Understanding the Two Main Types of Lava: Pahoehoe and 'A'a

The two primary types of basaltic lava flows are pahoehoe and 'a'a. Pahoehoe is characterized by its smooth, billowy, or rope-like surface, while 'a'a is distinguished by its rough, jagged, and "clinkery" texture consisting of sharp, broken lava blocks. Both originate from similar volcanic sources but differ due to their cooling rates and flow dynamics.

While the world of volcanology encompasses a vast array of eruptive styles and chemical compositions, the morphological distinction between pahoehoe and 'a'a remains the most fundamental classification for subaerial basaltic flows. These terms, of Hawaiian origin, have been adopted globally by geologists to describe how molten rock behaves once it reaches the Earth's surface. Understanding these two types requires looking beyond their surface appearance and into the physics of viscosity, shear stress, and thermal energy.

The Geological Foundation of Basaltic Lava

To understand why lava takes the form of either pahoehoe or 'a'a, one must first understand the material itself. Basalt is the most common rock type in the Earth's crust and makes up the entirety of the ocean floors. In 2026, our understanding of basaltic rheology—the study of how matter flows—has reached a point where we can predict flow transitions with high precision using satellite-based thermal imaging and real-time viscosity sensors.

Basaltic lava is typically high in iron and magnesium and relatively low in silica (silicon dioxide). This chemical makeup is crucial because silica acts as a thickening agent. Because basaltic lava has less silica than rhyolitic or andesitic lavas, it is "runny" or has low viscosity. This allows it to travel great distances from a volcanic vent, sometimes covering dozens of miles before cooling sufficiently to stop.

The temperature of these flows is immense, usually ranging between 1,100°C and 1,200°C (2,012°F to 2,192°F). At these temperatures, the lava glows a brilliant orange or yellow. As it moves and interacts with the atmosphere or the ground, it begins to lose heat, which triggers the structural changes that define its final type.

Pahoehoe: The Smooth Surveyor

Pahoehoe (pronounced pa-hoy-hoy) is the more fluid of the two types. The word in Hawaiian translates roughly to "smooth, unbroken lava." When you see images of "lava toes" advancing slowly across a road or "ropey" textures that look like twisted cable, you are looking at pahoehoe.

Formation and Appearance

Pahoehoe forms when the lava has a very low viscosity and a low rate of shear strain. As the molten rock flows, the surface in contact with the air cools rapidly, forming a thin, plastic "skin." Because the lava underneath this skin is still moving rapidly, it pushes against the cooling surface, causing it to wrinkle, fold, and billow.

This process results in several distinct sub-types of pahoehoe:

• Ropey Pahoehoe: Created when the surface skin is twisted or folded by the movement of the liquid lava below.
• Entrails: Long, narrow lobes that resemble biological organs, formed on steeper slopes.
• Pahoehoe Toes: Small, rounded lobes that break out from the front of a larger flow.

The Role of Lava Tubes

One of the most significant features of pahoehoe is its ability to form lava tubes. Because pahoehoe retains its heat so effectively beneath its insulating crust, the interior remains molten long after the surface has hardened. If the source of the lava is cut off, the molten interior may drain out, leaving behind a hollow, cave-like tube. In 2026, these tubes are recognized not just as geological curiosities but as vital components of volcanic plumbing systems that allow lava to travel miles underground without losing significant temperature.

'A'a: The Jagged Destroyer

'A'a (pronounced ah-ah) provides a stark contrast to the fluid grace of pahoehoe. The name is said to represent the sound one makes when walking barefoot across its razor-sharp surface. Unlike the smooth folds of pahoehoe, 'a'a consists of "clinkers"—broken, jagged, vesicular (holey) fragments of lava that can range from the size of a marble to the size of a house.

Formation and Mechanical Stress

'A'a forms when the lava has a higher viscosity or is subjected to high shear stress. This typically happens when the lava is flowing very quickly, is cooling down, or is losing its dissolved gases (vesiculation).

The mechanics of an 'a'a flow are similar to a tractor tread. The front of the flow is a steep wall of glowing, clattering blocks. As the molten core of the flow moves forward, it carries these blocks on top. Eventually, the blocks at the leading edge tumble down the front of the flow and are "run over" by the advancing lava. This creates a sandwich structure: a dense, massive molten core between two layers of jagged, fragmented clinkers.

The Sound of 'A'a

One of the most unique aspects of an 'a'a flow is the sound. Because the surface is composed of glass-like fragments, the movement of the flow creates a continuous metallic clinking or grinding noise. This "clinkery" nature makes 'a'a fields nearly impossible to traverse without heavy-duty footwear and extreme caution, as the rocks are often unstable and as sharp as industrial glass.

Comparing Pahoehoe and 'A'a

While both are basaltic, their physical properties and environmental impacts differ significantly. The following table summarizes the key distinctions between these two primary lava types.

The Irreversible Transition: From Pahoehoe to 'A'a

A fascinating aspect of volcanic activity is that a single lava flow can start as pahoehoe and transform into 'a'a. However, the reverse—'a'a turning back into pahoehoe—is virtually never observed in nature.

