Lava

The 2023 eruption at the Litli‑Hrútur vent produced vigorously agitated, bubbling lava at the surface, a near‑vent signature that reflects rapid volatile release and dynamic flow conditions during effusion. Such surface turbulence arises when gas escapes from ascending, volatile‑rich magma: bubbles nucleate, grow and rise, driving convective overturn, vesiculation and frequently localized spattering, processes that directly shape proximal lava morphology and short‑term emplacement patterns.

Most lavas that solidify at Earth’s surface are dominated by silicate minerals, which form the primary mineralogical framework of volcanic rocks. Common silicate constituents include feldspars and feldspathoids, olivine, pyroxenes, amphiboles, micas and quartz; their relative proportions determine key textural and chemical properties of the rock. Variations in the mineral assemblage—especially the balance between mafic phases (e.g., olivine, pyroxene) and more silica‑rich phases (e.g., feldspar, quartz)—record fundamental differences in magma composition, control melt viscosity and crystallization order, and thereby influence eruption style, flow behavior and the resulting volcanic landforms.

Lavas lacking a silicate matrix are rare and typically reflect unusual local conditions. They can form either by melting of preexisting nonsilicate mineral deposits or by magmatic differentiation in which immiscible liquids separate and a nonsilicate liquid is expelled. When present, nonsilicate lava products are diagnostically important: they point to atypical crustal mineralogy or uncommon magmatic processes and provide critical constraints on local crustal composition, magmatic evolution and the range of volcanic rock types produced in an eruptive sequence.

Silicate lavas

Silicate lavas are molten rocks whose chemical makeup is dominated by silicon and oxygen, with subordinate amounts of aluminium, calcium, magnesium, iron, sodium and potassium and trace quantities of other elements. Petrologists conventionally describe lava composition in terms of weight or molar fractions of the constituent oxides (for example SiO2, Al2O3, CaO, MgO, FeO, Na2O, K2O), which provides a practical summary of the major-element chemistry.

The physical behavior of silicate magmas is chiefly controlled by the silica component because silicon in the melt bonds to four oxygen atoms to form SiO4 tetrahedra, the basic structural unit of the liquid. When oxygen ions link two tetrahedra (so-called bridging oxygens), tetrahedra join into chains, rings or three‑dimensional networks; this linking, or polymerization, gives the melt a network-like structure. Aluminium, especially in the presence of alkali oxides (Na2O, K2O), can enter the tetrahedral framework and thus act as a network former, further promoting polymerization. By contrast, divalent and larger cations such as Fe2+, Ca2+ and Mg2+ bond less strongly to oxygen and function as network modifiers, disrupting Si–O–Si linkages and limiting the extent of polymerization.

The degree of polymerization exerts a primary control on melt viscosity: more extensively polymerized (higher‑silica) melts are much more viscous than less polymerized (lower‑silica) melts. Because silica content strongly influences viscosity and covaries with other eruptive properties (for example eruptive temperature), silicate lavas are chemically classified into four broad silica‑based groups—felsic, intermediate, mafic and ultramafic—reflecting progressively decreasing SiO2 and associated changes in rheology and eruptive behavior.

Felsic lava

Felsic (silicic) lavas, typified by rhyolite and dacite, are characterized by silica contents exceeding ~63%, a compositional control that strongly governs their rheology and eruptive behaviour. Their viscosities are extremely high—on the order of 10^8 cP (10^5 Pa·s) for very hot rhyolite (~1,200 °C) rising to about 10^11 cP (10^8 Pa·s) for cooler rhyolite (~800 °C)—many orders of magnitude greater than water (~1 cP; 0.001 Pa·s). Such viscous magmas commonly obstruct outflow and favour explosive fragmentation, producing pyroclastic deposits rather than extensive, free‑flowing lava sheets. Nonetheless, effusive emplacement does occur in felsic systems: rhyolitic eruptions can form lava spines, domes and short, thick flows (coulees) when magma is extruded more slowly or degassed. Extrusive rhyolite often fractures during emplacement, yielding blocky lava flows that commonly contain obsidian, the glassy product of rapid cooling in high‑silica melts. Although felsic magmas may erupt at temperatures as low as ~800 °C, unusually hot rhyolites exceeding ~950 °C can attain sufficient mobility to travel for many tens of kilometres, as documented in parts of the Snake River Plain.

Intermediate (andesitic) lava

Intermediate (andesitic) lavas contain roughly 52–63% SiO2 and, compared with felsic magmas, exhibit lower aluminium but relatively higher magnesium and iron contents. This chemistry affects both their physical behavior and outward appearance. Mineralogically they commonly develop conspicuous phenocrysts, with amphibole and pyroxene among the typical crystal phases, while the higher Fe–Mg content tends to produce a darker groundmass.

Morphologically, intermediate lavas are prone to forming blocky flows and andesitic domes and are frequently erupted from steep composite (stratovolcano) edifices — the volcanic chains of the Andes provide the archetypal setting. Thermally, their eruptive temperatures are higher than those of felsic magmas, typically in the range 850–1,100 °C, a factor that reduces melt viscosity relative to rhyolitic melts.

Rheologically, the combination of moderate silica and elevated temperature yields substantially lower viscosities than felsic melts. A representative laboratory measurement is about 3.5 × 10^6 cP (3,500 Pa·s) at 1,200 °C; this intermediate viscosity (roughly comparable to smooth peanut butter) explains why andesitic eruptions produce blocky, dome-forming flows rather than extremely fluid pahoehoe or the highly resistant extrusions characteristic of rhyolite.

