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Pillow lava gets its name from the way it looks — like pillows. These lava formations typically form as large, fat, domed shapes on the ocean floor. Almost all pillow lava formations can only be observed on the ocean floor because the ocean is the only place on Earth that provides the right conditions for them to occur. Volcanic activity doesn’t just happen on dry land, it also occurs underwater. The combination of the heat from the magma and the frigid temperatures on the ocean floor create an environment of extremes. These opposing conditions, when forced to interact, create pillow lava.
Earth’s crust is made up of hundreds of plates that move and press against each other. The cracks between these plates often offer the perfect place for magma to bubble to the surface of the crust. Pressure sometimes builds up beneath these plates and is released through the cracks in the form of gasses, which are usually followed by molten rock. When the superheated rock makes contact with the icy cold of the ocean floor, it hardens almost immediately.
Though the lava creates a relatively hard crust when it surfaces underwater, the magma behind it is still pressure-driven through the cracks. Instead of breaking the crust formed by the lava, the magma usually pushes the crust outward to create a bubble-shaped formation. The longer the pressure builds under the formation, the larger the bubble gets. Often, underwater explorers observe vast fields of these formations, all fused together at the edges. The explorers call this phenomenon pillow lava because it looks like a field covered in round ottoman-style pillows.
Underwater volcanoes don’t usually produce pillow lava because the magma rises to the top of a cone in large quantities and has a little time to spread out before it cools. This more often results in the creation of young islands or underwater mountain ranges. Pillow lava almost always occurs over flat fields.
Occasionally, the pressure beneath a field of hardened lava will become so great that some of the bubbles break open. The result usually looks something like a snowman because the magma creates bubbles of descending sizes stacked on top of each other. When this activity goes on for a long time, the pillow lava fields begin to look a bit like caves full of stalagmites. Generally, the pressure can only make formations that rise just a short distance from the ocean floor. Pillow lava doesn’t usually develop into volcanoes or small islands.