Basics

Lava

"Magma that has reached the Earth's surface through a volcanic vent."

Lava is molten rock that has breached the Earth’s surface through a volcanic vent or fissure. While beneath the crust, this molten material is termed magma; upon eruption, it is reclassified as lava. This distinction is crucial in geology not just for semantics, but because the loss of dissolved gases upon reaching the surface fundamentally alters the material’s physical properties and behavior.

Chemical Composition and Classification

Lava is primarily composed of silicate minerals, and its behavior is dictated by its silica (SiO₂) content. Geologists classify lava into three main chemical types:

  • Felsic (Rhyolitic): High silica content (>63%). These lavas are extremely viscous (thick) and have relatively low temperatures (650°C to 800°C). Because they trap gases easily, they are often associated with explosive eruptions rather than fluid flows. Rhyolitic lava, when it does flow, moves as slow, thick domes that can pile up to hundreds of meters in height.
  • Intermediate (Andesitic): Moderate silica content (52% to 63%). Common in stratovolcanoes at subduction zones, andesitic lava flows are thick and blocky, often not traveling far from the vent. The name comes from the Andes mountains, where this lava type is prevalent.
  • Mafic (Basaltic): Low silica content (45% to 52%) and high in iron and magnesium. These lavas are the hottest (1,000°C to 1,200°C) and least viscous, allowing them to flow rapidly over vast distances. The word “mafic” is derived from magnesium and ferric (iron).

Rheology and Viscosity

The viscosity of lava—its resistance to flow—is its most significant physical property. It determines the shape of the volcano, the style of eruption, and the hazard posed to surrounding communities.

Viscosity is governed by three key factors:

  1. Silica content: More silica creates stronger polymer chains within the melt, increasing viscosity dramatically. Rhyolite can be 10,000 times more viscous than basalt.
  2. Temperature: Hotter lava is always less viscous. As lava cools, it thickens, eventually solidifying.
  3. Gas content: Dissolved gases lower viscosity; as gas escapes, the lava thickens.

As lava cools, it becomes more viscous. However, the internal structure of the flow allows the core to remain hot and fluid while the crust hardens, creating lava tubes that insulate the flow and allow it to travel tens of kilometers from the source. This efficiency is why shield volcanoes, built from fluid basaltic lava, have such broad, gentle slopes.

Morphologies: Surface Textures

The way lava solidifies creates distinct surface textures, particularly in basaltic flows. The Hawaiian terms for these textures have become standard in volcanology worldwide:

  1. Pāhoehoe: Characterized by a smooth, billowy, or ropy surface. Pāhoehoe forms from hot, fluid lava with a relatively low flow rate. As the thin elastic skin cools, the continuing flow underneath drags it into rope-like folds or rolls it into smooth rounded lobes. Pāhoehoe flows can transition to ‘a’ā downslope as the lava cools and loses gas.
  2. ‘A’ā: A rough, jagged surface composed of loose, broken lava blocks called clinkers. ‘A’ā forms when the lava is slightly cooler, has a higher discharge rate, or has lost more gas, causing the surface to break rather than fold. Walking on ‘a’ā is notoriously difficult and painful—the sharp, glassy fragments shred footwear. The term comes from the Hawaiian expression of pain.
  3. Pillow Lava: Created when lava erupts underwater or subglacially. The rapid cooling causes the outer surface to solidify instantly while the interior remains liquid, and the continuing flow extrudes in rounded, pillow-shaped lobes (typically 30–100 cm across). Pillow lava is the most common type of lava formation on Earth, covering the vast majority of the ocean floor, and is a crucial rock type for understanding ancient tectonic environments.
  4. Blocky Lava: Common in andesitic or rhyolitic flows, where the lava is too thick and cool to form the smooth clinkers of ‘a’ā. Instead, it fractures into massive, smooth-sided, angular blocks with a relatively flat surface texture. Blocky flows move very slowly, sometimes only a few meters per day.

Lava Tubes: Nature’s Pipeline

One of the most important structural features associated with basaltic lava flows is the lava tube. As a pāhoehoe flow advances, its surface cools and hardens into a roof, while the lava below continues to drain through the insulated interior. When the eruption eventually stops, the liquid lava drains out, leaving a hollow tunnel.

Lava tubes allow lava to travel remarkable distances while staying hot enough to remain fluid. Kīlauea’s flows have traveled over 50 km to the sea through tube systems, building new land and ocean entries. Kazumura Cave on Hawaii’s Big Island is the longest lava tube in the world, extending more than 65 km. Globally, lava tube systems have been mapped on the Moon and Mars, where they are potentially much larger—with some estimated to be kilometers wide—making them candidates for future human habitats that would provide natural radiation shielding.

Lava as a Hazard

Despite moving slowly by most standards, lava flows pose significant hazards:

  • Property destruction: Basaltic flows, while rarely fast enough to outrun a person, can destroy buildings, infrastructure, and farmland through sheer inescapability. The 2018 lower East Rift Zone eruption of Kīlauea destroyed over 700 homes and buried entire communities under meters of lava. The 2021 Cumbre Vieja eruption on La Palma (Canary Islands) similarly destroyed over 3,000 structures and buried 1,200 hectares of land.
  • Gas emissions: Active lava flows outgas continuously, releasing SO₂ and CO₂ that can reach dangerous concentrations near the flow front. The interaction of lava with seawater creates laze (lava haze)—a corrosive cloud of hydrochloric acid and steam.
  • Laze and littoral explosions: When lava enters the ocean, the violent steam explosions (phreatic activity) at the ocean entry can hurl molten spatter and sharp glass fragments (“Pele’s hair” and “Pele’s tears”) hundreds of meters inland.

The Life Cycle of Lava

After eruption, lava acts as a constructive force. It adds new land mass to islands and continents. Successive flows from Hawaiian hotspot volcanoes have built islands from the seafloor over millions of years. Over geologic timescales, the weathering of basaltic lava produces some of the most fertile soils on the planet, rich in iron, magnesium, and phosphorus. This fertility is a primary reason why agricultural civilizations have historically settled near active volcanic zones despite the inherent risks—the volcanic soils of Java, Sicily, and the Pacific Northwest support extraordinarily productive agriculture.

Magma is the parent material of lava, distinguished by being underground. Pāhoehoe and ‘a’ā are the two primary surface textures of basaltic lava. Lava tube is the natural conduit formed within a flow. Tephra encompasses all fragmented volcanic material ejected into the air, as opposed to lava, which flows on the surface.