Pillow lava

Pillow lava

Pillow lavas are special rocks that form when hot, molten rock, called lava, flows underwater. The cold water makes the lava cool quickly, forming round, pillow-like shapes. These pillows can be as big as a meter across, about the height of a tall adult.

You can find pillow lavas on the ocean floor. They are important for scientists who study the Earth because they help us understand how the ocean floor was formed and how it changes. Pillow lavas are a key part of Layer 2 of the ocean floor's rocky surface.

Composition

Pillow lavas are usually made of a type of rock called basalt. Sometimes they can be made from other rocks like komatiite, picrite, boninite, basaltic andesite, andesite, dacite, or rhyolite. When the lava has more silica, it can make the pillows bigger because the lava doesn’t flow as easily.

Occurrence

Pillow lavas form when lava flows underwater. This happens near underwater volcanoes and where tectonic plates move apart. They are part of the ocean floor and help scientists learn about water on Earth long ago. These lavas can also appear near volcanoes under ice.

Formation

Pillow lavas form when magma reaches the surface underwater. The lava is much hotter than the water, so the outside cools quickly and hardens. More lava flows inside, making it grow like a balloon. When the pressure inside becomes too high, the skin breaks and new lava flows out. This process repeats, creating many pillow-shaped lumps. The outside of each pillow cools fast and becomes fine and glassy, while the inside cools more slowly and is a bit coarser.

Use as a way-up criterion

Pillow lavas help scientists learn how rocks were positioned long ago when they formed. By looking at the shape of pillow lavas, we can tell which way was "up" when the lava cooled underwater.

Here are three clues that show the original position:

1. Small holes or bubbles called vesicles are usually found toward the top of a pillow because gas rises.
2. The top of each pillow is often rounded or convex.
3. The bottom of a pillow might be pointy or tapered, fitting against the pillow below it.