Pillow lavas are mound-like tubular, nearly spherical, or pillow-shaped structures associated primarily with submarine or underwater basaltic volcanism. They usually measure 0.1-1 meter or more and may have a smooth, corrugated, striated, or cracked glassy rind with a more crystalline interior.
These structures often occur with other submarine eruptions, i.e., sheet flows, lobate lava, hyaloclastites, and ponded flows.
To give you a little bit of historical background, pillow lavas were first identified in the 1870s. Later, in 1902, Tempest Anderson, a British geologist, became the first person to observe an eruption, and by 1914, their origin was established.
Still, geologists didn’t know that they covered much of the ocean and were the most abundant volcanic rocks until the 1960s.
How they form is no longer an issue. Many scuba-diving geologists have documented their formation in Hawaii from the Kilauea volcano. You will even find videos showing how they develop as molten lava slowly enters and continues flowing under the water.
Learn about pillow lavas, including their appearance, characteristics, and morphology. We will also cover how and where they form.
What are pillow lavas?
Pillow lavas are near-spherical, tubelike, or pillow-shaped, mostly basaltic structures or mounds. These rock masses are usually 0.1m to 1m or larger and form from low-discharge effusive subaqueous eruptions.
However, they can be rounded, ellipsoidal, sack-like, spherical, flattened, elongated, oval, tongue-like, trapdoor-shaped, etc. Also, they may form an irregular pile, intertwined, branched, or lobed network of rock masses.
Besides the normal pillowy emplacement, some ancient emplacements may have thin, rugged, sheet-like flows known as para-pillow lava. These kinds form when extruded lava fails to inflate. Reasons for failure to inflate include imbalances in cooling rate, rate of flow, magma supply rate, or steep slopes with unsteady magma supply.
Although pillow lavas may appear as fluid-filled balloons, their cross-section will show interconnecting tubes. These tubes form from repeated budding as the pillows advance unless they break during emplacement.
The upper surface of these pillows is convex and slightly flat due to pressure from the water or ice above as they form. In contrast, their lower is concave with cusps or deformation.
Cusps, indentations, or deformations on the lower part form as the newly formed pillow, which is still fluid internally and has a plasticky crust, fills gaps or depression above preexisting solidified pillows.
It is worthwhile noting that lava discharge rates influence the structure of the pillows. For instance, low rates produce elongated ones with striation as the crust becomes thick and brittle, allowing budding to occur via irregular fractures. In contrast, quenched skin remains thinner without cracks at a higher discharge rate, allowing a smoother surface.
Let us talk more about pillow lava surface and internal morphology, hyaloclastites, and how they differ from pahoehoe.
1. Surface appearance
The pillow lava surface has basaltic (sideromelane and tachylyte) glass rinds less than an inch thick formed from the rapid quenching of hot lava.
These rinds may be smooth or have corrugations, linear grooves, striations or expansion, and contracting cracks or jointings.
For instance, the Juan de Fuca ridge has large, elongated pillows with striated surfaces. However, emplacements in other places may have smooth surface lobes resembling pahoehoe toes.
According to Gill (2010), irregular edges of cracks cause corrugations on extruded lava as a new pillow form. It is more like how squeezing mayonnaise or toothpaste from a tube takes the shape of the tube’s mouth.
Also, the surface may have small (5-15cm) smooth, often ellipsoidal or ovoid pillow buds. These buds result from lava leaks, often on the lower side. However, knobby pillow lava will have elongated buds and fingers on its entire surface.
2. Internal morphology
Slower cooling allows the interior of pillow lavas to be crystalline, with larger ones more crystalline. Thus, we can quickly identify these pillows by looking at their cross-section.
Their cross-section will show circular to elliptical concentric zoned fabric or banding. This zoned fabric or textural banding occurs due to progressive varying cooling rates from the outer glassy surface.
Also, they often show concentric vesicles parallel to the surface and radial fractures (cooling jointing) perpendicular to the surface.
In most cases, pillow lavas are almost entirely solid. However, some may have central cavities. These cavities vary from small tubular channels to mostly hollow to altogether hollow.
These voids form when lava drains from newly formed pillows or outflows from skin fractures. If such outflows occur on overhanging pillows, they can form rope-like structures that may be several meters long.
Lastly, some pillow lava may have narrow tubular shelves formed from discrete drainage.
3. Hyaloclastites
Hyaloclastites surround many pillow lavas. These are angular detrital basaltic glass fragments formed from abrupt quenching lava.
The rapid and dramatic cooling when hot lava directly encounters cold water causes contraction stress. This stress results in thin, brittle, glassy rind surrounding pillows.
However, hyaloclastites are unstable. Thus, they may undergo alteration to form palagonite, a green-brownish, orangish-brown, or yellowish mass, filling interstices or spandrels between pillows.
As Carey (2010) notes, not all pillows have hyaloclastites. Some don’t have because of insulating steam from boiling water that forms around the chilled crust, making it remain plastic and cool slowly.
However, natural instability and flow current may collapse this steam layer. This allows water to encounter molten lava, forming hyaloclastites.
4. Pillow lava vs. pahoehoe
Pillow lava is analogous to subaerial lava flow known as pahoehoe. They both have interconnecting tubes and may have similar-looking toes and a billowy appearance.
However, pillow lavas have a few vesicles, a convex top, and a concave bottom with cusps and often lack ellipsoidal tubes. In contrast, the pahoehoes have open ellipsoidal tubes and radial pipe vesicles, and their lower part molds to the surface.
