Lava (Volcanic Rock): Physical & Optical Characteristics
Linas JuozenasShare
Lava: Physical and Optical Characteristics
Lava is molten rock that reaches Earth’s surface and cools into volcanic, or extrusive, rock. Its appearance depends on chemistry, gas content, cooling rate, and environment: a basalt flow may become dense charcoal stone, frothy scoria, underwater pillows, or glassy tachylite, while silica-rich eruptions may quench into obsidian or expand into pumice.
What lava becomes after cooling
Lava is not one mineral. It is the surface expression of magma, and after cooling it becomes a volcanic rock made from crystals, glass, vesicles, and sometimes later mineral fillings. That is why the physical and optical behavior of “lava rock” varies so widely from dense basalt to pumice that can float.
Composition matters first. Basaltic lava is iron- and magnesium-rich, usually dark, relatively fluid, and commonly composed of plagioclase, pyroxene, olivine, and iron-titanium oxides. Andesitic lava is intermediate in composition and viscosity, often gray to brown and porphyritic. Rhyolitic lava is silica-rich, highly viscous, and especially prone to forming volcanic glass, obsidian, and extremely vesicular pumice.
Basaltic lava
Low-viscosity mafic lava that commonly forms fine-grained basalt, ropey pāhoehoe surfaces, broken ʻaʻā clinkers, scoria, and underwater pillows.
Andesitic lava
Intermediate lava with medium viscosity, often carrying visible feldspar, pyroxene, or amphibole phenocrysts in a fine gray-brown groundmass.
Rhyolitic lava
Silica-rich lava that can quench into obsidian, expand into pumice, or preserve fine flow bands and devitrification textures.
Physical and optical properties
The values below describe common volcanic rock material, especially basaltic lava. Individual pieces may vary because lava is an aggregate of minerals, glass, bubbles, and alteration products.
| Property | Typical lava material | Interpretive note |
|---|---|---|
| Material type | Extrusive igneous rock | May contain crystals, volcanic glass, vesicles, oxidation rinds, and secondary minerals. |
| Common minerals | Plagioclase, pyroxene, olivine, magnetite, ilmenite, volcanic glass | Andesitic lava may include amphibole; rhyolitic glass may be mineral-poor at hand-specimen scale. |
| Color | Black, dark gray, brown, brick-red, cream, pale gray, or glassy black | Fresh basalt is commonly black to charcoal; red scoria reflects iron oxidation; pumice is often pale. |
| Texture | Aphanitic, vesicular, glassy, porphyritic, frothy, scoriaceous, ropy, blocky, or pillowed | Texture records cooling rate, gas content, eruption style, and whether the lava cooled in air or water. |
| Mohs hardness | About 5 to 6.5 overall | Obsidian is commonly about 5 to 5.5; pumice may be friable despite glassy material near Mohs 6. |
| Specific gravity and apparent density | Framework commonly about 2.7 to 3.1; apparent density can be much lower when porous | Pumice may float because sealed vesicles lower bulk density; scoria is lighter than compact basalt. |
| Luster | Dull, earthy, sub-vitreous, silky-matte, or vitreous | Obsidian is vitreous; basalt may show a subtle sheen on fresh fractures; pumice is usually matte. |
| Fracture | Uneven, granular, rough, splintery, or conchoidal in glass | Obsidian breaks with sharp conchoidal edges; vesicular rocks break along bubble walls. |
| Streak | Grayish to dark, generally not diagnostic | Rocks are mixtures, so streak is less useful than texture, magnetism, density, and context. |
| Magnetism | Weakly magnetic to locally magnetic | Magnetite and ilmenite can produce a response, especially in basaltic material. |
| Acid reaction | No fizz from the lava itself in weak acid | Late carbonate coatings or fillings may react, but the volcanic host is not limestone. |
| Optical character | Variable: birefringent crystals in groundmass, isotropic glass in obsidian | Thin sections reveal plagioclase, pyroxene, olivine, glass, and microlites; obsidian is primarily isotropic. |
| Fluorescence | Usually inert | Some coatings, zeolites, calcite, or amygdales may fluoresce, but the lava host generally does not. |
Optical behavior
Lava’s optical behavior is controlled by cooling rate. Fast cooling can preserve glass; slower cooling, even within a lava flow, produces fine crystals. Gas bubbles scatter light, while glassy surfaces can return a mirror-like reflection.
Obsidian is volcanic glass. It is amorphous and optically isotropic, so it behaves differently from crystalline volcanic rocks. A polished or freshly broken edge can show a glossy, mirror-like luster, conchoidal fracture, flow banding, and, in some varieties, sheen effects or snowflake spherulites.
