust just above the sheet dikes complex: The Pillow Basalts. The production of rocks is fed from below by the dikes emerge into the deep sea and erupt on the surface of the seafloor. The bulbous form that we traditionally see is due to the eruption of magma fed from the dikes below, but rapidly cooled on the sea floor. The forms are rounded on top and pinched at the base as they fill the convex space between those basalts already erupted. In the cordillera we can see portions of the Cambrian ocean floor. Image courtesy of Professor Burchfiel.