How Does Lava Build a Pile of Rock "Pillows" Underwater?

1. How Lava Underwater Grows a Skin, Swells, and Splits

Lava that erupts underwater, or flows into water, loses heat fast. The outside of each blob of lava cools rapidly into a thin, glassy skin. The U.S. Geological Survey (USGS) calls this chilled skin a great insulator: inside it, the lava stays molten and can keep inflating the lump. [4]

New lava keeps pushing in from below. Pressure builds until the skin breaks, and more lava squeezes out through the crack. That lava chills into a new skin and swells into a new lump. The USGS glossary describes the cycle repeating until a thick pile builds up. [3]

A simple way to picture it is squeezing toothpaste out of a tube, except that here each new push of lava is wrapped in a skin, so the pile grows as a string of lumps. (This is only a picture, not the USGS's explanation.) The National Park Service (NPS) adds that pillows form especially when the eruption rate is relatively low, and that pieces can break off, roll to the front of the flow, and start new pillows there. [2]

Pillow lava is usually basalt, a dark lava that flows easily, which is why many sources use the name "pillow basalt." [2] So what does the finished rock look like inside?

2. What a Pillow Looks Like When You Cut It Open

Because a pillow cools from the outside in, it records that history. The NPS says pillow basalts are typically glassy because of rapid cooling, and that the few gas bubbles they contain often line up in a radial pattern, spreading out from the center, as the pillow cooled from all sides. [2]

The underside has a clue too. The NPS notes that a "keel," a tail-like point, reaches downward from the pillows. [2] This matters later: a tail that hangs down is a hint about which way was up when the rock formed. The USGS describes whole piles of these shapes: rounded, pillow-sized lumps closely packed on top of each other in thick stacks. [4]

Pillow lava makes a lot of ocean floor. But how do scientists know it is that common, and that it forms the way described above?

3. How Divers Watched Pillow Lava Form in Hawaii in 1971

For a long time, geologists only had a good guess. Most researchers agreed that pillows form when fluid lava chills in contact with water. But in a 1975 paper, USGS geologist James G. Moore wrote that, before his team's study, pillow formation "had never been directly observed." [1]

The chance came from Kilauea volcano on the island of Hawaii. In June 1969, lava from a new vent called Mauna Ulu flowed about 12 km (7.5 miles) down the volcano's flank and entered the ocean. Similar flows reached the sea for a few weeks each year through 1973. [1]

In April 1971, scuba divers became the first to investigate lava flowing underwater. Conditions were harsh: hot water, explosive concussions, strong convection currents, and murky water full of suspended sediment. Even so, the abstract says, "valuable observations were made." [1] That gave people their first close look at lava meeting water.

4. Why Pillow Lava May Be the Most Common Volcanic Rock on Earth

Moore's abstract says most of the new ocean floor created where tectonic plates pull apart, such as along the Mid-Atlantic Ridge, is made of pillowed basalt flows. Submarines looking closely at a depth of 2.7 km (about 1.7 miles) in the ridge's rift valley found that virtually all the lava erupted there is pillowed. The undersea parts of the big ocean volcanoes, including the ridge beneath the Hawaiian Islands, are also built largely of pillow lava. [1]

He concludes that pillow lava is probably the most abundant form of volcanic rock on Earth, though most of it is hidden under the oceans and buried by younger sediment. [1] That helps explain why the shape had gone unwatched for so long: nearly all of it forms in places people almost never see.

5. Why Pillow Lava Shows Up in Lakes and on Dry Land Too

The ingredient is water, not specifically the ocean. The NPS says pillow basalts may form in any underwater setting, including lakes. One example is the Hopi Buttes Volcanic Field near Petrified Forest National Park in Arizona, where many eruptions took place within a shallow lake. [2] The USGS shows a pillow-lava flow in northern California, near the former Copco Lake, and says it tells geologists that the lava entered a body of water and chilled the same way. [4]

This turns pillow lava into a clue. The USGS glossary says that when geologists find pillow basalts in ancient rock, they may conclude the area was once underwater. [3] Some of the seafloor has been lifted onto land by the movement of the plates, and the pillows came along.

The NPS lists several parks where you can find ancient pillow basalts. In Denali National Park in Alaska they erupted about 200 million years ago. In Golden Gate National Recreation Area in California, it was about 190 million years ago. In Olympic National Park in Washington State, the Crescent Formation formed roughly 40 to 50 million years ago. [2] At Kenai Fjords National Park in Alaska, pillows formed 57 to 58 million years ago underwater above a subduction zone, then were attached to the continent. [2]

The keel helps here too. If a rock has since been tilted or flipped, the direction its tails point may be one clue to where "down" was when the lava cooled. This is a reading of the keel description above, not a claim the NPS page makes in so many words.

6. How You Can Look at Pillow Lava Yourself

If you live near one of the parks above, or anywhere with a road cut or sea cliff of dark volcanic rock, look for rounded, cushion-shaped lumps stacked together. Notice whether each lump has a smooth, glassy-looking edge and a pointed tail underneath. Ask a park ranger or a teacher where pillow basalt can be seen nearby, and stay back from cliff edges and loose rock.

Then ask the question the 1971 divers answered: what was here when this rock was liquid? Seeing pillow lava on a hillside can be a clue that the rock formed under water, even though the land is dry today.

Sources

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