Basalt and andesite contain tiny crystals in a fine groundmass. In thin section, plagioclase laths may show polysynthetic twinning, pyroxene commonly has high relief, and olivine may appear as altered or fresh phenocrysts. Pumice and scoria are optically dominated by vesicles: light scatters across bubble walls, creating a matte, foam-like appearance.
Color and surface stability
Lava color is a record of chemistry and weathering. Fresh basalt is black to charcoal because of iron-rich silicates, volcanic glass, and iron-titanium oxides. Scoria may be black, mahogany, or brick-red as iron oxidizes along vesicle walls. Pumice is commonly cream, pale gray, or white when the glass is clean, with tan to rusty staining where iron-bearing fluids have passed through.
Obsidian is typically black, but varieties may be mahogany, snowflake, banded, or sheen-bearing depending on iron oxidation, devitrification, crystal growth, and microscopic inclusions. Most volcanic rocks are stable in ordinary indoor display, though glassy surfaces can develop hydration rinds over very long periods. Rapid heating and cooling should be avoided because volcanic glass can craze or fracture under thermal shock.
Black and charcoal
Typical of fresh basalt, tachylite, and obsidian. A fresh fracture is often darker and cleaner than weathered exterior surfaces.
Red and brown oxidation
Common in scoria, cinders, and weathered basalt where iron-bearing minerals have oxidized in air or near-surface water.
Pale pumice
Usually tied to silica-rich frothy glass with abundant sealed vesicles. Its low apparent density is structural, not a separate mineral identity.
Textures and morphologies
Volcanic rocks are often identified more reliably by texture than by color alone. Bubble walls, glassy fracture, flow banding, pillows, and crystal size all record the conditions of eruption and cooling.
| Texture or form | How it forms | Visible character | Common material |
|---|---|---|---|
| Aphanitic basalt | Rapid cooling produces crystals too small to see easily without magnification. | Dense dark gray to black stone, sometimes with subtle fracture sheen. | Basalt and fine-grained andesite. |
| Vesicular lava | Dissolved gases expand as pressure drops, leaving bubbles in the cooling rock. | Rounded or stretched holes; light for its size when vesicles are abundant. | Scoria, vesicular basalt, pumice. |
| Scoria | Mafic to intermediate lava froths, then cools with thicker bubble walls than pumice. | Dark, rusty, or red-brown porous rock with rough surfaces. | Basaltic to andesitic cinders and flows. |
| Pumice | Gas-rich silica-rich magma expands into frothy glass. | Pale, lightweight, highly vesicular material that may float until water enters the pores. | Rhyolitic to dacitic pumice. |
| Obsidian | Silica-rich lava quenches quickly enough to preserve glass rather than crystals. | Vitreous luster, conchoidal fracture, sharp edges, flow bands, or spherulites in some pieces. | Rhyolitic volcanic glass. |
| Pāhoehoe | Fluid basalt develops a flexible cooling skin that folds as lava continues to move below. | Ropy, smooth, folded, or billowed surfaces. | Basaltic flow crusts. |
| ʻAʻā | More disrupted flow breaks into angular clinker as the moving lava is torn apart. | Jagged, rough, blocky surfaces with sharp fragments. | Basaltic flows and cinders. |
| Pillow lava | Lava erupts underwater and quenches into rounded lobes with glassy chilled margins. | Bulbous, stacked, pillow-like forms, often with radial cracks or chilled rims. | Submarine basalt. |
| Lava bombs | Molten or plastic fragments are ejected and shaped during flight. | Aerodynamic, twisted, spindle, ribbon, or bread-crust forms. | Basaltic to andesitic ejecta. |
| Amygdaloidal lava | Vesicles are later filled by secondary minerals. | Rounded fillings of chalcedony, agate, calcite, zeolites, or other minerals in the host rock. | Basalt and andesite with later fluid alteration. |
Identification and look-alikes
Correctly identifying lava means combining texture, density, magnetism, fracture, and context. Color alone is unreliable: black industrial glass, furnace slag, coal clinker, dyed porous beads, and artificial landscaping materials can all resemble volcanic rock at a glance.
Simple observations
- Look for vesicles: bubbles may be round, stretched, open, or mineral-filled.
- Check weight: pumice feels unusually light, scoria is lighter than dense basalt, and compact basalt feels solid.
- Use a small magnet: basalt may respond weakly because of magnetite or ilmenite.
- Observe fracture: obsidian has sharp conchoidal glass fracture; basalt breaks more granularly.
- Weak acid should not fizz on the volcanic host, though carbonate fillings may react.
Industrial slag
Slag may be vesicular and dark, but it often shows unnatural glassy colors, metallic inclusions, iron smears, very angular artificial surfaces, or context tied to foundries, old rail beds, fire pits, or industrial waste rather than a volcanic field.
