If you’ve ever looked at a picture of the ocean floor or a slice of ancient rock, you might have seen structures that look like piles of sandbags or stone pillows. This unique formation is known as pillow lava. It’s a fascinating type of volcanic rock that tells a clear story about where and how it formed: underwater. This article will explain everything you need to know about this incredible geological feature, from how it’s made to where you can find it.
What Is A Pillow Lava
Pillow lava is a rock formation created when basaltic lava erupts under water or flows into it. As the hot lava meets the cold water, its outer surface chills almost instantly, forming a thin, flexible glassy skin. The lava continues to flow inside this skin, inflating it like a balloon until the pressure breaks it. Then, new lava spills out and forms another “pillow” right next to it. This process repeats, creating piles and mounds of these characteristic bulbous, pillow-shaped lobes.
The Science Behind the Shapes
So why do they form these specific shapes? It all comes down to physics and cooling rates. The instant cooling creates a viscous, elastic rind that contains the fluid lava inside. The lava doesn’t just flow freely; it’s extruded through a tube-like system. Each new pulse of lava pushes out through the broken crust of the previous pillow, creating a chain of connected lobes. The final size of a pillow is usually between 30 cm and about 1 meter in diameter, though they can vary.
Key Conditions for Formation
- Subaqueous Eruption: The lava must erupt under a significant body of water, like an ocean, lake, or even under ice.
- Basaltic Composition: The lava needs to be mafic (rich in magnesium and iron), like basalt. This type has the right viscosity—fluid enough to flow but not too runny—to form pillows.
- Rapid Cooling: The drastic temperature difference between the lava (over 1100°C) and the water (often near freezing) is essential for the quick crust formation.
Where Can You Find Pillow Lavas?
You can find pillow lavas in two main contexts: modern seafloor settings and ancient rock sequences on land. The most abundant pillow lavas on Earth are forming right now along mid-ocean ridges, where tectonic plates are pulling apart. Underwater volcanoes (seamounts) also produce them. On land, you find them in “ophiolite” sequences. These are slices of ancient ocean crust that have been pushed up onto continents by tectonic forces. Places like Cyprus, Newfoundland, and the California Coast Ranges have spectacular examples.
Step-by-Step: How a Pillow Lava Forms
- Eruption Begins: Basaltic lava erupts from a vent on the seafloor.
- Instant Crusting: The lava’s outer surface contacts cold seawater and freezes into a glassy hyaloclastite skin within seconds.
- Inflation: Molten lava inside continues to flow, inflating the flexible crust like a water balloon.
- Rupture: Pressure builds until the skin ruptures, usually at the tip or a weak point.
- Extrusion: A new lobe of lava extrudes through the rupture, and the process repeats, forming a chain or pile.
- Stacking: Hundreds to thousands of these events build large mounds or sheet-like flows of pillow lava.
Identifying Features and Characteristics
When you look at an outcrop of pillow lava, there are several telltale signs to look for. Recognizing these will help you identify them in the field or in photographs.
External Morphology
- Bulbous and Elliptical Shapes: They look like stacked sacks, pillows, or sometimes like toothpaste squeezed from a tube.
- Radial Cracks: The outer surface often has a pattern of radial and concentric cracks from the cooling and contraction.
- Glassier Rind: The very outer edge is a quickly cooled glassy margin, often paler in color.
- Tube Connections: You might see smaller “toes” or tubes connecting one pillow to the next.
Internal Structure
If you break open a pillow, you’ll see a distinct internal layering. The very edge is the fine-grained glassy crust. Inside that, you often find radial fractures that fan inward from the crust. The very center of the pillow is usually the coarsest-grained, as it cooled the slowest. Sometimes, you can even see vesicle bands—layers of tiny gas bubbles—that show how the pillow inflated in pulses.
Why Are Pillow Lavas So Important to Geologists?