Also, pillow lavas have mostly smooth, cracked, striated, or corrugated surfaces, while pahoehoe has a surface with more character. It may be ropy, swirly, hummocky, or gently undulating.
Composition
Most pillow lavas form from low-viscosity basaltic lava. However, Archean pillows are from komatiitic lavas.
Also, they can occur in picrite, basaltic andesite, andesite, boninite, dacite, and rhyolite lava flows. For instance, a few andesitic pillows occur at Semail Ophiolite in Orman.
However, andesitic and more silicic lavas like dacitic and rhyolitic rarely form subaqueous pillows. Instead, they will form volcanic breccia as the rapid cooling causes fracturing.
Where do pillow lavas form?
Most pillow lavas occur in effusive submarine eruptions with low discharge flow rates. Higher discharge rates will form lobate lava or sheet flow with increasing effusion rates.
Besides submarines, they can also form in subaqueous conditions not as deep as oceans, such as when lava flows into seas (observed in Hawaii), rivers, lakes, or water. However, high discharge rates in such settings may result in steam explosion forming scoria instead.
Lastly, they can form beneath glaciers. Mount Murphy in Antarctica, is an example of subglacial basaltic pillow and hyaloclastite breccia emplacement.
How does pillow lava form?
Pillow lavas form when highly fluid, molten lava effusively erupts from fissures in a subaqueous environment with a low discharge rate.
Upon encountering water, it rapidly cools, forming a plastic glassy rind or skin that temporarily prevents the advance of still-molten interior lava.
The injection of more lava under the skin will make it inflate. This forms a rounded, spherical, pillow-like, or elongated lava blob.
However, with time, the skin will cool and become brittle, i.e., it cannot expand. Any more injected lava will break the skin and extrude incandescent lava that forms the next pillow.
The process goes on, producing numerous successive breakouts characterized by intermitted moments of pulses (when skin restrains flow) and flows (when the skin breaks to form a new lobe).
New lobes can form in front or sides of preexisting pillows. Also, they can change direction and create interconnected complex structures with multiple pillows spreading laterally. Some may even stack on each other.
Winter (2014) explains observed pillow formation off Hawaii’s coast at a 20° slope, ten meters under water. During the observation, toothpaste-like molten lava protrudes from cracks in the previous pillow. This molten lava may then detach, roll down, or remain connected, producing another bud. However, formation in flat terrain is unrecorded.
Lastly, flowage downwards and outwards may make pillows elongate from their weight. Also, a continuous lava supply will form tubes that will create a new pillow if they meet an obstacle.
Here is a video of pillow lava forming underwater:
Where are pillow lavas found?
Pillow lavas form much of the sea or ocean floors. They also occur in midocean ridges and seamounts and are part of ophiolite sequences.
Here is more on where these pillows are found:
1. Ocean floor
These pillows occur on ocean floors beneath sediments, above sheeted dikes, and gabbro. They may appear as intertwined networks of irregular piles 1-5 meters high, conical piles 5–20 meters tall, or pillow volcanoes 100-200 meters high, notes Batiza & White (2000).
2. Mid-ocean ridge basalts (MORB)
Pillow lavas occur in mid-ocean ridges basalts, especially on slow-spreading ridges. Here, they may form hammocks about 50 meters high and 500 meters in diameter and hummocky ridges 1-2 km long.
Furthermore, they can create small seamounts tens to hundreds of meters high and hundreds to thousands of meters in diameter, according to Perfit & Davidson (2000).
Besides slow spreading centers, these structures can occur together with sheet flow in moderate spreading ridges.
Lastly, fast-spreading ridges have mostly sheet flows, especially those with high discharge rates and broad cross sections. However, a few pillows may occur in seamounts flanked by lava sheets. Also, they can be abundant a few kilometers from the axis where the magma supply is low.
3. Seamounts
Pillow lava also occurs in seamounts or pillow volcanoes in hotspots, near tectonic plate boundaries, and mid-ocean ridges. Those that are on steep-side seamounts will have tubular pillows-oriented downslope.
However, seamounts may have sheet flows, lobate tubes, and ponded flows depending on effusion rates and volumes.
La Palma (Spain) is an example of such a seamount.
4. Ophiolites and greenstone belts
Ophiolites like Vishnu Schist in the Grand Canyon, USA, and Troodos in Cyprus have pillow lavas. Others like Cedros Island (a back arc-basin fill in Mexico), Archean Abitibi Greenstone Belt in Canada, and Troodos have pillow seamounts.
Pillow lava significance
Pillow lavas are evidence of submarine volcanism. Also, when on land, they indicate the area was once underwater. It may have undergone uplift or obduction (forms ophiolites).
Secondly, their convex top and concave lower side with cusps may help know the orientation of tectonically tilted terrain with pillow lava sequence.
Lastly, some thick (several kilometers) pillowy lava sequences may contain economically valuable volcano-hosted massive sulfide deposits.
References
- Batiza, R. & White, J. D. L. (2000). Submarine lavas and hyaloclastite. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. pp. 364-367) San Diego: Academic Press.
- Carey, S. N. (2010). Understanding the physical behavior of volcanoes. In Marti, J., & Ernst, G. (Eds). Volcanoes and the environment (1st ed. pp. 29-43). Lighning Source, UK, Ltd.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Paula-Hahn, M.M., Meschede, M. and Blakey, R. (2011) Plate tectonics: Continental Drift and mountain building. Heidelberg: Springer.
- Perfit, M. R. & Davidson, J. P. (2000). Plate tectonics and volcanism. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. pp. 96-98) San Diego: Academic Press.
- Geshi, N. (2005). Lava. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5, pp. 326-328). Elsevier Academic.