Aa vs. Pahoehoe

What's the Difference?

Aa and Pahoehoe are two types of lava flows commonly found in volcanic regions. Aa lava is characterized by its rough and jagged texture, formed by the slow movement and cooling of highly viscous lava. It often creates a rough and uneven surface, making it difficult to walk or drive on. On the other hand, Pahoehoe lava flows are smoother and have a rope-like appearance due to the rapid movement and low viscosity of the lava. Pahoehoe flows can form intricate patterns and can sometimes resemble twisted ropes or braids. While both types of lava flows originate from the same source, their different textures and appearances are a result of variations in lava viscosity and flow dynamics.


DefinitionA type of lava flow characterized by a rough, blocky surfaceA type of lava flow characterized by a smooth, ropy surface
ViscosityHigh viscosityLow viscosity
TemperatureHigher temperatureLower temperature
Surface TextureRough and blockySmooth and ropy
FormationForms from highly viscous lavaForms from low-viscosity lava
Commonly FoundHawaii, Iceland, and other volcanic regionsHawaii, Iceland, and other volcanic regions

Further Detail


Volcanic eruptions are fascinating natural phenomena that shape the Earth's surface. One of the most striking features of volcanic activity is the formation of lava flows. Lava can take on different forms, with two common types being Aa and Pahoehoe. These terms, derived from the Hawaiian language, describe the textures and characteristics of lava flows. In this article, we will explore and compare the attributes of Aa and Pahoehoe, shedding light on their distinct features and the processes that give rise to them.

Formation and Composition

Aa and Pahoehoe lava flows are both formed from basaltic magma, which is rich in iron and magnesium. However, their different attributes arise from variations in the cooling and flow dynamics of the lava. Pahoehoe lava flows are characterized by their smooth, ropy texture, which forms when the lava is relatively fluid and flows rapidly. This allows the surface to solidify into a continuous, undulating crust. In contrast, Aa lava flows have a rough, jagged appearance due to the slower movement and higher viscosity of the lava. As the Aa lava advances, the outer surface cools and solidifies, while the molten interior continues to flow, causing the crust to break into sharp, angular fragments.

Surface Texture and Appearance

The surface texture of Aa and Pahoehoe lava flows is one of the most noticeable differences between the two types. Pahoehoe lava has a smooth, billowy appearance, often resembling twisted ropes or braids. The ropy texture is a result of the continuous, laminar flow of the lava, which creates elongated, sinuous structures. In contrast, Aa lava flows have a rough, clinkery surface composed of sharp-edged fragments. These fragments, known as clinker, are formed when the solidified crust of the lava flow breaks apart due to the continued movement of the molten interior. The clinker gives Aa lava flows a rugged and jagged appearance.

Flow Dynamics and Speed

The flow dynamics and speed of Aa and Pahoehoe lava flows are closely related to their surface textures. Pahoehoe lava flows are characterized by their relatively fast movement and low viscosity. The lava flows smoothly and can travel long distances before solidifying. This fluidity allows Pahoehoe lava to form extensive, continuous sheets. In contrast, Aa lava flows move more slowly and have a higher viscosity. The lava is more resistant to flow, resulting in a fragmented and blocky appearance. The slower movement of Aa lava also limits its ability to travel far from the vent, causing it to pile up in thick, short flows.

Temperature and Cooling

The temperature and cooling processes of Aa and Pahoehoe lava flows play a significant role in their formation and appearance. Pahoehoe lava flows are hotter than Aa lava flows, with temperatures ranging from 1,100 to 1,200 degrees Celsius (2,000 to 2,200 degrees Fahrenheit). The high temperature of Pahoehoe lava allows it to remain fluid and flow rapidly, resulting in its smooth surface texture. Aa lava flows, on the other hand, have lower temperatures, typically ranging from 800 to 1,000 degrees Celsius (1,500 to 1,800 degrees Fahrenheit). The lower temperature contributes to the higher viscosity of Aa lava, causing it to move more slowly and solidify into a rough surface.

Volcanic Hazards

Both Aa and Pahoehoe lava flows pose unique hazards to surrounding areas during volcanic eruptions. Pahoehoe lava flows, due to their fluid nature, can travel at higher speeds and cover larger areas. This makes them potentially more dangerous to human settlements and infrastructure. However, Pahoehoe lava flows are generally easier to navigate and traverse due to their smooth surface texture. In contrast, Aa lava flows, with their rough and jagged surface, present significant challenges for movement. The clinker fragments can be sharp and unstable, making it difficult to walk or drive across an Aa lava flow. Additionally, the slower movement of Aa lava flows can result in the accumulation of thick lava masses, which can block roads and hinder evacuation efforts.


In conclusion, Aa and Pahoehoe lava flows are two distinct types of lava with contrasting attributes. Pahoehoe lava flows are characterized by their smooth, ropy texture, fast movement, and low viscosity. On the other hand, Aa lava flows have a rough, clinkery surface, slower movement, and higher viscosity. These differences arise from variations in cooling, flow dynamics, temperature, and composition. Understanding the attributes of Aa and Pahoehoe lava flows is crucial for assessing volcanic hazards and studying the geological processes that shape our planet. By studying these fascinating lava flows, scientists can gain valuable insights into the behavior of volcanoes and the formation of volcanic landscapes.

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