Pillow Lava: Formation, Occurrence, Importance

Pillow lava, also known as submarine lava or underwater lava, is a type of volcanic rock that forms when lava is extruded underwater. It is characterized by its distinctive pillow-shaped morphology, which is caused by the rapid cooling of the lava by the surrounding water. 

Pillow Lava Definition

Pillow Lava: A type of lava formation characterized by bulbous, pillow-shaped structures that occur when molten lava erupts into or flows into water, particularly in underwater volcanic environments. As the lava contacts the water, it rapidly cools and solidifies on the outside, forming a crust while the still-molten interior expands or breaks through to form new pillows, creating a stacked or overlapping series of these formations. Pillow lava is most commonly associated with basaltic compositions and is indicative of volcanic activity at places like mid-ocean ridges or near submarine volcanoes.

Formation of Pillow Lava

When lava erupts underwater, it comes into contact with the cold water, which causes the outer surface of the lava to solidify quickly. This forms a thin, glassy rind around the lava, which acts as an insulator and prevents the interior of the lava from cooling too quickly. As the lava continues to flow, it forms a series of pillow-shaped lobes, each of which is surrounded by a glassy rind.

Pillow Lava - Thailand. Photo: Waruto Sama Studio/Shutterstock

Composition of Pillow Lava

Pillow lava is typically composed of basaltic magma, but it can also be composed of other types of magma, such as komatiite, picrite, boninite, basaltic andesite, andesite, dacite, or even rhyolite. The composition of pillow lava can vary depending on the source of the magma and the conditions under which it erupts.

Characteristics of Pillow Lava

Pillow lavas are typically small, ranging from a few centimeters to a meter in diameter. They are often smooth and glassy on the surface, and they may have cracks or fractures that are caused by the expansion and contraction of the lava as it cools. Pillow lavas may also have lineations or scrape marks on their sides, which are caused by friction between the lava and the surrounding rock.

Pillow lavas exhibit a range of characteristics that reflect their unique formation process:

Size: Pillow lavas typically range in size from a few centimeters to a meter in diameter. However, exceptionally large pillow lavas have been observed, reaching up to 10 meters in diameter.

Shape: The defining feature of pillow lava is its distinctive pillow-shaped morphology. These bulbous, spherical, or tubular lobes are formed as the outer rind of the lava solidifies, while the interior remains molten.

Surface Texture: The surface of pillow lavas is often smooth and glassy due to the rapid cooling process. However, it may also exhibit cracks or fractures caused by the expansion and contraction of the lava as it cools. Additionally, lineations or scrape marks may be present on the sides of pillow lavas, resulting from friction between the lava and the surrounding rock.

Internal Structure: The interior of pillow lavas is typically crystalline, reflecting the slower cooling rate of the molten lava within the solidified rind. The internal crystalline structure may vary depending on the composition of the lava and the cooling conditions.

Pillow lava: These diagrams show the formation of pillow lava. The pillows vary in shape but tend to be elongated tube-like structures.

Occurrence of Pillow Lava

Pillow lavas are found in a variety of environments, including:

Mid-ocean ridges: Mid-ocean ridges are the sites of active plate spreading, and they are where most of the Earth's new oceanic crust is formed. The upwelling of magma from the Earth's mantle and its subsequent eruption into the cold abyss of the deep ocean create ideal conditions for pillow lava formation.

Submarine volcanoes: Submarine volcanoes are volcanoes that erupt underwater. They can be found in a variety of locations, including near mid-ocean ridges, on seamounts, and in volcanic arcs. As magma rises from the depths and encounters the cool seawater, it undergoes rapid cooling, giving rise to the characteristic pillow-shaped morphology.

Subglacial volcanoes: Subglacial volcanoes are volcanoes that erupt beneath glaciers or ice sheets. They are relatively rare, but they can be found in a variety of locations, including Antarctica, Greenland, and Iceland.When magma erupts through the ice, it interacts with the frigid meltwater, leading to the formation of pillow structures.

The formation of pillow lava during subaqueous eruptions.

Importance of Pillow Lava

Pillow lavas serve as valuable indicators of past volcanic activity, offering clues about the location and intensity of past eruptions. Their presence in various geological settings helps to reconstruct the history of volcanic processes and the evolution of the Earth's oceans and continents.

Pillow lavas play a crucial role in understanding various geological processes and phenomena:

Indicators of Past Volcanic Activity: Pillow lavas serve as valuable indicators of past volcanic activity, particularly in deep-ocean environments where other evidence may be limited. Their presence provides insights into the location and extent of past underwater eruptions.

Understanding Ocean Crust Formation: Pillow lavas are essential components of the oceanic crust, the layer of rock that forms the Earth's ocean floor. Studying pillow lavas allows scientists to better understand the processes of ocean crust formation and the evolution of the Earth's oceans.

Evidence of Underwater Environments: Pillow lavas provide physical evidence of past underwater environments, including the depth and temperature of the water at the time of eruption. This information is crucial for reconstructing ancient seafloor environments and understanding Earth's paleogeography.

Habitats for Marine Organisms: Pillow lavas provide unique habitats for a variety of marine organisms, including invertebrates, fish, and even some specialized algae. The complex shapes and crevices of pillow lavas create microhabitats that support diverse marine life.

In conclusion, pillow lava stands as a remarkable geological formation,  reflecting the intricate relationship between volcanic processes and  aquatic environments.

A hollow pillow lava that formed during the 2015 eruption. Photo: University of Washington/OOI-NSF/CSSF-ROPOS

Pillow lava formations in Catania, Italy, showcasing the rounded, pillow-shaped structures and the dark, textured surface of the volcanic rock. Photo: Strombolicchia

Pillow Basalt

Pillow Basalt is A specific type of pillow lava where the rock composition is basalt. It forms when basaltic lava is extruded underwater, rapidly cooling upon contact with water to create distinctive pillow-shaped structures. These structures are typically found at mid-ocean ridges, representing the rapid formation of new oceanic crust, or near any underwater volcanic activity where basaltic magma is present. Basalt is a very common type of lava, so most pillow lava is, in fact, made of basalt. 

Pillow basalt typically features a glassy outer surface due to rapid cooling upon contact with water, while its interior can be more crystalline. These structures often show radial cracks formed by the cooling and contracting lava. Although the outer layer is usually dense, the interior might have a vesicular texture with holes from gas bubbles trapped during the slightly slower internal cooling process.

Pillow basalt formations on the central coast of California, near Pismo Beach, featuring the distinctive rounded shapes and dark, textured surface of the volcanic rock.

Pillow lava, Llanddwyn (J Conway)