Pillow Basalt
Pillow basalt is basaltic lava that erupted underwater and cooled into rounded, pillow-shaped lobes with glassy rinds. It covers much of the sea floor and is a signature of submarine volcanism, even in rocks now exposed on land.
Pillow basalt is basaltic lava that erupted underwater and cooled into rounded, pillow-shaped lobes with glassy rinds. It covers much of the sea floor and is a signature of submarine volcanism, even in rocks now exposed on land.
🪨 Geology Guide Specimen: Open Pillow Basalt in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Underwater Eruption: How Do Pillows Form?
When lava meets cold water, its surface chills instantly into a thin glassy skin. Fresh lava keeps pushing out through breaks in the skin, forming a new lobe, and the process repeats, stacking sack-like pillows tens of centimeters to a meter or more across. Radial cracks form as the pillow's interior contracts.
Ocean Crust: Where Are Pillow Lavas?
Pillow lavas form the upper layer of oceanic crust at spreading ridges, and they cap seamounts and submarine volcanoes. They sit above the dikes and gabbro of the deeper crust, and appear on land in ophiolites and in places where the sea floor has been uplifted.
Ancient Evidence: What Do Pillows Tell Geologists?
Finding pillows in very old rocks shows that lava erupted underwater at the time, as in Archean greenstone belts billions of years old. Pillows also sag downward at their bases and are pinched between neighbors, so they show which way up the layers were originally, even if folded.
Pillow Basalt at a Glance
| Property | Value |
|---|---|
| Type / variety | Subaqueous Quenched Pillow Lava |
| Composition | 48–51% SiO₂ • Glassy Sideromelane Rind + Radial Cooling Joints |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Mafic Volcanic (Basaltoids) |
| Mohs hardness | Mohs 6 |
| Texture / habit | Bulbous lobes with glassy chilled rinds and radial cracks |
| Density | 2.90–3.00 g/cm³ |
| Lustre and cleavage | Dark aphanitic interior with dark glassy outer skin |
| Geological setting | Mid-ocean ridge spreading centers and submarine seamounts |
| Diagnostic feature (Geology Guide) | Stacked, bulbous sack-like lobes of dark fine-grained lava with dark glassy outer skins and radial contraction cracks, formed when molten rock extrudes directly into cold ocean water. |
| Rind | Glassy chilled skin |
| Setting | Submarine eruptions |
See Also
- Basalt — the rock pillow lava is made of
- Pahoehoe Basalt — basalt flowing in air instead of water
- Gabbro — the deeper layer of ocean crust