Mafic (basaltic) lavas are defined by a high proportion of mafic oxides—notably magnesium oxide (MgO) and iron oxide (FeO)—and by relatively low silica contents, typically in the range of about 45–52 wt%. This chemical signature controls their mineral assemblage and largely determines their physical behavior: basaltic magmas erupt at elevated temperatures, commonly between ~1,100 and 1,200 °C, which together with their composition produces markedly low viscosities (on the order of 10^4–10^5 cP, or roughly 10–100 Pa·s). Although still many orders of magnitude more viscous than water, this low-viscosity regime favors mobile, largely laminar flows with rapid surface extension and efficient heat transport relative to higher-silica magmas.

The mobility of basaltic lavas underpins characteristic landforms. Their ability to travel long distances before solidifying promotes the construction of broad, low-angle shield volcanoes and the emplacement of extensive flood-basalt plateaus. On gentle slopes individual flows may undergo inflation: a thin, solidified crust develops while continued supply of molten lava beneath it lifts and thickens the flow interior, producing final flow thicknesses substantially greater than the instantaneous molten stream.

Surface morphologies of basaltic flows reflect variations in effusion rate, cooling history, and shear: the two dominant facies are pāhoehoe (smooth, often ropy surfaces) and ʻaʻā (fragmented, rubbly surfaces), whereas blocky lavas are comparatively uncommon in true basaltic compositions. In submarine settings basaltic eruptions typically form pillow lavas—rounded, lobate bodies produced by rapid quenching—that are broadly analogous in form and formation dynamics to pāhoehoe lobes on land, demonstrating similar flow and cooling behavior across subaerial and submarine environments.

Ultramafic lava

Ultramafic lavas are magmas characterized by very low silica (<45 wt%) and include highly magnesian types such as komatiites and the magmas that produce boninite. Komatiites represent an extreme end‑member, with MgO contents in excess of ~18 wt% that record exceptionally mafic, mantle‑derived compositions. Their eruptions are inferred to have occurred at temperatures approaching 1,600 °C, a thermal regime in which silicate polymerization is minimal and the melts become unusually mobile. Estimated viscosities for komatiite magmas are extremely low for silicate melts (roughly 100–1,000 cP, or 0.1–1 Pa·s), comparable to light motor oil, which explains their capacity for rapid flow and the development of distinctive flow morphologies. The stratigraphic record of ultramafic lavas is predominantly ancient—most occurrences are no younger than the Proterozoic—although a few Phanerozoic examples (notably in Central America) have been linked to anomalously hot mantle upwellings. The absence of modern komatiite eruptions is attributed to long‑term cooling of the mantle, which has lowered upper‑mantle temperatures below those required to generate the very high‑Mg magmas that produce komatiite.

Alkaline lavas

Alkaline silicate magmas are distinguished by relatively high concentrations of the alkali oxides (Na2O + K2O) and are generated preferentially in tectonic environments such as continental rifts, intraplate hotspot provinces, and regions influenced by deeply subducted lithosphere. Compared with subalkaline magmas, many alkaline melts originate at greater mantle depths and thus sample less-degassed, more primitive mantle domains; their bulk compositions span the full SiO2 range from ultramafic to felsic (e.g., nephelinites, basanites, tephrites → trachytes).

Representative compositions illustrate how major-element chemistry reflects source and evolution. Nephelinite, an ultramafic, strongly alkaline lava (SiO2 ≈ 39.7 wt%), combines high CaO and MgO with appreciable Na2O (≈3.8 wt%) and is interpreted as deriving from unusually deep mantle sources. By contrast, very mafic/ultramafic picrite shows an extreme MgO content (≈20.8 wt%), characteristic of high-temperature, primitive melts closely linked to mantle peridotite melting. Typical tholeiitic basalt has intermediate silica (≈54 wt%), modest MgO (~4 wt%) and moderate alkalis (Na2O ≈3.0 wt%, K2O ≈1.5 wt%), representing a more evolved mafic end-member relative to the ultramafic types.

Progressive differentiation produces intermediate and felsic compositions with higher SiO2 and, in some cases, significant alkali enrichment. Andesite (SiO2 ≈60 wt%, Al2O3 ≈16 wt%) reflects more evolved melts often associated with subduction settings, whereas rhyolite (SiO2 ≈73 wt%) records extensive fractionation or crustal melting: it has the highest K2O (≈4.1 wt%) and very low MgO and total Fe, consistent with late-stage, highly evolved chemistry. Together these examples demonstrate that alkali enrichment can occur across a wide compositional spectrum and that major-oxide systematics (SiO2, MgO, Fe, Na2O, K2O) provide key constraints on source depth, melting conditions, and magmatic evolution.

Non‑silicate lavas

Lava need not be silicate: in different planetary settings the term applies to molten materials dominated by carbonates, oxides, sulfur, or volatile‑rich ice mixtures (the latter occurring as “lava” analogues on the icy satellites of the giant planets). These non‑silicate magmas exhibit bulk chemistries and physical behaviours that contrast strongly with common basaltic and rhyolitic lavas.

Carbonatite magmatism is rare in the modern volcanic realm, with Ol Doinyo Lengai (Tanzania) as the only known active carbonatite volcano, but carbonate‑rich rocks are preserved in the geological record. Such rocks typically contain around three‑quarters carbonate minerals, with subordinate silica‑undersaturated silicates (e.g., micas, olivine) and accessory phases like apatite, magnetite and pyrochlore; primary carbonate proportions in preserved deposits are frequently overprinted by post‑eruptive hydrothermal alteration and removal of alkali carbonates. Isotopic and other geochemical data commonly tie carbonatites to contemporaneous alkaline silicic magmas, supporting models in which carbonatite melts segregate from an alkaline parental melt by immiscible liquid separation rather than deriving solely from direct primary mantle partial melts.

