Hardened lava goes by many names, and the one that applies depends on two things: what the molten rock was made of chemically and how quickly it cooled once it reached the surface. The most common answer is basalt, a dark, dense rock that makes up the bulk of Earth’s ocean floor and most volcanic landscapes. But the same general process that produces basalt can also yield obsidian, pumice, scoria, andesite, rhyolite, and a handful of rarer types, each with a distinct look and feel shaped by the conditions of its formation.
Basalt, the Most Common Hardened Lava
If you pick up a dark, heavy rock near a volcano and it feels dense with a fine, uniform grain, you are almost certainly holding basalt. It forms when lava with relatively low silica content cools at the surface. Because basalt lava is comparatively runny, it can travel long distances before solidifying, which is why basalt flows often cover enormous areas. The mid-ocean ridges that snake along the floors of every ocean are built almost entirely of basalt, making it far and away the most abundant volcanic rock on Earth.
Basalt is typically dark gray to black, and its crystals are too small to see with the naked eye. That fine texture comes from the fact that surface lava loses heat faster than magma sitting underground, so crystals do not have much time to grow. If the same low-silica magma cooled slowly deep beneath the surface instead, it would produce gabbro, a coarser rock with visible mineral grains. The chemical recipe is effectively the same; the cooling environment makes the difference.
How Silica Content Creates Different Rock Types
Geologists sort volcanic rocks along a spectrum defined mainly by how much silica they contain. Silica is the compound behind quartz and glass, and its concentration in the original magma controls both how the lava behaves during an eruption and what kind of rock it leaves behind.
At the low end, basalt contains roughly 45 to 52 percent silica by weight. It flows easily and tends to erupt relatively gently. In the middle of the spectrum sits andesite, named after the Andes mountains where it is common. Andesite forms from intermediate-composition magma typically found at subduction zones, where one tectonic plate dives beneath another. The 2015 eruption of Calbuco volcano in southern Chile, for instance, produced andesitic pyroclastic flows and fallout in a dramatic sub-Plinian event that illustrated how these intermediate magmas behave at active plate boundaries.1Copernicus Publications. Petrology of the April 2015 eruption of Calbuco volcano, southern Chile Andesite is grayer than basalt, often with small visible crystals of feldspar or pyroxene embedded in a finer groundmass.
At the high-silica end, above about 70 percent silica, you get rhyolite. Rhyolite lava is extremely viscous. Its thickness traps gas bubbles as they form inside the melt, effectively building up pressure until the volcano erupts explosively. This is why many of the most violent eruptions in the geologic record are associated with rhyolitic magma. Where rhyolite does manage to flow out quietly, it typically forms short, thick, slow-moving lava domes rather than the sprawling flows basalt produces.
Obsidian Is Volcanic Glass, and It Forms Differently Than Most People Think
Obsidian is the rock most people picture when they think of dramatic volcanic glass: jet-black, shiny, and smooth enough to reflect light. It forms when silica-rich lava solidifies without developing an organized crystal structure, leaving behind a natural glass rather than a typical rock with mineral grains. The popular explanation for decades has been that obsidian results from extremely rapid cooling, the idea being that the melt freezes before crystals can nucleate. Recent research tells a more nuanced story.
A 2025 study published in Nature Communications demonstrated that obsidian actually requires relatively slow cooling, on the order of 10⁻⁴ to 10⁻⁸ degrees Celsius per second, to allow remnant gas bubbles to be resorbed back into the melt. If the cooling were truly rapid, those bubbles would be trapped, producing a frothy rock like pumice rather than the dense, bubble-free glass that defines obsidian.2PubMed Central. Obsidian forms by slow cooling The absence of crystals, it turns out, is not because they did not have time to grow but because the melt composition and conditions suppressed crystal nucleation through a different mechanism than sheer speed. This is one of the more satisfying corrections in volcanology: the textbook explanation was tidy but wrong in a way that careful bubble physics revealed.
What makes obsidian useful, historically and practically, is its fracture behavior. It breaks conchoidally, meaning it fractures along smooth, curved surfaces rather than along flat crystal planes. This property arises because obsidian is a glass with no preferred planes of weakness, unlike crystalline rocks that tend to split along mineral boundaries.3Journal of Human Evolution. How do stone knappers predict and control the outcome of flaking? Implications for understanding early stone tool technology The result is edges sharper than surgical steel. Ancient peoples on every continent where obsidian was available used it to make blades, arrowheads, and scrapers. Some modern surgeons have experimented with obsidian scalpels for the same reason: the edge is thinner than anything you can achieve with metal.
