Lava: Formation, Geology & Varieties
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Lava: From Mantle Melt to Volcanic Rock
Lava is magma that reaches Earth’s surface, loses heat and gas, and becomes volcanic rock. Its final form depends on how the melt was generated, where it erupted, how much silica and gas it carried, and whether it cooled in air, under water, beneath a crust, or as airborne fragments.
What counts as lava?
Lava is molten or partly molten rock that erupts at the surface. While it is still below the surface it is called magma; once it emerges from a vent, fissure, or fracture, it becomes lava and begins to cool into extrusive igneous rock.
Rapid cooling gives lava its characteristic fine-grained, glassy, or porous textures. Dense basalt, vesicular scoria, pale pumice, glossy obsidian, blocky dome rock, and rounded submarine pillow lava can all be volcanic products, even though they look dramatically different. Their differences come from melt chemistry, gas content, temperature, viscosity, crystal content, and cooling environment.
Lava flow
A coherent body of molten rock moving across the surface. Basaltic flows may travel far; silica-rich flows are usually short, thick, and steep-sided.
Lava fragment
A piece of lava thrown, spattered, torn, or broken from a flow. Bombs, spatter, cinders, and scoria preserve the motion and gas content of eruption.
Lava glass
A quenched melt that cooled too quickly for crystals to grow. Obsidian and tachylite are important glassy volcanic materials.
How magma forms
Magma forms when conditions allow solid rock to partially melt. The three major pathways are decompression, volatile addition, and heat transfer.
Decompression melting
Hot mantle rises and pressure drops faster than the material cools. This allows partial melting without needing a large temperature increase. Decompression melting feeds mid-ocean ridges, continental rifts, and many hotspot systems, commonly producing basaltic magma.
Flux melting
Water and other volatiles released from a subducting plate lower the melting point of the overlying mantle wedge. This process is central to volcanic arcs, where andesitic and dacitic magmas are common.
Heat-transfer melting
Hot mafic magma intrudes cooler crust and transfers heat into it. In continental settings this can help generate silica-rich melts, including rhyolitic magma associated with calderas, domes, and obsidian-bearing systems.
How magma evolves before eruption
After melting begins, magma may change through fractional crystallization, assimilation of surrounding rock, magma mixing, volatile loss, and storage in crustal reservoirs. These processes help explain why one volcanic province may erupt basalt, andesite, dacite, and rhyolite at different times.
Tectonic settings
Lava composition and eruption style are strongly tied to tectonic setting. Each setting supplies a different balance of heat, pressure, water, crustal interaction, and melt storage.
| Setting | Melting process | Typical lava products | Geological expression |
|---|---|---|---|
| Mid-ocean ridges | Decompression melting of upwelling mantle. | Tholeiitic basalt, pillow lava, sheet flows, dikes. | Creation of oceanic crust and submarine volcanic ridges. |
| Subduction zones | Flux melting from slab-derived water and volatiles. | Basalt, andesite, dacite, rhyolite, domes, blocky flows. | Island arcs, continental arcs, stratovolcanoes, and explosive centers. |
| Hotspots | Decompression melting in mantle plumes or long-lived thermal anomalies. | Basaltic shields, alkalic basalts, lava tubes, pāhoehoe, ʻaʻā. | Ocean islands, shield volcanoes, and long volcanic chains. |
| Continental rifts | Extension, decompression, and crustal heat transfer. | Basalts through rhyolites, obsidian flows, domes, and alkaline lavas. | Rift valleys, fissure systems, volcanic fields, and caldera complexes. |
| Large igneous provinces | High-volume mantle melting and fissure eruption. | Flood basalts, thick flow sequences, lava plateaus. | Layered volcanic plateaus and broad basalt provinces. |
Chemistry, temperature, and viscosity
Silica content is one of the strongest controls on lava behavior. Low-silica basaltic lava is hotter and more fluid; high-silica rhyolitic lava is cooler, stickier, and more likely to trap gas or quench into glass.