The natrocarbonatite lavas of Ol Doinyo Lengai illustrate the extreme end‑member behaviour of carbonate magmas. These lavas are dominated by sodium carbonates, contain calcium carbonate at roughly half the abundance of the sodium component and potassium carbonate at an intermediate proportion, together with minor halides, fluorides and sulfates. They are extraordinarily fluid—viscosities only slightly greater than water—and erupt at anomalously low temperatures for lavas (measured ≈491–544 °C), leading to flow and cooling dynamics unlike those of silicate lavas.

Other non‑silicate volcanic products include iron‑oxide and sulfur flows. Iron‑oxide magmas have been invoked to explain Proterozoic iron ore bodies (e.g., Kiruna, Sweden) and more recent iron‑oxide lava flows (e.g., the Pliocene El Laco complex, Chile–Argentina); these are interpreted to result from immiscible separation of an iron‑rich oxide liquid from calc‑alkaline or alkaline parental magmas and can produce economically important ore concentrations. Sulfur flows, such as those documented at Lastarria volcano (Chile), form by local melting of sulfur accumulations and can travel tens to hundreds of metres at very low temperatures (reported down to ≈113 °C).

Two recurrent petrogenetic themes underlie these examples: first, unusual bulk chemistries (carbonate‑, oxide‑ or sulfur‑rich) impart markedly different physical properties—notably much lower viscosities and/or eruption temperatures—than silicate lavas; second, immiscible liquid separation during magmatic evolution and subsequent hydrothermal modification are key processes that generate the distinctive compositions and, in some cases, concentrate economically valuable minerals.

Rheology governs how lava deforms and therefore controls emplacement style, landform development, and hazards. Silicate lavas span a wide range of physical states: mafic basalts erupt at temperatures near 1,200 °C (≈2,190 °F) with viscosities around 10^4 cP (≈10 Pa·s), whereas felsic melts erupt nearer 800 °C (≈1,470 °F) and may reach viscosities on the order of 10^11 cP (≈10^8 Pa·s), a difference of roughly seven orders of magnitude. Chemical composition is the principal control on viscosity; absolute temperature and the instantaneous shear rate during flow are important secondary controls that can modify effective resistance to deformation.

Viscosity in turn dictates eruptive behavior and planetary occurrence: low-viscosity magmas favour effusive emplacement and produce most extrusive units on Earth, Mars and Venus, whereas high-viscosity magmas are prone to fragmentation and explosive eruption. This bias toward low-viscosity eruptions is reflected in global statistics: about 90% of observed lava flows are mafic or ultramafic, ~8% are intermediate and ~2% are felsic. Field observations exemplify these relationships — for example, advancing toes of pāhoehoe from basaltic flows on the east rift zone of Kīlauea at Kalapana illustrate how low-viscosity lava can inundate inhabited terrain.

Morphology and transport are direct expressions of rheology. Low-viscosity mafic lavas typically form thin, laterally extensive sheets and distinctive surface types such as pāhoehoe and ʻaʻā, producing broad shield volcanoes with gentle slopes. Intermediate compositions tend to alternate effusive beds and explosive tephra to build steep stratovolcanoes. High-viscosity silicic magmas, when erupted effusively, generate high‑aspect‑ratio flows, blocky surfaces, lava domes and frequently glassy (obsidian) margins because rapid quenching arrests crystal growth and flow.

Natural lavas are suspensions rather than simple Newtonian fluids: they contain crystals, xenoliths and fragments of earlier lava that impart complex, non‑Newtonian behaviour. Most lavas exhibit yield strength and shear‑dependent viscosity and are commonly modelled as Bingham fluids; a finite yield stress must be exceeded before flow initiates, producing plug flow with shear concentrated in thin boundary layers (analogous to toothpaste extrusion). Thixotropic behaviour and shear thinning hinder crystal settling and modify mobility during transient forcing. As crystal content increases toward roughly 60 vol.% the mixture loses fluidity and transitions into a crystal mush that behaves mechanically as a solid.

Flow velocities are controlled principally by viscosity and slope. Typical Hawaiian basaltic flows advance at order 0.4 km h−1 (0.25 mph), with rapid emplacement on steep slopes of order 10–48 km h−1 (6–30 mph); exceptional events such as the post‑lava‑lake collapse flows at Mount Nyiragongo have reached 32–97 km h−1 (20–60 mph). Scaling arguments show mean advance speed scales approximately with flow thickness squared divided by viscosity; consequently, a rhyolitic (high‑viscosity) flow would need to be on the order of 10^3 times thicker than a basaltic flow to attain a comparable front‑advance rate, illustrating why silicic effusive emplacement is typically localized and high‑relief.

Temperature

Most lavas erupt at extremely high temperatures, typically between about 800 °C and 1,200 °C (1,470–2,190 °F). At eruption they are near their most fluid state; as they lose heat their viscosity increases markedly, so that a modest drop in temperature can change flow behavior substantially. Newly extruded lava cools first by radiative heat loss, producing a thin solid crust that insulates the interior; subsequent cooling of the flow proceeds mainly by slow conduction through this crust, which greatly retards cooling of the deeper molten portion.

Field studies illustrate the effectiveness of this insulation. Drill data from the 1959 Kīlauea Iki eruption show that after three years the surface crust was only about 14 m thick and had a base temperature near 1,065 °C (1,949 °F) despite the lava lake being roughly 100 m deep, and pockets of residual liquid persisted at depths of order 80 m even nineteen years later. Such slow, inward-directed solidification produces characteristic volumetric shrinkage and fracture patterns within a single flow.

Shrinkage-driven fracturing varies vertically: the upper zone commonly develops irregular, downward‑splaying cracks (the entablature), while the lower zone fractures into a regular array of vertical joints (the colonnade) that isolate polygonal columns—most often five- or six‑sided—exemplified by sites such as the Giant’s Causeway. Surface striations on column faces, often called chisel marks, are natural periodic fracture traces produced during joint propagation rather than human tool marks.