Obsidian does not last forever in its glassy state. Over geological time, it slowly crystallizes, a process called devitrification. The glass gradually rearranges into fine-grained mineral crystals, eventually losing its glossy, conchoidal character. This is why truly ancient obsidian is rare; most obsidian found in the field is geologically young, typically less than a few tens of millions of years old.
Pumice and Scoria, the Bubbly Volcanic Rocks
Not all hardened lava is dense. When gas-rich magma erupts violently, dissolved volatiles come out of solution and form bubbles throughout the melt. If the lava solidifies while those bubbles are still present, the result is a rock riddled with tiny holes, called vesicles. The two most familiar vesicular volcanic rocks are pumice and scoria, and telling them apart is straightforward once you know what to look for.
Pumice is pale, often white or cream, and so full of air pockets that it frequently floats on water. It forms from high-silica, felsic magma, and analyses of pumice from Slamet Volcano in Central Java show silica contents in the range of 60 to 64 percent by weight.4Journal of Applied Sciences, Management and Engineering Technology. Petrology and Geochemical Comparation of Pumice and Scoria Rocks of Slamet Volcano, Central Java Scoria, by contrast, is dark brown to black and comes from lower-silica, mafic to intermediate magma. The same study found scoria from Slamet with silica contents around 49 to 50 percent, firmly in the basaltic range.4Journal of Applied Sciences, Management and Engineering Technology. Petrology and Geochemical Comparation of Pumice and Scoria Rocks of Slamet Volcano, Central Java Scoria is denser than pumice and sinks in water, but it still feels surprisingly light for its size compared to solid basalt.
Both rocks are porous, but the character of that porosity differs. Physical analysis of scoria and pumice samples from volcanic fields in Saudi Arabia showed that both have mesoporous structures, with scoria surface areas varying depending on the specific source location.5Journal of Physics Communications. Physical properties of mesoporous scoria and pumice volcanic rocks That porosity makes both rocks commercially useful. Pumice is widely used as an abrasive in everything from cosmetic exfoliants to stone-washed denim. Scoria is a common landscaping material and is sometimes crushed for use as lightweight aggregate in concrete.
Surface Names for the Same Rock
Sometimes what you call a hardened lava depends not on its chemistry but on how its surface looks. Basalt lava, in particular, comes in two dramatically different surface forms that carry Hawaiian names still used by geologists worldwide.
Pahoehoe (pronounced pah-HOY-hoy) has a smooth, ropy surface that looks like coils of thick rope laid side by side. It forms when lava flows slowly with relatively low viscosity, allowing the outer skin to fold gently as the flow moves. Aa (pronounced AH-ah) has a rough, jagged, clinkery surface that is brutal to walk across. It forms when the same type of lava flows more quickly or is more viscous at the time of eruption. Research on lavas from Izu-Oshima volcano in Japan showed that pahoehoe and aa from that volcano had nearly identical chemical compositions in their groundmass, yet their crystal textures differed by about two orders of magnitude in plagioclase population density. The researchers proposed that a slight difference in the degree of undercooling before final eruption was enough to produce the large textural difference between the two lava types.6Journal of Volcanology and Geothermal Research. Textural difference between pahoehoe and aa lavas of Izu-Oshima volcano, Japan — an experimental study on population density of plagioclase In other words, the same magma can produce either surface type depending on subtle differences in eruptive conditions.
When lava erupts underwater, it takes on yet another distinctive form: pillow lava. Contact with water chills the outer surface almost instantly, forming a glassy rind, while the interior stays molten and continues to advance, bulging out into rounded, pillow-shaped lobes. A study of the Watuadeg pillow lava in Yogyakarta, Indonesia, used crystal size distributions to estimate how quickly different parts of the pillows cooled. The outer rim solidified in roughly 12 hours, while the core took around 91 hours, confirming the steep temperature gradient between the water-chilled exterior and the insulated interior.7Journal of Applied Geology. Cooling history (from magma ascent to lava extrusion) of the Watuadeg pillow lava, Berbah, Yogyakarta, Indonesia Pillow lavas are a key indicator that geologists use to identify ancient underwater eruptions in rock sequences that are now on dry land.