| Lava type | Typical SiO2 | Typical eruption temperature | Relative viscosity | Common products |
|---|---|---|---|---|
| Basaltic | About 45-52 wt% | About 1100-1250 °C | Low | Pāhoehoe, ʻaʻā, lava tubes, sheet flows, pillow lava, scoria. |
| Andesitic | About 52-63 wt% | About 900-1100 °C | Medium | Blocky flows, composite-cone lavas, spatter, breccias. |
| Dacitic | About 63-69 wt% | About 800-950 °C | High | Short thick flows, domes, spines, pumiceous margins. |
| Rhyolitic | Greater than about 69 wt% | About 650-850 °C | Very high | Obsidian, pumice, flow-banded lava, domes, coulees. |
Why gas changes everything
Volatiles such as water, carbon dioxide, and sulfur dioxide dissolve in magma at depth. As magma rises and pressure falls, these volatiles form bubbles. If the lava is fluid, gas can escape more easily. If the lava is viscous, gas may remain trapped, producing pumice, explosive fragmentation, or pressure-driven dome growth.
Surface and submarine flow styles
Lava-flow style is a direct expression of viscosity, slope, effusion rate, cooling rate, crystal content, and crust formation. Basaltic systems can produce both smooth and jagged forms, while silica-rich lavas commonly build short, thick, blocky masses.
Pāhoehoe
Fluid basalt develops a thin, flexible crust that wrinkles and folds as lava continues moving beneath it. The result is smooth, ropy, billowed, or shelly surfaces.
ʻAʻā
A disrupted basaltic flow breaks into angular clinker and moves with a rough, grinding surface. It commonly forms when lava is cooler, more crystalline, or moving under higher strain.
Blocky flows
Andesitic to rhyolitic lava often forms thick flows with fractured block surfaces. Their interiors may remain hot and ductile while outer carapaces break into angular slabs.
Lava domes
Very viscous dacitic or rhyolitic lava may pile up near a vent instead of flowing far. Domes can grow as lobes, spines, or coulees, and their collapse may generate block-and-ash deposits.
Pillow lava
Underwater eruption quenches lava into rounded lobes with glassy chilled rims. Pillows record submarine or subglacial eruption and are common in oceanic basalt.
Lava tubes
A basalt flow may crust over while liquid lava drains through a thermally insulated interior. When the flow empties, it can leave a cave-like tube.
Geological varieties of lava
Lava varieties are best understood as combinations of composition and texture. A name such as basalt, andesite, or rhyolite describes chemistry and mineralogy; a name such as scoria, pumice, obsidian, or pillow lava describes texture or eruption environment.
| Variety | Composition or process | Visible character | What it records |
|---|---|---|---|
| Basalt | Mafic, low-silica lava. | Dark, fine-grained, sometimes vesicular or porphyritic. | Hot, fluid lava common at ridges, hotspots, rifts, and flood-basalt provinces. |
| Andesite | Intermediate lava, often associated with arcs. | Gray to brown, commonly porphyritic, blocky, or brecciated. | More viscous lava influenced by water-rich subduction systems and crustal evolution. |
| Dacite | Silica-rich intermediate to felsic lava. | Light gray to brown, blocky, dome-forming, sometimes pumiceous. | High viscosity, high gas retention, and short, thick flows or domes. |
| Rhyolite | High-silica lava. | Pale to reddish, flow-banded, glassy, pumiceous, or dome-forming. | Silica-rich melts that cool as obsidian, pumice, domes, or banded flows. |
| Obsidian | Rapidly quenched volcanic glass, usually rhyolitic. | Glossy black, brown, gray, or banded glass with conchoidal fracture. | Cooling so rapid that crystals did not have time to grow. |
| Scoria | Gas-rich mafic to intermediate lava fragments. | Dark, red, or brown porous rock with thick bubble walls. | Degassing, oxidation, and cinder-producing eruption styles. |
| Pumice | Gas-rich felsic lava expanded into frothy glass. | Pale, highly vesicular, lightweight material that may float initially. | Volatile-rich explosive or effusive silicic activity. |
| Spatter and bombs | Molten fragments ejected from a vent. | Welded blobs, twisted ribbons, spindle bombs, bread-crust forms. | Fragmentation and shaping while lava was still plastic or molten. |
Cooling structures and post-flow features
Once lava stops moving, cooling continues to write new structures into the rock. These features help geologists reconstruct flow direction, cooling history, water interaction, and later alteration.
Columnar joints
Thick flows and lava lakes may contract into polygonal columns as they cool. The columns grow roughly perpendicular to cooling surfaces.
Flow banding
Silica-rich lava and obsidian may preserve streaks, folds, and bands from movement of slightly different melt layers before final cooling.
