Pillow lavas form when lava erupts underwater. Nearly all are basalt, so they are usually called pillow basalts. Most form on the ocean floor, but they can form in any body of water — including lakes.
Pillow basalts in Kenai Fjords National Park, tilted about 45° since they erupted. The keel is the part of a pillow that points downward, filling the space between older pillows below. NPS photo by Chad Hults.
How they form
Pillows form most readily when lava erupts underwater at a relatively low rate. The water instantly chills the outer surface, but lava keeps flowing in, so the blob inflates into a pillow shape. Pillows may break off and roll to the front of the flow as new ones form behind them.
How to recognize them
- Shape: spherical, bulbous or tube-like, rounded on top, with a keel or tail pointing down
- Glassy, because of rapid cooling
- Few gas bubbles (vesicles) — and those present often radiate outward, because the pillow cools from all sides
- Palagonite, a mix of clay and other minerals formed when basaltic glass reacts with water, is common — and helps tell pillows apart from the similar-looking pāhoehoe toes at the front of lava flows on land
Pillow basalt collected from Kamaʻehuakanaloa (formerly Lōʻihi Seamount) at 3,870 ft (1,180 m) depth; it is glassy because seawater quenched it quickly. NPS photo by Ed Shiinoki.
Where they form today
- Mid-ocean ridges and subduction zones
- The underwater first stage of Hawaiian shield volcanoes, such as Kamaʻehuakanaloa, about 22 miles (35 km) off the east coast of the island of Hawaiʻi — the youngest volcano in the chain
- Lakes: in the Hopi Buttes Volcanic Field near Petrified Forest National Park, many eruptions took place in a shallow lake, and pillows formed in the lake that once filled Aniakchak caldera in Alaska
A pillow’s chilled glassy margin and radial vesicles, Kenai Fjords National Park. NPS photo by Chad Hults.
National parks with pillow basalts
| Park | What’s there | Age |
|---|---|---|
| Denali, Alaska | pillows in volcanic rock added to North America, on the Eielson Bluffs at park road milepost 68 | about 200 million years |
| Golden Gate, California | pillows in the Franciscan Complex, altered to greenstone, at Bonita Point in the Marin Headlands | about 190 million years |
| Kenai Fjords, Alaska | near the top of the Resurrection ophiolite, formed above a subduction zone and added to the continent | 57–58 million years |
| Olympic, Washington | in the Crescent Formation on the east and north sides of the Olympic Mountains | about 40–50 million years |
| Petrified Forest, Arizona | in the Bidahochi Formation, laid down in a basin that sometimes held a shallow lake; the nearby Hopi Buttes field is one of the most extensive maar-diatreme fields on Earth | — |
| Yukon-Charley Rivers, Alaska | the spectacular Devonian Woodchopper Volcanics along the Yukon River | Devonian |
Pillow basalt in Petrified Forest National Park, Arizona. NPS photo.
Pillow basalts are also found in Channel Islands, Crater Lake and other parks.
Pillow basalts along the Denali park road on the Eielson Bluffs. NPS photo.
Sources
Based on "Pillow Basalts," National Park Service; a work of the United States government in the public domain. Photographs from the same page. Pictures an earlier version of this page left out were restored on 2026-09-26.
In these publicationsGolden Gate National Recreation AreaDenali National Park & PreserveCrater Lake National ParkGrand Canyon National ParkHawaiʻi Volcanoes National ParkChannel Islands National Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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