Pillow lava

Pillow lavas are special kinds of rock that form when hot, molten rock, called lava, flows underwater. Because the water is very cold compared to the hot lava, the lava cools quickly and forms round, pillow-like shapes. These pillow-shaped masses can be as big as a meter across, which is about the height of a tall adult.

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

Composition

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

Occurrence

Pillow lavas form when lava flows underwater, such as near underwater volcanoes and along the edges where tectonic plates move apart. They are part of the ocean floor's structure and help scientists understand when water existed on Earth long ago. These lavas can also appear near volcanoes that erupt under ice.

Formation

Pillow lavas form when magma reaches the surface underwater. Because the lava is much hotter than the water, the outer layer cools quickly, creating a hard skin. More lava flows into this skin, making it grow like a balloon. When the pressure inside gets too high, the skin breaks, and new lava starts to flow out. This process repeats, creating many pillow-shaped lumps. The outside of each pillow cools fast, becoming very fine and glassy, while the inside cools slower and is a bit coarser.

Use as a way-up criterion

Pillow lavas help scientists figure out how rocks were positioned when they formed long ago. 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.