As solidification proceeds from the margins, volatile phases are expelled and concentrated at the upper and lower flow boundaries as pipe‑stem vesicles or amygdales, while interstitial liquids rejected from the crystallizing mush may migrate upward to form vertical vesicle cylinders. Where these cylinders merge near the flow top they create sheets of vesicular basalt sometimes capped by gas cavities; such cavities are favorable loci for later mineral precipitation, producing amygdaloidal fillings and, in extreme cases, large mineral-lined geodes (e.g., amethyst) in some flood basalt provinces. Textural outcomes also depend on composition and crystallization kinetics: low‑silica flood basalts that remain largely molten during emplacement tend to lack pronounced internal flow textures, whereas more silica‑rich (felsic) flows commonly display conspicuous flow banding and other flow-related structures because of their higher viscosity and different crystallization behavior.

Lava morphology

Lava morphology denotes the visible surface form and texture of erupted molten rock; it arises chiefly from variations in composition and viscosity and, in turn, determines how flows move, cool and construct volcanic landforms. Silica-poor, low-viscosity basalts behave as highly fluid lavas that spread as extensive, thin, sheet-like flows with relatively smooth, planar surfaces. This mobile style of effusion permitslava to travel long distances and is a principal process in building broad volcanic edifices and enlarging islands. In contrast, silica-rich rhyolitic lavas are highly viscous and resist deformation, so they advance only short distances and accumulate as blocky, knobby masses or dome-like constructs that commonly form close to their vents.

When lava is erupted beneath or directly into water, its morphology is strongly modified by rapid quenching and interaction with the ambient fluid. Submarine and littoral eruptions commonly produce glassy, quenched surfaces, highly fragmented deposits and rounded or lobate forms that differ markedly from subaerial sheet flows and viscous domes. A modern example of these processes is the Pacific littoral of the Island of Hawaiʻi, where active basaltic lava entering the sea cools and solidifies to add new volcanic rock to the shoreline, progressively lengthening and reshaping the island’s coast.

ʻAʻā

ʻAʻā is a basaltic lava flow type characterized by a dense, internally active core overlain by a loose, rubbly surface of broken blocks called clinker. The word—borrowed from Hawaiian (literally “stony, rough lava,” and also “to burn” or “blaze”) and incorporated into geological usage by Clarence Dutton—appears in several orthographies (aa, aʻa, ʻāʻ a, a-aa) and is pronounced [ʔəˈʔaː] or /ˈɑː(ʔ)ɑː/.

Morphologically, ʻaʻā forms a highly irregular, sharp, and spiny topography that substantially impedes travel on its surface. Mechanically, the flow comprises a pasty, plastic interior that moves downslope and entrains or transports the cooled clinker layer; at the advancing front cooled fragments tumble down steep margins and become buried by subsequent lava, producing layered clinker horizons both at the top and base of the flow. Rounded, welded aggregates of clinker—accretionary lava balls—are common and may reach sizes of about 3 m.

Rheologically, ʻaʻā is generally more viscous than pāhoehoe; pāhoehoe may transform into ʻaʻā when shear and turbulence increase—for example where flows encounter obstacles or descend steep slopes—so the two morphologies represent end-members on a continuum controlled by slope, shear, and surface disruption. Typical eruption temperatures for ʻaʻā range roughly from 1,050 to 1,150 °C (and sometimes higher). The angular, blocky surface is a strong radar reflector, producing bright returns in orbital radar imagery (e.g., Magellan), and field examples such as advancing ʻaʻā fronts over pāhoehoe on Kīlauea’s coastal plain illustrate the dynamic interaction and contrast between these basaltic flow types.

Pāhoehoe (also spelled pahoehoe; pronounced approximately paːˈhoweˈhowe) is a basaltic lava type characteristic of Hawaiian volcanism, notably producing the flows of Kīlauea. The Hawaiian name—literally referring to smooth, continuous lava—was introduced into geological usage by Clarence Dutton.

Morphologically, pāhoehoe is distinguished by smooth, billowy, undulating and ropy surface textures. These forms develop when very fluid basalt continues to flow beneath a progressively solidifying surface crust, generating a range of sculptural landforms. At the flow front, advancement typically occurs by repeated breakouts of small lobes or “toes” from a cooled skin; concurrently, insulated lava tubes commonly form and permit the preservation of high internal temperatures and low viscosity, enabling fluid transport over considerable distances.

Thermal state and rheology control surface texture and flow behavior. As pāhoehoe cools and loses heat away from the vent, viscosity rises and the flow may convert to the rougher ʻaʻā style; laboratory studies place this pāhoehoe→ʻaʻā transition roughly around 1,170–1,200 °C, although the threshold depends on shear rate and other dynamic conditions. Measured eruption temperatures for pāhoehoe generally lie between about 1,100 and 1,200 °C, situating it at the high-temperature, low-viscosity end of common terrestrial basalts.

In extent, most individual lava flows are under ~10 km, yet pāhoehoe can form flows exceeding 50 km, whereas some flood-basalt events preserved in the rock record contain individual flows that traveled for hundreds of kilometres. The smooth, rounded surface of pāhoehoe produces weak radar backscatter, so these flows are often poor radar reflectors and appear dark in orbital radar imagery (for example, images from the Magellan mission), complicating remote detection.

The interplay of basaltic composition, elevated emplacement temperatures, low viscosity, tube-confined transport, and lobate emplacement style explains pāhoehoe’s capacity to generate extensive, long-lived, and morphologically diverse lava fields in settings such as Kīlauea.

