Lava

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.

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.
• Intermediate (Andesitic): Moderate silica content (52% to 63%). Common in stratovolcanoes, andesitic lava flows are thick and blocky, often not traveling far from the vent.
• Mafic (Basaltic): Low silica content (45% to 52%) and high in iron and magnesium. These lavas are the hottest (1000°C to 1200°C) and least viscous, allowing them to flow rapidly over vast distances.

Rheology and Viscosity

The viscosity of lava—its resistance to flow—is its most significant physical property. It determines the shape of the volcano and the style of eruption. Viscosity is governed by temperature, chemical composition, and gas content.

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:

1. Pahoehoe: Characterized by a smooth, billowy, or ropy surface. Pahoehoe forms from hot, fluid lava with a relatively low flow rate. As the skin cools, the continuing flow underneath drags it into rope-like folds.
2. ‘A’a: A rough, jagged surface composed of loose, broken lava blocks called clinkers. ‘A’a forms when the lava is slightly cooler and has a higher discharge rate. The crust breaks into sharp chunks as the faster-moving core drags it along.
3. Pillow Lava: Created when lava erupts underwater. The rapid cooling causes the lava to extrude in rounded, pillow-shaped lobes. This is the most common type of lava formation on Earth, covering the vast majority of the ocean floor.
4. Blocky Lava: Common in andesitic or rhyolitic flows, where the lava is too thick to form clinkers. Instead, it fractures into massive, smooth-sided blocks.

The Life Cycle of Lava

After eruption, lava acts as a constructive force. It adds new land mass to islands and continents. Over geologic time scales, 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.