Imagine stepping onto a beach, but instead of sand, you find smooth, rounded, almost organic-looking rocks. That's a bit like what you'd encounter if you could somehow visit the ocean floor where lava meets water. These are pillow lavas, and they're far more common than you might think – in fact, they're considered the most abundant geologic landform on Earth's surface.
So, what exactly are these peculiar formations? Pillow lavas are essentially blobs of molten rock, or lava, that erupt underwater. When hot lava encounters the much colder ocean water, its outer surface rapidly cools and solidifies, forming a crust. The still-molten lava inside continues to push outwards, inflating the crust like a balloon. As this happens, the crust can crack, and more lava oozes out, creating a new lobe or 'pillow' attached to the first. This process repeats, building up these distinctive, rounded shapes that resemble stacked pillows or even giant, lumpy cushions.
We've actually seen this happen! Scientists have observed pillow lava forming on the shallow underwater slopes of Hawaii, where lava flows from land spill into the sea. They've also been spotted on seamounts, which are essentially underwater mountains formed by volcanic activity. These direct observations, along with studies of lava found on the seafloor, in ancient volcanic deposits, and even in rocks that were once deep beneath the ocean (called ophiolites), have given us a pretty good understanding of how they form.
These individual pillows aren't always perfectly round; they can be quite squashed or elongated, especially if the lava is flowing down a steep slope. Their surfaces often have interesting textures, too. You might see parallel grooves and ridges, which are like scars left as the pillow expands and its crust breaks. Sometimes, you'll notice smaller, bud-like extrusions, similar to what you'd see on certain types of lava flows on land (called pahoehoe lava).
The growth of a single pillow stops when its outer crust becomes too thick to allow more lava to inflate it. Once that happens, the crust might break, and the flow will find a new path to create another pillow nearby. This tendency for lava to form these discrete, self-contained units means that pillow lava often creates accumulations that don't spread out too far from their original source. Instead, they tend to build up into high-relief mounds, hummocks, and ridges, creating a unique underwater topography.
Interestingly, pillow lavas aren't just found in the deep ocean. They can also form in environments where volcanoes erupt beneath glaciers or ice sheets. In these 'glaciovolcanic' settings, the ice acts much like the water, chilling the lava and causing it to form pillows. These can create distinctive mounds and ridges, often hidden within larger volcanic structures called tuyas. While most pillow lavas are made of basalt (a common type of volcanic rock), it's possible for other types of lava, like andesite or dacite, to form pillows too, though their shape and the resulting landforms might differ due to the different flow properties of these more evolved magmas.
So, the next time you think about volcanoes, remember these fascinating underwater sculptures. Pillow lavas are a testament to the dynamic and often surprising ways our planet shapes itself, creating unique geological features right beneath the waves.