This transition is governed by the relationship between the lava's viscosity and the rate of "deformation" (how fast it is being stretched or pushed). As pahoehoe moves away from the volcanic vent, several things happen:

1. Heat Loss: The lava cools, which naturally increases its viscosity.
2. Degassing: Bubbles of gas (mostly water vapor and carbon dioxide) escape the melt. This loss of gas makes the lava "stiff."
3. Crystallization: Small mineral crystals begin to grow within the liquid, further increasing internal friction.

Once the lava reaches a critical point where it can no longer deform plastically to create smooth folds, the surface skin snaps and tears under the pressure of the moving interior. This tearing creates the first "clinkers" of 'a'a. Once this fragmentation begins, the increased surface area leads to even faster cooling, making the transition irreversible.

Secondary Lava Types and Sub-Classifications

While pahoehoe and 'a'a are the "Big Two" for land-based basaltic volcanoes, there are other forms of lava that occur under specific environmental conditions. Understanding these provides a more complete picture of volcanic morphology.

Pillow Lava: The Underwater Variant

When basaltic lava erupts underwater—such as at mid-ocean ridges or when a land flow enters the sea—it forms pillow lava. The instant contact with cold seawater creates a quenched, glassy rind around the lava. The internal pressure then causes the rind to crack, allowing a new "pillow" of lava to squeeze out, looking much like a stack of large, overstuffed cushions. This is the most common form of lava on Earth, as it covers most of the deep ocean floor.

Blocky Lava: The High-Silica Cousin

While not technically a "basaltic" type in the same sense as pahoehoe or 'a'a, blocky lava is often confused with 'a'a. Blocky flows occur when the lava is much higher in silica (such as andesite or rhyolite). Because the lava is so viscous, it doesn't form the small, spinose clinkers of 'a'a. Instead, it breaks into large, smooth-sided blocks. These flows are much thicker—sometimes hundreds of feet high—and move at an imperceptibly slow pace.

Safety and Environmental Impact in 2026

Living near or studying active lava flows requires a deep respect for their physical properties. In 2026, volcanologists use advanced LIDAR and drone-based photogrammetry to map the growth of 'a'a and pahoehoe fields in real-time, providing early warnings for communities in places like Hawaii, Iceland, and the Canary Islands.

Hazard Comparison

'A'a flows are often considered more destructive to property in the short term because they tend to be thicker and can act like a bulldozer, pushing over structures and trees. However, pahoehoe is more "insidious." Because it forms lava tubes, it can transport molten rock long distances without being visible from the surface. A seemingly "dead" lava field can have active pahoehoe flowing just a few feet beneath the crust.

Atmospheric Hazards: Laze

When both types of lava enter the ocean, they create a phenomenon known as Laze (lava haze). This is a plume of hydrochloric acid steam and fine volcanic glass particles. While 'a'a creates more surface area for the reaction, any contact between molten rock and seawater is hazardous to the lungs and eyes.

Frequently Asked Questions

Can you walk on lava?

While you cannot walk on liquid lava due to the extreme heat (you would catch fire before touching it), you can walk on cooled lava. Pahoehoe is generally safe to walk on once hardened, resembling a dark sidewalk. 'A'a, however, remains dangerous even after it has cooled. The fragments are unstable, and a fall onto 'a'a often results in deep lacerations.

Why is 'a'a sharper than pahoehoe?

'A'a is sharper because it forms through the mechanical breaking and shearing of cooling rock. This process creates jagged edges and "spinose" (spine-like) features. Pahoehoe, by contrast, cools while the rock is still fluid enough to stretch, resulting in rounded, smooth surfaces.

Which type of lava flows faster?

Pahoehoe is typically more fluid and can flow faster in concentrated channels or tubes, sometimes reaching speeds of 30 mph (50 km/h) in extreme cases. However, 'a'a flows can appear more aggressive as they move as a massive, surging wall of rock.

Is all lava either pahoehoe or 'a'a?

Most basaltic lava on land falls into one of these two categories. However, there are transitional forms where a flow might exhibit characteristics of both. Additionally, lavas with different chemical compositions (like those found in stratovolcanoes) form different structures entirely, such as lava domes or blocky flows.

Where can I see these lava types today?

As of 2026, the most consistent places to observe both pahoehoe and 'a'a are the Big Island of Hawaii (Hawaii Volcanoes National Park) and the Reykjanes Peninsula in Iceland. Both locations feature basaltic volcanism that frequently produces well-defined examples of both morphologies.

Practical Recommendation for Observation and Study

If you are planning to visit a volcanic site to see these formations, focus your attention on the edges of the flow. The transition zones—where smooth pahoehoe begins to tear and crumble into 'a'a—offer the best insight into the cooling process. Always wear thick-soled, high-top hiking boots, as the glass-rich crust of both types can easily shred thin sneakers. Furthermore, when traversing a pahoehoe field, be mindful of "skylights" (holes in the roof of a lava tube) and "lava blisters" (hollow swells in the crust), as these can be structurally weak and lead to dangerous falls into underlying cavities.