Block-lava flows are produced chiefly by andesitic magmas erupted from stratovolcanoes; their high viscosity yields an armored surface of smooth-faced, angular blocks rather than the loose clinker typical of basaltic ʻaʻā. Although they are morphologically akin to ʻaʻā in overall flow form, the greater resistance to deformation produces a coherent, blocky crust, markedly slower downslope translation, and substantially greater vertical thickness than lower-viscosity counterparts.

Internally, these flows characteristically retain a still-molten core that is thermally insulated beneath the solidified blocky crust. Advancement is maintained as the hot interior slides forward beneath the crust and over a talus of detached blocks and rubble that accumulates at the flow front, producing a rubble apron and episodic frontal collapse.

These surface and mechanical attributes reflect the andesitic composition and stratovolcanic source, accounting for the thicker, slower-moving deposits observed in the field. The Fantastic Lava Beds adjacent to Cinder Cone in Lassen Volcanic National Park provide a representative example, where the blocky morphology and flow mechanics illustrate the typical behavior of block-lava eruptions.

Pillow lava

Pillow lava comprises bulbous, lobate bodies produced when erupting basaltic or other viscous magmas are emplaced directly into a fluid environment such as seawater or glacial meltwater. It is most familiar from the ocean floor—e.g., around volcanic islands and mid-ocean ridges—but also forms where subglacial eruptions occur or where subaerial flows enter standing water.

Three principal tectono-environmental settings generate pillow lava: effusion from submarine vents on the seafloor, eruptions beneath ice where meltwater provides the quenching medium, and coastal or deltaic situations where terrestrial flows are forced into seawater. In each case, the eruption interacts with an external fluid that rapidly extracts heat from the lava.

The formation mechanism is governed by simultaneous mechanical and thermal processes. Contact with cold water or meltwater induces rapid skinning of the lava surface, creating a glassy or quenched crust. Continued injection of molten material beneath that crust produces internal pressure, causing the crust to rupture and fresh molten lobes to extrude. Repetition of crusting, cracking, and extrusion yields the characteristic rounded lobes.

Morphologically, individual pillows are discrete, roughly hemispherical to elongate lobes bounded by quenched crusts; they commonly overlap and interlock into stacked packages that record episodic emplacement and stepwise advance of the flow front. Internal features such as radial jointing, chilled margins, and void spaces commonly reflect the cooling history and growth dynamics.

Pillow lavas are abundant because much of Earth’s volcanism interacts with water—oceans cover the majority of the planet and many volcanic centers are submarine or adjacent to water and ice. Consequently, pillow-forming eruptions are a pervasive component of the volcanic stratigraphic record.

Because they form only where lava meets a fluid medium, pillow lavas serve as a diagnostic indicator of subaqueous or subglacial eruption. Their presence in modern and ancient successions provides direct evidence for past eruption environments, aids reconstruction of paleogeography, and constrains the interplay between volcanism and surface water or ice.

Lava landforms (viscous magmatism)

The physical behavior of high-viscosity magmas—typically silica-rich and often volatile-laden—fundamentally governs both eruption style and the forms that accumulate at the surface. High viscosity impedes flow, favoring slow, dome-like extrusion and short, thick lobate flows, while simultaneously inhibiting efficient gas escape; the resulting overpressure promotes explosive fragmentation and the generation of pyroclastic density currents. Thus rheology and volatile content together dictate whether emplacement is dominantly effusive, explosive, or a combination, and control the spectrum of deposits produced.

At landscape scales viscous eruptions build steep, blocky architectures: lava domes and spines, steep stratovolcanic edifices, coulees, and short flows bounded by lobes and levees. Explosive removal of large volumes of viscous magma can produce extensive pyroclastic sheets—ignimbrites and welded tuffs—and may culminate in caldera collapse when roof support is removed. These large-scale constructs commonly produce locally high relief and rugged topography where dense or welded units cap the terrain.

Meso- to local-scale surface morphologies record emplacement dynamics. Exposed flow surfaces are often angular and blocky with talus aprons, fractured flow fronts and autobrecciated margins; flow-front terraces, collapse pits and obsidian veneers mark quenched outer zones, whereas pumice-rich fallout and surge deposits reflect explosive fragmentation. Pyroclastic density currents create hummocky deposits, valley-ponded sheets, and welded margins adjacent to vents, producing characteristic depositional geometries and heterogeneity in grain size and compaction.

At the microscopic and petrographic level viscous lavas and their pyroclasts preserve signatures of rapid cooling and restricted crystal growth: glassy matrices (obsidian), abundant microlites and spherulites, and phenocrysts set in glass. Vesicularity varies widely—producing buoyant pumice where gas escape was extensive and dense obsidian where vesiculation was minimal—and welded textures develop in hot, compactive ignimbrites. Cooling contraction in thick coherent masses may produce polygonal columnar jointing.

The geomorphic consequences are scale-dependent and compositionally controlled. Dense, welded or glassy units are resistant to erosion and tend to preserve local relief, whereas pumice-ash terrains are readily reworked into low-relief plains and soils. Emplacement modifies drainage and sediment routing by creating natural dams, basins and abrupt slope changes that influence slope stability and sediment flux. Repeated episodes of growth and collapse produce complex stratigraphies of interbedded lava, breccia, ash and welded tuff, with both local collapse-generated block-and-ash flows and regionally extensive tephra dispersal.

Over geological timescales viscous-volcano-derived features may either be long-lived—when capped by erosion-resistant flows or ignimbrites—or rapidly erased where loose pyroclastic material dominates. Intrusive counterparts (dikes, plugs) associated with viscous magmatism commonly persist as topographic highs after surrounding rock is removed by erosion.

