Lava, the molten rock expelled by a volcano during an eruption, presents a remarkable spectrum of forms and behaviors. Its appearance and movement are determined by a precise chemistry interacting with environmental conditions, creating a dynamic and visually stunning natural phenomenon. Understanding the different kinds of lava is essential for grasping how volcanoes build landscapes and how they pose risks to the surrounding environment.
Viscosity: The Primary Determinant
The most fundamental way to categorize lava is by its viscosity, or its resistance to flow. This property is primarily controlled by silica content, temperature, and gas pressure. Low-viscosity lavas flow easily, traveling great distances and forming gentle slopes, while high-viscosity lavas pile up near the vent, creating steep, precarious structures. This simple concept explains the majority of the diversity observed in volcanic landforms.
Pahoehoe Lava: The Ropey River
Pahoehoe (pronounced pah-ho-ho) lava is characterized by its low viscosity, allowing it to flow smoothly and efficiently. Its surface is typically smooth, ropy, or billowy, resembling twisted rope or hardened chocolate. This type of lava can form lava tubes as the surface cools and solidifies while the molten interior continues to flow, creating remarkable natural tunnels. Pahoehoe is generally associated with Hawaiian-style eruptions, where it moves quickly and poses a threat primarily through its speed and volume rather than explosive violence.
Aa Lava: The Jagged Clinker
In contrast, aa (pronounced ah-ah) lava is a rough, clinkery rock formed from high-viscosity basaltic lava. Its brittle surface fractures into a chaotic field of sharp, jagged blocks that can be painful to walk on, hence its name. Aa flows much more slowly than pahoehoe and tends to advance in a series of short, thick lobes. The rough texture of aa actually insulates the hotter, fluid interior, allowing it to travel considerable distances despite its high viscosity.
The Role of Silica and Gas
Silica acts as a hardening agent in magma; the more silica present, the higher the viscosity. This is why felsic lavas, which are rich in silica, are so much more explosive and viscous than mafic lavas, which are silica-poor. Gas content is the other critical factor. When gas-rich, highly viscous magma decompresses during ascent, the dissolved gases exsolve, creating pressure that leads to violent fragmentation. This process is responsible for the most dramatic and dangerous volcanic events.
Andesite and Dacite: The Destructive Intermediates
Andesite and dacite are intermediate lavas that sit between the benign basalts and the catastrophic rhyolites in terms of viscosity and explosivity. Andesite, common in stratovolcanoes around the Pacific Ring of Fire, can produce thick, blocky flows that build steep cones. Dacite, with its higher silica content, is even more viscous and is often involved in the formation of lava domes. These domes are unstable masses that can collapse, triggering pyroclastic flows, or they can extrude slowly, creating monstrous, spine-like protrusions.
Rhyolite: The Glassy Giant
Rhyolite is the most silica-rich lava on Earth, making it extremely viscous and prone to explosive eruptions. Because it is so thick, it rarely flows far, often plugging the volcanic vent and leading to catastrophic pressure build-up. When it does erupt, it can form obsidian, a naturally occurring volcanic glass, or contribute to the formation of vast ash deposits and pyroclastic ignimbrites. Its high viscosity prevents gas from escaping easily, making it one of the most dangerous types of lava.