Pillow lavas are not just cool-looking rocks; they are critical pieces of evidence for understanding Earth’s history. Their presence in a rock formation is a definitive indicator of an underwater volcanic environment. This helps scientists reconstruct ancient geographies, proving that an area now high in mountains was once deep under an ocean. They are a key component of ophiolites, which are our primary window into the composition and structure of the oceanic crust we can’t easily visit.
Furthermore, studying their chemistry helps us understand mantle processes and the cycling of elements through the Earth’s system. They also provide clues about the temperature and depth of the water they erupted into. In the search for ancient life, the porous interiors of pillow lavas are thought to have provided protected habitats for early microbes, making them a target in the study of astrobiology on other planets.
Pillow Lavas Beyond Earth
Incredibly, we have evidence of pillow-like structures on other planetary bodies. Satellite imagery and rover data suggest that pillow lavas may exist on Mars. The conditions in Mars’s past, with possible widespread water oceans or lakes, could have been perfect for their formation. Some scientists also interpret certain features on the Moon and Venus as possible submarine volcanic deposits, though the evidence is less clear. Finding pillow lavas on another planet would be strong proof of past standing water.
Common Misconceptions and Clarifications
Let’s clear up a few common mix-ups about pillow lavas. First, they are not formed from highly viscous lava like rhyolite; that lava is too sticky and would form a dome, not a pillow. Second, while they are most famous from deep-sea settings, they can form in any water deep enough to prevent the water from flashing to steam explosively—this includes lakes and shallow seas. Finally, the spaces between pillows are not empty; they are filled with fragmented debris called hyaloclastite, which is bits of the glassy crust broken off during formation.
How to Study Them: Field and Lab Techniques
Geologists use a combination of methods to study pillow lavas. In the field, they map their distribution, orientation, and size. They carefully log the sequence and look for way-up indicators (like the convex-up shape of pillows) to understand the original orientation of the rock layers. Samples are collected for thin section analysis under a microscope to study mineralogy and texture. Geochemical analysis in a lab determines the exact elemental composition, which reveals the lava’s source and history.
Frequently Asked Questions (FAQ)
What is the difference between pahoehoe lava and pillow lava?
Both are formed from fluid basaltic lava, but the environment is key. Pahoehoe forms on land in air, creating smooth, ropy, or billowy surfaces. Pillow lava forms underwater (or under ice), creating the distinct stacked, bulbous shapes due to rapid water cooling.
Can pillow lavas form in freshwater?
Yes, absolutely. The critical factor is the presence of a significant volume of water to cause rapid chilling, not the water’s salinity. Pillow lavas have been documented in ancient lake bed deposits.
Are pillow lavas a type of igneous rock?
Yes, they are a volcanic igneous rock. More specifically, the rock itself is almost always basalt. “Pillow lava” describes the distinctive shape and structure the basalt formed into, not a different rock type.
How do you know if a rock is pillow lava?
Look for the classic pillow shapes—rounded, bulbous lobes stacked together. Check for a glassy rind and radial fracture patterns on the surface. The context is also a huge clue; if the surrounding rocks are deep-sea sediments like chert, it’s a strong indicator.
What does the presence of pillow basalt indicate?
It is one of the best indicators of an ancient submarine eruption environment. If you see pillow basalt in a mountain range, it tells you that those rocks were once part of an ocean floor that was later uplifted by tectonic forces.
Why are they called pillow lavas?
They are called that because their rounded, overlapping shapes closely resemble a pile of sacks or pillows. The term was first used by geologists in the late 19th century who saw the resemblance.
Conclusion: Windows into Earth’s Underwater Fires
Pillow lavas are a perfect example of how geology connects process, environment, and form. Their unique shape is a direct snapshot of a dramatic moment—scalding lava meeting icy water. They serve as a crucial tool for scientists, acting as a definitive signpost for ancient oceans and helping us piece together the puzzle of plate tectonics and Earth’s evolution. Next time you see a picture of these strange formations, you’ll know your looking at a frozen moment from a deep-sea volcanic eruption, a fundamental building block of the oceanic crust beneath our waves.