Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Pillow Lava

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite, dacite or even rhyolite are known. In general, the more felsic the composition, the larger the pillows, due to the increase in viscosity of the erupting lava.

Pillows form when the outer surface of flowing lava is quickly chilled by seawater (Fig.1). Rock is an excellent insulator, so the lava inside this very thin rind remains molten. The surface of the incandescent lava is quickly quenched by seawater, but the internal pressure from the ongoing extrusion of lava at the vent very quickly causes the hard rind to split open, forming a lobe. The tongue continues to lengthen and inflate with more lava until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting lobate shapes that are pillow-like in cross-section. Lava pillows are found in a wide variety of shapes including near-spherical bulbous pillows, flattened pillows, elongate and tubular pillows, and trapdoor pillows. In three-dimensional exposures on land, pillows commonly consist of interconnected tubes. They vary in diameter from several tens of centimeters to several meters, with typical sizes of 0.5-1 m.

pillow2020(2).jpg

Fig.1: Stages, left to right, of the advance of a pillow lava lobe. From Dave Tucker.

The interiors of pillows cool more slowly than the quenched glass rind, and so are more crystalline. Progressive crystallization at slower cooling rates toward the interior produces a variety of rock textures, and the interiors of large pillows may be almost entirely crystalline. Vesicles are common and are usually concentrated in concentric layers parallel to the outer surface. In on-land pillows, radial pipe vesicles are common. Many pillows are completely solid, although it is common for pillows to have central cavities ranging from small tubular channels in a mostly solid pillow to pillows that are entirely hollow. These cavities form when lava drains back out of a newly formed pillow, or when lava pushes aside a portion of the outer pillow crust and drains out through the hole to form a spilled pool outside the broken pillow.

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Boatman's Harbour pillow lavas, Oamaru, New Zeland. From Geotrips.

Chilled margins in pillow lavas, Oamaru, New Zeland. From Stefan Winkler.

Vesicles in a pillow, Mardihalli hill, Chitradurga, India. From R. Thomas.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase and pyroxene (interstitial) crystals in a Pillow. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. XPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)
Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 20x (Field of view = 1mm)
Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a Pillow. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase and magnetite crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase and magnetite crystals in a Pillow. PPL image, 20x (Field of view = 1mm)	Skeletal plagioclase and magnetite crystals in a Pillow. PPL image, 20x (Field of view = 1mm)