A multiscale investigative approach is therefore necessary: field mapping and geomorphic analysis across meters to kilometers to delineate domes, coulees and ignimbrite extents; stratigraphic, structural and sedimentological logging to infer emplacement processes; petrographic and electron-microscopy studies of thin sections to document glass, microlite and phenocryst textures; and remote sensing together with DEM analysis to quantify morphological footprints and landscape change.

Volcanoes

Volcanoes are edifices constructed over time by repeated extrusion of lava and emission of fragmented pyroclastic material; their resulting landforms reflect the style, composition and recurrence of eruptive activity. Broad, gently sloping shield volcanoes form chiefly from low-viscosity basaltic lavas during dominantly effusive eruptions, producing extensive flowfields. By contrast, stratovolcanoes (composite cones) accumulate steep-sided profiles through alternating deposits of viscous lava and pyroclastic layers, a record of eruptions involving more silicic, higher-viscosity magmas.

The dominant difference between effusive and explosive behaviour is governed by magma composition and viscosity: mafic, low-viscosity magmas favour outpouring flows, whereas intermediate to felsic magmas with higher viscosity inhibit degassing and promote explosive fragmentation and emplacement of ash, blocks and dome-building lavas. These contrasts determine both the external morphology and the internal stratigraphy of volcanoes.

Calderas form when the support above a magma reservoir fails following substantial magma withdrawal. Collapse may occur catastrophically after large explosive eruptions or more gradually through incremental subsidence as magma is withdrawn; the latter process is common at many shield volcanoes and need not involve a single calamitous explosion. As a stratovolcano, Costa Rica’s Arenal exhibits the steep slopes and layered ash–lava sequences characteristic of intermediate–felsic systems and therefore has the potential, under significant magma evacuation, to develop collapse-related features such as calderas, crater lakes or post-collapse lava domes.

Cinder and spatter cones are small, discrete volcanic edifices that form by the accumulation of erupted material around a single, localized vent on a larger volcano. Both produce cone-shaped deposits adjacent to their source, but they differ fundamentally in the nature of the ejecta and the eruption dynamics that produce them. Cinder cones are constructed largely of fragmental pyroclastic material—tephra, ash, and related tuffs—ejected explosively and deposited as loose, unconsolidated fragments that build a permeable cone. Spatter cones, by contrast, grow through the fall and welding of semi-molten slag and hot cinders; these plastic clasts deform and stick together on emplacement, producing agglutinated, cohesive accumulations.

The contrast between the two types reflects differences in eruption temperature and eruptive style: more explosive, fragment-producing eruptions yield the loose, easily erodible deposits of cinder cones, whereas hotter, more fluid ejections produce welded spatter cones with greater mechanical coherence. Consequently, cinder and spatter cones exhibit distinct behavior under erosional and mechanical stress, governed by their respective degrees of consolidation and the physical properties of their constituent materials.

Kīpuka (from Hawaiian, used in Hawaiian English) denotes an elevated remnant of pre‑existing terrain—for example a hill, ridge or older lava dome—that protrudes above and remains surrounded by younger lava surfaces. Morphologically, a kīpuka is an in situ patch of older land that was not buried by subsequent effusive activity; its persistence reflects either sufficient topographic relief to avoid being overtopped or resistance to emplacement and erosion that left it standing above adjacent flows.

Kīpuka form when later lava advances inundate lower ground while leaving higher blocks exposed, thereby isolating those blocks from contiguous older terrain and producing island‑like features within a matrix of fresh lava. Visually and ecologically they commonly appear as vegetated, often forested, “islands” set in a surrounding expanse of relatively barren, recently emplaced lava. This contrast arises because soils and biotic communities on the remnant have had more time to develop and recover, whereas the surrounding surfaces are geologically young and sparsely colonized.

Within volcanic landscapes, especially in places such as the Hawaiian Islands, kīpuka contribute important landscape heterogeneity: they function as refugia for plants and animals, preserve records of pre‑flow environments, and serve as practical markers for reconstructing lava‑flow history and relative surface ages during mapping and ecological studies.

Lava domes are steep, bulbous volcanic edifices that develop when highly viscous, silica-rich magmas are extruded so slowly that they accumulate at or immediately above the vent instead of forming long, fluid flows. Growth may proceed by surface extrusion (building successive lobes) or by internal emplacement: in the latter case magma intrudes into the dome and inflates it from within, producing a swollen, pillow‑like mass often termed an inflation or endogenous dome. Inflating and extruding domes commonly undergo surface cracking and spalling; these structural failures release blocks and rubble from the interior and generate a mantle of breccia and ash that cloaks the dome margins, promoting localized mass‑wasting. When domes are emplaced on slopes they may advance as short, thick, viscous flows called coulées, which typically travel only a few kilometres from their source. Field examples range from the forested silicic dome occupying the center of the Valle Grande meadow in New Mexico’s Valles Caldera to well‑documented activity at Novarupta and the sequence of successive domes erupted at Mount St. Helens, illustrating both single‑dome and multi‑dome eruptive behavior.

Lava tubes develop when an initially fluid lava flow forms a rigid, cooled roof while the molten core continues to move beneath it, producing an enclosed conduit through which liquid magma can be conveyed away from the vent. The hardened upper layer serves as an effective thermal barrier, reducing heat loss from the interior and allowing the mobile lava to remain hot and mobile over long distances. Sustained flow beneath this insulated cover can extend the conduit for many kilometres, enabling lava to be emplaced far downslope of the source.

When supply to the tube ceases, the interior lava commonly drains back toward the vent or downslope, leaving behind elongated voids that persist within the solidified flow as cave-like passages. Active tube formation is observable at Kīlauea, providing a present-day example of the process and its role in shaping volcanic terrain. Ancient, well-preserved examples occur in North Queensland (Tertiary age), where some tubes measure on the order of 15 km in length, demonstrating both the potential spatial extent of these features and their capacity for long-term preservation in the rock record.