Chilled margins
Lava in contact with water, wet sediment, ice, or cold air may develop glassy rims or fine-grained skins.
Jointing and fractures
Cooling contraction, flow inflation, and later stress create cracks that can guide fluids and secondary mineral growth.
Lava inflation
Fluid basalt may continue feeding beneath a crust, lifting the surface and creating tumuli, pressure ridges, and hollow cavities.
Amygdales
Vesicles may later be filled by minerals such as calcite, quartz, chalcedony, zeolites, chlorite, or epidote, forming amygdaloidal lava.
Vesicles, amygdales, and gas records
Vesicles are frozen gas bubbles. Their size, shape, abundance, and alignment reveal how gases escaped, how fast the lava moved, and how the flow cooled.
- Round vesicles form when bubbles are preserved without much stretching.
- Elongated vesicles record flow movement or shear while the lava was still soft.
- Vesicle-rich flow tops often show gas gathering near the upper part of a basalt flow.
- Amygdales show that fluids later moved through the rock and deposited secondary minerals.
- Pumice foam represents extreme vesiculation in silica-rich glass.
Identification and look-alikes
Lava is identified by texture, context, mineralogy, density, magnetism, and fracture. Color alone is not reliable, because industrial slag, furnace clinker, manufactured glass, coal waste, and dyed porous materials can resemble volcanic rock.
Useful clues
- Vesicles may be rounded, stretched, open, or mineral-filled.
- Basalt is commonly dense, dark, and weakly magnetic because of iron-titanium oxides.
- Obsidian shows glassy luster and conchoidal fracture.
- Pumice is unusually light because of abundant sealed pores.
- Volcanic context strongly supports identification.
Slag and clinker
Slag can be dark and vesicular, but it may contain metallic droplets, unnatural colors, industrial glass surfaces, or context linked to foundries, rail beds, furnaces, or waste dumps.
Natural glass versus manufactured glass
Obsidian and manufactured glass can both break conchoidally. Flow banding, spherulites, volcanic inclusions, and geological context help support an obsidian identification.
Care and handling
Dense basalt and many lava specimens are stable for display, but porous and glassy forms need more careful handling. Pumice and scoria can shed grains from thin bubble walls, while obsidian can have very sharp edges. Avoid thermal shock, boiling water, direct flame, and heavy oils or waxes that can soak into porous material and change its surface.
Cleaning
Use a soft brush, air bulb, or dry cloth. Stable basalt can be briefly rinsed and dried thoroughly, but porous scoria and pumice should not be left wet.
Storage
Wrap obsidian and other sharp glassy pieces so edges do not cut skin or scratch neighboring specimens. Support fragile pumice and scoria from beneath.
Display
Side lighting reveals vesicles, flow lines, glassy luster, and mineral-filled amygdales better than harsh direct glare.
Frequently asked questions
Is lava always basalt?
No. Basalt is the most widespread lava type at Earth’s surface, especially in oceanic and hotspot settings, but lava can also be andesitic, dacitic, rhyolitic, or more unusual in composition.
Why do some lava flows look smooth while others look jagged?
Smooth pāhoehoe and jagged ʻaʻā can both be basaltic. The difference comes from temperature, crystallinity, gas content, slope, flow rate, and the way the outer crust breaks or folds while the interior continues moving.
How does lava become obsidian?
Obsidian forms when silica-rich lava cools so quickly that crystals do not have time to grow. The result is volcanic glass with glossy luster and conchoidal fracture.
Why can pumice float?
Pumice contains so many sealed gas bubbles that its bulk density can be lower than water. Once water enters the pore network, a piece that once floated may eventually sink.
What are amygdales in lava?
Amygdales are former gas bubbles later filled by minerals carried by fluids. Common fillings include calcite, quartz, chalcedony, zeolites, chlorite, and epidote.
Can lava form underwater?
Yes. Submarine eruptions are common at mid-ocean ridges and oceanic volcanic settings. Lava that erupts into water often forms pillow structures with glassy chilled margins.
The formation story in one view
Lava is the visible end of a deep geological process: rock partially melts, magma rises, gases expand, and molten material emerges into air, water, ice, or open ground. From that moment, cooling begins to turn motion into texture. Ropy basalt, jagged ʻaʻā, pillow lava, obsidian glass, pumice foam, scoria, domes, tubes, columns, vesicles, and amygdales are all records of the same transformation: Earth’s heat becoming a permanent surface language.