Understanding Pāhoehoe vs ʻAʻā Lava Flows

What Are Pāhoehoe and ʻAʻā Lava?

Pāhoehoe lava is characterized by its smooth, ropey texture. It flows in a relatively fluid state with low viscosity, allowing it to form thin, continuous sheets or ropes as it cools and solidifies.

ʻAʻā lava, on the other hand, is rough and clinkery. It has higher viscosity due to its cooler temperature when compared to pāhoehoe lava, resulting in a more fragmented flow with sharp edges and jagged surfaces.

The Role of Viscosity

Viscosity is a measure of a fluid's resistance to gradual deformation by shear or tensile stress. In the context of lava flows, higher viscosity means that the lava resists flow more easily, leading to more fragmented and clinkery textures like ʻaʻā.

Conversely, lower viscosity allows for smoother, more continuous flows such as pāhoehoe, which can travel greater distances before cooling and solidifying.

Cooling Rate and Its Impact

The rate at which lava cools significantly influences its texture. As lava cools, it loses heat to the surrounding environment or water (if present). Faster cooling rates can lead to rapid solidification, resulting in a rougher surface with more cracks and fragments.

Slower cooling allows for the formation of smoother textures as the lava has time to flow more continuously before solidifying.

Real-World Examples

Pāhoehoe flows are common in Hawaiian volcanoes, where the high temperature and low viscosity of basaltic lava allow for smooth, ropey textures. These flows can travel long distances before cooling completely.

ʻAʻā flows are more typical in eruptions with cooler lava or when the flow is obstructed, leading to rapid cooling and fragmentation. Examples include the 2018 Kīlauea eruption where both types of lava were observed.

Frequently asked questions

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