As geomorphological elements, lava tubes strongly affect flow morphology by facilitating distal emplacement of molten material and by generating subsurface void networks that contribute to cave systems within volcanic provinces. Their presence thus influences the distribution, thickness, and surface expression of lava flows and leaves a durable imprint on volcanic landscapes.

Lava lakes

Exposed intrusive features such as Shiprock, New Mexico—a resistant volcanic neck with a conspicuous radiating dike—illustrate how volcanic plumbing can be revealed at the surface and contrast with systems in which magma is retained in near-surface depressions. When molten rock ponds within a cone or caldera instead of being emplaced as distal flows, the surface accumulation is termed a lava lake: a body of molten lava occupying a topographic basin and representing magma momentarily retained at the surface. Lava lakes are typically transient because the contained melt and pressure can be lost by return flow into the subsurface magma reservoir, a process commonly triggered by gas venting through the caldera. Alternatively, a lava lake may be evacuated at the surface during eruptive episodes—either by effusion that generates lava flows or by explosive, pyroclastic removal that transports the former lake material as eruptive deposits. Persistent, long-lived lava lakes are uncommon; well-documented examples include Mount Erebus (Antarctica), Erta Ale (Ethiopia), Nyiragongo (DRC) and Ambrym (Vanuatu). The presence or absence of a lava lake therefore signals ongoing magmatic and volatile behavior and has direct implications for volcanic monitoring, continuous gas emissions, and local hazard potential at these and analogous volcanic systems.

Lava deltas

Lava deltas are constructional coastal landforms that develop where subaerial lava flows enter standing water bodies—lakes, lagoons or the sea—linking a terrestrial volcanic source with adjacent bathymetry. Upon contact with water, hot lava cools almost instantaneously and undergoes mechanical disintegration, producing angular, fragmental clasts that accumulate at the shoreline and on the seafloor.

This rapid cooling and fragmentation progressively infills local seabed depressions and irregularities, creating a shallower substrate that permits subsequent subaerial flows to extend further offshore. The process is most effective during large, effusive basaltic eruptions because low-viscosity basalt can produce long, voluminous flows capable of reaching and building out across water margins. Repeated emplacement yields a landward-to-offshore stratigraphy characterized by broken lava breccia and interleaved flow lobes; as seabed relief is filled, the coastline progrades and a steep, constructional delta front develops, underlain by successively younger subaerial flow units.

By transforming deeper bathymetry into a shallower platform, lava deltas modify coastal profiles and local hydrodynamics. The newly deposited, mechanically weak fragmental material and the oversteepened delta fronts are prone to collapse and mass-wasting, making lava deltas inherently unstable as they grow during ongoing eruptive activity.

Lava fountains

Lava fountains are energetic, largely non‑explosive expulsions of molten magma from vents, craters, or linear fissures, in which material is discharged as either rapid, repetitive pulses or as sustained jets. Mechanically, they represent high-velocity effusion driven by volatile exsolution and decompression of low-viscosity magma, producing ballistic trajectories of incandescent lava rather than the fragmentation and ash production associated with explosive eruptions.

Observed examples illustrate the wide range of fountain scales and durations. Hawaiian eruptions commonly generate fountains of several hundred metres; for example, Kīlauea has produced fountains on the order of 450 m. More extreme events can reach kilometre-scale heights: the largest recorded lava fountain occurred at Mount Etna on 23 November 2013, when a sustained column reached about 2,500 m for roughly 18 minutes and briefly peaked near 3,400 m.

Across volcanic environments, lava fountains share the attributes of high-energy but effusive discharge, variable temporal behavior (from short pulses to prolonged jets), and dependence on magma properties and conduit dynamics. While they are most frequently associated with Hawaiian-style effusive activity, comparable fountain phenomena can occur in diverse tectonic and volcanic settings when conditions favor rapid volatile release and low magma viscosity.

Hazards

Lava flows cause extensive destruction to infrastructure within their paths, but human fatalities are relatively uncommon because most flows progress slowly enough to permit evacuation. Flow velocity, however, varies strongly with lava viscosity: low‑viscosity lavas can achieve much higher speeds and pose substantially greater immediate danger. When escape routes are cut off, when people approach active flows too closely, or when unusually rapid advances occur, flow fronts can become lethal.

Most deaths and serious injuries associated with lava are attributable to secondary phenomena rather than the advancing molten rock itself. Ballistic ejecta, pyroclastic flows generated by collapsing lava domes, lahars (volcanic mudflows), toxic gases that can travel ahead of a front, and steam‑driven or explosive interactions when lava contacts water all present acute hazards. A notable example is the Mount Nyiragongo event of 10 January 1977, when a breached crater wall allowed a fluid lava lake to drain in under an hour; the resulting high‑speed flows descended steep slopes at speeds reported up to ~100 km/h, overrunning villages while residents slept. Because of its eruptive style and proximity to population centers, Nyiragongo subsequently attracted international attention and was designated a Decade Volcano in 1991.

Hazard persists long after emplacement. Newly formed lava land is often unstable and prone to collapse—particularly where flows enter the sea, producing hazardous, breakable lava benches. Cooled flow surfaces also retain physical dangers: deep fissures and chasms are common, and ʻaʻā flows create extremely sharp, jagged terrain; falls onto such surfaces can produce severe lacerations, so sturdy protective clothing (rugged boots, long trousers, gloves) is advised for ground access.

Active engineering to divert lava is challenging but not impossible. Historical partial success—most notably efforts in Vestmannaeyjar, Iceland—shows diversion under favorable conditions can reduce impacts, and development of simple, low‑cost barriers to redirect flows remains an area of applied research. Effective volcanic hazard assessment therefore requires integrating the direct thermal and inundation effects of lava with the broad suite of secondary hazards, the post‑emplacement instability of flow fields, human behavioral risk factors (e.g., sleeping populations, obstructed egress), and the limited but occasionally effective engineering options for mitigation.

