Lava Types in Iceland: Pahoehoe vs Aa

Iceland's volcanic landscape tells the story of millions of years of eruptions through the frozen language of lava rock. Across the island's 103,000 square kilometres, two distinct types of basaltic lava flows dominate the terrain: the smooth, ropey pahoehoe and the rough, jagged aa. Understanding these fundamental lava types transforms any journey across Iceland from mere sightseeing into geological detective work, where every rock formation reveals the dramatic circumstances of its birth.

The Formation Process: Temperature and Flow Dynamics

Both pahoehoe and aa lavas begin their journey identically, erupting from Icelandic volcanoes at temperatures between 1,100°C and 1,200°C. The critical difference lies not in their chemical composition—both are basaltic lavas with similar mineral content—but in their behaviour as they cool and flow across Iceland's varied topography.

Pahoehoe, pronounced "pah-hoy-hoy" from the Hawaiian term meaning "smooth, unbroken lava," forms when lava flows at relatively low speeds, typically 1-10 metres per hour, maintaining temperatures above 1,080°C. The outer skin cools gradually, creating a flexible crust that wrinkles and folds like thick treacle, producing the characteristic ropey texture that gives pahoehoe its distinctive appearance.

Aa lava (pronounced "ah-ah," Hawaiian for "hard on the feet") develops when flows exceed 10 metres per hour or when temperatures drop below 1,080°C. The increased shear stress breaks the cooling crust repeatedly, creating a chaotic jumble of sharp, angular blocks called clinker. These fragments tumble along the flow front, building the distinctive rough, impassable terrain that inspired its descriptive Hawaiian name.

Iceland's Pahoehoe Landscapes

The Reykjanes Peninsula

The Reykjanes Peninsula, just 40 kilometres southwest of Reykjavík, showcases some of Iceland's most accessible pahoehoe formations. The recent eruptions at Fagradalsfjall, beginning in March 2021, created extensive pahoehoe fields that visitors can explore safely. The smooth, undulating surfaces stretch for hundreds of metres, their bronze and black hues shifting with the light throughout the day.

These flows, easily reached via a 3-kilometre hike from the Fagradalsfjall car park, demonstrate pahoehoe formation in real-time. The lava's slow advance rate—rarely exceeding 5 metres per hour during the eruption—allowed the characteristic skin to develop, creating surfaces so smooth that locals joked about their suitability for skateboarding.

Snæfellsnes Peninsula Flows

The Snæfellsnes Peninsula, 120 kilometres north of Reykjavík, contains pahoehoe flows dating back 8,000 years. Near Hellnar, ancient pahoehoe formations create natural sculptures along the coastline, where centuries of Atlantic storms have carved the smooth lava into dramatic arches and pillars. The Lóndrangar basalt pinnacles, rising 75 metres from the sea, began as pahoehoe flows before millennia of erosion isolated these striking formations.

Aa Lava Territories

Eldhraun: The World's Largest Aa Flow

The Eldhraun lava field in southern Iceland represents the planet's largest historical aa flow, covering 565 square kilometres. Created during the catastrophic Laki eruption of 1783-1784, this vast expanse of jagged aa extends along Route 1 between Kirkjubæjarklaustur and Skaftafell National Park.

The sheer scale of Eldhraun overwhelms visitors driving the 50-kilometre stretch through its heart. The rough, clinkery surface, broken by occasional moss-covered hummocks, stretches to the horizon in every direction. This flow advanced at rates exceeding 1 kilometre per day during peak eruption phases, too rapid for stable crust formation, ensuring the development of classic aa morphology.

Krafla's Aa Fields

The Krafla volcanic system in northern Iceland, 7 kilometres northeast of Lake Mývatn, contains extensive aa flows from eruptions spanning 1975-1984. The Leirhnjúkur geothermal area provides hiking access through these relatively fresh aa formations, where visitors can appreciate both the challenging terrain aa creates and the way geothermal activity continues to shape the landscape.

Distinguishing Features in the Field

Surface Characteristics

Pahoehoe surfaces exhibit smooth, billowy forms often compared to coiled rope or folded drapery. The lava preserves flow lines and pressure ridges that indicate direction and rate of movement. Many pahoehoe flows develop hollow interior spaces called lava tubes, where the molten interior drained away while the crust remained intact. Iceland contains numerous accessible lava tubes formed in pahoehoe flows, including Raufarhólshellir, stretching 1,360 metres southeast of Reykjavík.

Aa surfaces present sharp, angular fragments ranging from gravel-sized pieces to blocks exceeding 10 metres in diameter. The rough texture makes walking across aa flows extremely difficult and potentially dangerous, earning aa the nickname "bulldozer lava" for the heavy machinery required to clear paths through it.

Cooling Patterns

Pahoehoe flows cool uniformly from the outside inward, creating smooth, glassy surfaces that often preserve gas bubbles and flow textures. The gradual cooling process allows minerals to form larger crystals, giving pahoehoe a denser, more homogeneous structure.

Aa flows cool chaotically, with constant mixing of hot interior material and cool surface fragments. This irregular cooling creates a more porous structure filled with irregular voids and sharp-edged vesicles.

Planning Your Lava Flow Exploration

Best Viewing Locations

The Golden Circle route provides excellent aa flow viewing at Þingvellir National Park, where 9,000-year-old aa formations fill the rift valley between the North American and Eurasian tectonic plates. Entry costs 750 ISK (€5) for parking, and the visitor centre provides detailed geological interpretation.

For pahoehoe exploration, the Blue Diamond Circle route on the Reykjanes Peninsula offers multiple viewing opportunities within 90 minutes of Reykjavík. The Reykjanes UNESCO Global Geopark provides free access to most sites, with detailed geological trails marked throughout the peninsula.

Safety Considerations

Both lava types present unique hazards. Pahoehoe's smooth surface can hide deep cracks and unstable sections, particularly near lava tubes. Aa terrain poses obvious dangers from sharp surfaces and unstable footing. Always wear sturdy hiking boots with ankle support, carry first aid supplies, and inform others of your planned route.

Recent volcanic areas may retain dangerous levels of toxic gases. Check safetravel.is for current volcanic activity and access restrictions before visiting any geological sites.

Optimal Visiting Conditions

Early morning light enhances the textural differences between pahoehoe and aa flows, while late afternoon sun creates dramatic shadows that emphasise surface variations. Winter conditions can obscure lava field details under snow, making summer visits (June-September) optimal for geological observation.

The interplay between pahoehoe and aa lavas has shaped Iceland's dramatic landscape over millions of years, creating the diverse terrain that makes the island a world-class destination for geological exploration. Understanding these fundamental volcanic processes adds profound depth to any Icelandic adventure, transforming simple lava fields into windows into the dynamic forces that continue to build and reshape this remarkable island.

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