Pillow lava is formed when lava is rapidly cooled in the ocean. Pillow lava is steep slides of igneous rock that are filled with broken lava fragments that will result in shallow-water eruptions. It is formed because the lava, when coming in contact with the water, rapidly cools in certain spots when it remains hot in others. This is what causes pockets of lava to form as other lava around it hardens, and turns into igneous rock.