Towns and villages worldwide have been directly destroyed by lava emplacement, producing outcomes that range from permanent abandonment to later reconstruction and demonstrating both acute and recurrent risks. In Hawai‘i, Kīlauea flows demolished more than a hundred houses in Kalapana in 1990 and led to the community’s abandonment; earlier and separate Kīlauea eruptions in January 1960 leveled the settlements of Koae and Kapoho, which were also abandoned. Kapoho’s vulnerability was underscored again in June 2018, when renewed flows inundated the area and completely destroyed the Vacationland subdivision, illustrating the propensity for low‑lying coastal developments to suffer repeated inundation. Similar long‑term impacts are recorded elsewhere: the Nisgaʼa villages of Lax Ksiluux and Wii Lax Kʼabit in northwestern British Columbia were buried by thick Tseax Cone flows in the 1700s, reflecting the persistent hazard that local volcanism poses to indigenous communities. In the Philippines, Mayon’s 1814 eruption buried the town of Cagsawa under a combination of effusive and pyroclastic deposits, showing that lava flows often act together with other volcanic products to obliterate built infrastructure. By contrast, some historic port and municipal centers have been rebuilt on formerly inundated ground: Garachico on Tenerife was destroyed by Trevejo lava in 1706 and subsequently reconstructed, and San Sebastiano al Vesuvio was demolished by Mount Vesuvius in 1944 during the modern historical period before being rebuilt. Together these cases—spanning centuries and continents—illustrate the range of social and spatial consequences of lava flow hazards, from one‑time catastrophic loss to cycles of destruction and reconstruction.

Towns damaged by lava flows

Between 1669 and 2021, documented eruptions demonstrate recurrent interactions between active volcanic systems and nearby settlements across a range of tectonic and island/continental contexts. These events—occurring at stratovolcanoes, volcanic islands and rift‑zone volcanoes—produced outcomes that span rebuilding and restoration to permanent abandonment, and they illustrate how eruption duration, style and proximity determine the long‑term fate of towns.

Early modern and twentieth‑century examples include the 1669 Mount Etna eruption that inundated parts of Catania and required extensive post‑eruption reconstruction, and the 1928 Etna event that nearly destroyed Mascali before the town was rebuilt. On oceanic island chains, the six‑year Matavanu eruptions (1905–1911) on Samoa substantially altered settlement patterns at Sale’aula, while the 1973 Eldfell eruption on Heimaey (Vestmannaeyjar, Iceland) combined explosive and effusive hazards yet allowed for subsequent restoration and reoccupation of the island community. Réunion’s Piton Sainte‑Rose was similarly affected by lava incursions in 1977, underscoring vulnerability in hotspot island settings.

Prolonged and monogenetic eruptions have reshaped landscapes and settlements over decadal timescales: the Parícutin eruption (1943–1952) buried and displaced the eponymous village and nearby San Juan Parangaricutiro, remaking local land use and settlement locations. By contrast, the 1986–87 activity of Kīlauea inundated the Royal Gardens neighbourhood in Hawai‘i and led to its abandonment, demonstrating that some lava‑affected residential areas become permanently uninhabitable. The 2002 Nyiragongo eruption in the East African Rift severely impacted the city of Goma, illustrating the acute threat fast‑moving summit lava flows pose to densely populated continental urban centres. Most recently, the 2021 Cumbre Vieja eruption on La Palma (Canary Islands) produced flows that destroyed neighbourhoods in Los Llanos de Aridane (notably Todoque) and El Paso (El Paraíso), a well‑documented case of modern island municipal losses.

Taken together, these cases show: (1) repeated human settlement on and immediately adjacent to active volcanic systems in diverse settings; (2) a continuum of post‑eruption outcomes—reconstruction, relocation or abandonment—shaped by eruption intensity, duration and local preparedness; (3) the outsized influence of multi‑year eruptions on long‑term landscape and societal change; and (4) the imperative for context‑sensitive hazard mitigation, land‑use planning and recovery strategies tailored to the volcanic and social characteristics of regions such as Italy, Samoa, Mexico, Iceland, Réunion, Hawai‘i (USA), the Democratic Republic of Congo and the Canary Islands.

Towns destroyed by tephra

Tephra—the fragmental material ejected during explosive volcanism, ranging from fine ash through lapilli to larger bombs and blocks—can blanket and bury landscapes, causing abrupt loss of habitability, large-scale landscape modification, and, in some cases, exceptional preservation of archaeological and built environments.

Classic archaeological cases illustrate these outcomes. The Late Bronze Age settlement at Akrotiri on Santorini was entombed beneath pumice and ash from the island’s Minoan eruption (c. 1600 BCE), demonstrating how heavy tephra fallout can bury entire coastal communities and preserve settlement layouts. Similarly, the Roman towns of Pompeii and Herculaneum were overwhelmed during Mount Vesuvius’s AD 79 eruption; thick ash and related pyroclastic deposits both destroyed and conserved architecture, artifacts, and organic remains beneath their deposits. In El Salvador, the village of Cerén was buried by tephra from the Ilopango eruption (within the interval 410–535 CE), providing a tightly dated example of a rural community preserved under ash. The 1815 eruption of Tambora on Sumbawa Island produced extensive tephra dispersal across the island and beyond, illustrating the broad geographic reach of large explosive events. More recently, tephra deposition and associated volcanic hazards from Soufrière Hills forced permanent abandonment of Plymouth, Montserrat, in 1995; the town, formerly the island’s capital and principal port, remains officially the de jure capital despite being uninhabitable.