Columnar Jointing and the Geometry of Cooling
One of the most visually striking things that can happen to hardened lava is columnar jointing, the formation of tall, roughly hexagonal columns that look like they were carved by a sculptor. The Giant’s Causeway in Northern Ireland and Devils Tower in Wyoming are famous examples. These columns form when a thick basalt flow cools and contracts. As the rock shrinks, stress builds up, and cracks propagate inward from the cooling surface in a pattern that naturally tends toward hexagonal geometry because that shape distributes stress most evenly.
Modeling work has shown that these fractures propagate through the cooling lava in a way that can be described using nonlinear elasticity, with the crack network fragmenting the flow into prismatic columns as it advances.8Mathematical Models and Methods in Applied Sciences. A Model of Columnar Jointing The size of the columns depends on how quickly the lava cools: faster cooling produces thinner columns, and slower cooling produces thicker ones. Columns can range from a few centimeters across in rapidly cooled flows to over a meter in thick, slowly cooled sills. The process is not limited to basalt, but basalt’s uniformly fine-grained texture makes it especially prone to forming clean, regular columns.
Lavas That Break the Mold
Most volcanic rocks fit neatly into the basalt-to-rhyolite spectrum defined by silica content. A few do not, and they are among the most fascinating products of volcanism.
Carbonatite lava is the most extreme outlier. Instead of being made primarily of silicate minerals, carbonatite is dominated by carbonate minerals, the same family of compounds found in limestone. Only one volcano on Earth currently erupts carbonatite: Oldoinyo Lengai in Tanzania. Its sodium-rich lava is strikingly different from any silicate lava. Temperature measurements taken during an active eruptive period in 1988 found that carbonatite flows ranged from about 491 to 519 degrees Celsius, with the highest reading of 544 degrees Celsius measured at a lava lake. Those temperatures are several hundred degrees lower than any silicate lava. At those temperatures, carbonatite melt was less viscous than even the most fluid basaltic lavas, making it runnier than any other known lava on Earth.9PubMed. Temperature measurements in carbonatite lava lakes and flows from Oldoinyo Lengai, Tanzania When carbonatite lava solidifies and is exposed to rain, it weathers rapidly, turning from black to white within weeks, because the sodium carbonate minerals react with water. The rock looks nothing like what most people imagine when they think of lava.
Komatiite occupies the other end of the unusual-lava spectrum. These are ultramafic volcanic rocks with very high magnesium oxide content, up to about 30 percent MgO, which means the original melt had to be extraordinarily hot, likely above 1,600 degrees Celsius, to stay liquid. Komatiites are characterized by a distinctive texture called spinifex, where elongated, blade-like or skeletal olivine crystals form during cooling, creating a pattern that looks somewhat like frost on a window. Almost all known komatiites date to the Archaean eon, more than 2.5 billion years ago, when Earth’s interior was significantly hotter than it is today.10Journal of Volcanology and Geothermal Research. Komatiites: An early precambrian phenomenon The enhanced geothermal gradient of early Earth is the most widely accepted explanation for why such ultra-hot eruptions occurred then and essentially stopped as the planet’s mantle gradually cooled. Finding komatiite in the field is a strong signal that you are looking at very ancient crust.
Lava on Other Worlds
Hardened lava is not exclusive to Earth. The Moon’s dark, flat maria, the features that make up the “face” visible from your backyard, are enormous basalt plains formed by ancient volcanic eruptions billions of years ago. Lunar basalt is similar in broad composition to its terrestrial counterpart but differs in detail: it tends to have higher iron and titanium content and formed in an environment with no water and almost no oxygen, which affects its mineral chemistry.
Mars has its own volcanic basalt on a staggering scale. Olympus Mons, the largest known volcano in the solar system, is a shield volcano built from basaltic lava flows stacked over hundreds of millions of years. The Martian surface is littered with features that resemble terrestrial lava tubes, collapsed channels, and flow fronts, all pointing to a volcanic past that may have persisted until geologically recent times. Venus, beneath its thick cloud cover, appears to be almost entirely resurfaced by basaltic volcanism, and there is radar evidence suggesting that some volcanic activity may still be happening there today.
Even the icy moons of the outer solar system have a version of volcanism. On Jupiter’s moon Io, sulfur-rich lavas erupt at temperatures high enough to suggest ultramafic or at least very mafic compositions, making Io the most volcanically active body in the solar system. Saturn’s moon Enceladus and Neptune’s moon Triton show cryovolcanism, where the “lava” is water, ammonia, or nitrogen ice rather than molten rock. The hardened product looks nothing like basalt, but the underlying process of hot material erupting onto a cold surface and solidifying is the same basic phenomenon that gives us every volcanic rock on Earth.