Obsidian and manufactured glass
Both are glassy and conchoidal. Natural obsidian often shows flow banding, mineral inclusions, spherulites, or a plausible volcanic context; manufactured glass may show mold marks, bubbles of different character, or non-geological color consistency.
Basalt and andesite
Basalt is usually darker and richer in olivine, pyroxene, and iron-titanium oxides. Andesite is commonly gray to brown and may show more visible feldspar or amphibole phenocrysts. Laboratory methods are needed for exact classification.
Microscope clues
Thin sections may show plagioclase laths with twinning, high-relief pyroxene, olivine phenocrysts, Fe-Ti oxides, glassy groundmass, and microlites. Obsidian is dominantly isotropic glass, while snowflake obsidian contains radiating spherulites formed during devitrification.
Care, handling, and storage
Volcanic rocks range from tough dense basalt to fragile pumice and razor-edged volcanic glass. The safest care approach is simple: remove dust gently, avoid unnecessary oils or chemicals, and support porous pieces from beneath.
Cleaning
Use a soft brush, air bulb, or dry microfiber cloth. A brief gentle rinse can be used for stable basalt, scoria, or pumice, but pieces should dry thoroughly so moisture does not remain trapped in vesicles. Oils and heavy soaps can darken porous surfaces and attract dust.
Sharp glass
Obsidian shards and natural volcanic glass fibers can cut skin. Handle rough obsidian by broad faces, wrap sharp pieces during storage, and keep fragile glass fibers away from eyes, pets, and fabrics.
Porous material
Pumice and scoria can shed grit or crumble at thin bubble walls. Do not clamp vesicular edges tightly. Store with cushioning so vibrations do not grind the walls.
Heat and moisture
Avoid ovens, boiling water, direct flame, and rapid temperature changes. Volcanic glass may crack under thermal shock, and porous rock can trap moisture that expands when heated.
Observing and photographing lava
The best way to study lava visually is to use light that matches the texture. Dense basalt benefits from soft side light. Scoria and pumice need raking light that casts tiny shadows in vesicles. Obsidian often needs one broad reflection to reveal flow bands and a second, softer light to keep the glass from becoming featureless black.
Use raking light for pores
A low side angle makes bubble walls, rough scoria surfaces, ʻaʻā clinker, and amygdales easier to see.
Control reflections on glass
Obsidian is best shown with a broad, clean reflection that reveals gloss without overexposing the surface.
Show scale and density clues
Pair a whole view with a close view of bubbles, fracture, and mineral fillings. For pumice, a carefully staged floating test can illustrate low apparent density when appropriate.
Keep color edits restrained
Basalt, scoria, pumice, and obsidian are easily misrepresented by excessive contrast or saturation. Neutral backgrounds make texture more legible.
Frequently asked questions
Is lava rock a single mineral?
No. Lava rock is a volcanic rock, meaning it is a mixture of minerals, glass, pores, and sometimes secondary fillings. This is why its hardness, density, luster, and magnetic response vary from piece to piece.
Why does some lava float?
Highly vesicular pumice contains so many sealed bubbles that its bulk density can be lower than water. Over time, water may enter the pores and the same piece may eventually sink.
Is obsidian the same as ordinary lava rock?
Obsidian is a volcanic glass, usually formed from silica-rich lava that cooled too quickly for crystals to grow. It is part of the volcanic-rock family, but it behaves optically and physically more like natural glass than like crystalline basalt.
Does lava fluoresce under ultraviolet light?
The volcanic host is usually inert. Occasional fluorescence may come from later minerals in vesicles or coatings, such as calcite or certain zeolites, rather than from the lava itself.
Can lava rock be used in aquariums or terrariums?
Dense basalt and many porous lava rocks are commonly used in some aquascapes and terrariums after thorough rinsing and brushing. Sharp obsidian should be avoided where animals could be cut, and sensitive habitats should be tested for water chemistry and physical safety.
How can slag be distinguished from vesicular lava?
Slag often shows industrial context, unnatural glass colors, metallic inclusions, unusually heavy zones, or artificial-looking flow surfaces. Natural lava is better supported by volcanic context, mineral texture, oxidation rinds, and geological associations.
The essential character of lava
Lava is surface-born rock in motion made permanent. Its physical and optical range is unusually broad: basalt can be dense and dark, scoria can be red-brown and porous, pumice can be pale and buoyant, and obsidian can be black volcanic glass with a mirror-bright fracture. The unifying story is rapid cooling at Earth’s surface, where gas, glass, crystals, oxidation, and water all leave visible traces in the finished stone.