Pillow lava is formed when hot magma extrudes from below the Earth's crust through injection dykes (vents) in the plates that make up the crust. Extruded lava cools rapidly when it contacts water, which promotes the forming of balloon-like or pillow shapes. the size of the pillow depends on how rapidly the lava extrudes.  Rapid extrusion causes a series of bubble bursts' Pillow lava can also form under glaciers. Quartz and serpentine  are common minerals found in igneous pillow lava rocks. At Port San Luis in Avila Bay, the bluffs north of the pier display an amazing array of pillow lava formations.