Lava Types Explained: Your Guide to Exploring Lava in Iceland

Discover the main lava types in Iceland, from fast-flowing basalt to explosive rhyolite and all the lavas in between from the volcanoes you see as you travel the island.

At the Lava Show, you can experience real, molten lava as it would be directly coming out of the earth. You can feel the lava’s heat, here it break and shatter, and see the differences in appearance as it cools. It is a unique experience to see lava up-close, in a controlled environment, with a specialist to answer your questions. This unique experience is avaialble in our locations in Reykjavik or Vík.

But what about lava out in the wild: what is it made of, what geologic properties control its formation, and what does it look like along the road and at the main sites you visit in Iceland?

Lava can be complicated. There are so many different properties that control the appearance and composition of lava. But, we promise, it can be simplified! Here in this article, we will talk about the basic science of lava and how you can become a lava expert and wow all your family and friends as you travel through Iceland.

Ropy pāhoehoe lava at Þingvellir, formed by smooth, flowing basalt during gentle volcanic eruptions. Photo by Jessica Poteet.

Lava Basics: what controls the nature of lava

If you remember from previous Lava Academy articles, lava is magma that has erupted from the ground. The composition and conditions magma undergoes is what controls lava. And one of those main properties is rheology (a fluid’s ability to flow or not flow, i.e. viscosity). So what are the main rheological properties that make different magmas, and thus make different lavas:

Jagged ʻaʻā lava field on the Reykjanes Peninsula, where rough, broken basalt creates a rugged and uneven landscape. Photo by Jessica Poteet.

Silica content

The more silica in the lava, the more its chemical elements are stuck together and tangled, which makes the lava thicker and harder to flow (more viscous). The more viscous a lava, the more it is prone to explosive eruptions.

Temperature

The hotter the lava, the more runny (less viscous) it is. Temperature is controlled by tectonic setting (i.e. are the tectonic plates spreading apart or colliding) of the erupting volcano and depth of magma storage chambers.

Columnar basalt formations at Reynisfjara, where cooling lava has fractured into striking hexagonal pillars. Photo by Jessica Poteet.

Silica content

Crystal content (or age of system)

The more solid crystal material in the lava, the more viscous a lava will be. Magma is usually made up of some percentage of fully melted material (melt), solidified material (mineral crystals), and gas (bubbles). And what causes a magma to solidify, thus making a more viscous lava? Time sitting underground (age), and cooling of the magma system.

Gas content (or pressure of system)

If trapped gas bubbles can escape from magma as it rises to the surface to erupt as lava, this also lowers the viscosity (runniness) of the lava. The more pressure in a volcanic system as the magma rises, the more gas tends to stay in the magma, causing it to explode and create thicker lava deposits.

As you can see, there are lots of physical properties that cause lava to form and behave differently. But this is just one way geologists and volcanologists classify lava. Above we talk about the silica content, and this is the key characteristic that scientists use when classifying and naming lavas.

Glymur waterfall cascading through layered basalt cliffs, revealing dark, thin lava flows built up over time. Photo by Jessica Poteet.

Silica content

Silicate minerals: the backbone of the earth’s crust and its lavas

If you look at the chemical composition of the earth’s crust, it is predominately made up of silica-rich minerals (SiO2). There will be no chemistry tests in this article, but simply know that the percentage of these silicate minerals is how scientists classify lava, and usually that percentage is controlled by the type of rock being melted initially to form magma which in turn forms the lava we see on earth. Here is an easy, generalized breakdown of different lavas:

Mafic Lavas

• Low silica (~45–52%)

• Rich in iron and magnesium

• High temperature (~1100–1200°C)

• Low viscosity

Typical rock: basalt
Tectonic settings: mid-ocean ridges, hotspots (e.g., Iceland, Hawaii)
Flow type: mafic lavas are fluid and capable of traveling long distances.

Intermediate Lavas'

• Moderate silica (~52–63%)

• Intermediate viscosity and temperature (~900–1100°C)

Typical rock: andesite
Tectonic settings: subduction zones (e.g. Pacific Northwest, Japan, Indonesia)
Flow Type: mixed behavior, both effusive and explosive depending on gas content.

Felsic Lavas

• High silica (>63%)

• Rich in silica, sodium, and potassium

• Lower temperatures (~650–900°C)

• Very high viscosity

Typical rock: dacite, rhyolite

Tectonic settings

Continental arcs, evolved volcanic systems (Philippines, Indonesia, New Zealand, continental USA)

Flow Type

Sluggish, gloopy, and tend to form domes or thick, short flows.

Most of the lava you will see in Iceland will be basalt: large fields of hardened lava that formed hot, runny molten lava that came out of a fissure eruption or shield volcano. That said, there are areas of rhyolitic lava, formed from magma chambers that sat in the earth a long time and partially solidified or interacted with cooling waters from glaciers before erupting.

But as you are driving or hiking around Iceland, how will you know the difference? Let’s look at the different morphologies (rock shape, appearance) of lava that you can see here on the island, and teach you how to identify different lavas.

Easy-to-use Iceland Lava Field Guide

Here are the different lava types that you will see in Iceland, and here is how to differentiate between them as you encounter lava rocks in the wild.

Brimketill lava pool on Iceland’s Reykjanes Peninsula, where waves have carved a natural basin into rugged basalt rock.

Basalt (the default rock in Iceland)

First impression: dark, dense, and widespread.

What Basalt lava looks like?

• Color: black to dark gray (fresh), weathers to brown

• Texture: very fine-grained, often with gas bubble holes (vesicles)

• Field clues: widely expansive, dark, looks like elves could live there

• Special surface types:

• Smooth, ropy, called pāhoehoe

• Rough, jagged, called 'a‘ā

• Vertical, smooth columns, called stuðlaberg in Iceland

  
  

Where to find Basalt lava

the lava fields at Fagradalsfjall and Sundhnúkur, the lava fields between Vík and Kirkjabæjarklaustur, columnar basalt at Reynisfjara and Svartifoss.

Rhyolite (the obvious contrast rock)

First impression: light-colored and visually striking.

What it looks like?

• Color: light gray, pink, beige, sometimes yellow, can alter to green

• Texture: very fine-grained, may contain visible crystals, can also be glassy

• Field clues: thick masses, layered or flow bands, colorful

• Special surface types:

• Smooth, black glass, called obsidian

• Very light, almost frothy texture, called pumice
  
  

Obsidian rock in Hrafntinnusker, where rapidly cooled rhyolite lava forms Iceland’s striking black volcanic glass.

Where to find:?

The colorful Highland mountains in Landmannalaugar and Kerlingarfjöll, many mountains in east Iceland, that one reddish mountain just east of Esjan in the Reykjavik area, called Mosfellshnjúkar.

Vibrant rhyolite mountains in Landmannalaugar, shaped by evolved lava flows and rich mineral composition. Photo by Jessica Poteet.

Silica content

And here is a quick list for easy identification of the different lava types seen here in Iceland.

• Dark, smooth or jagged, also columnar, and widespread? → Basalt

• Flat, smooth, ropey texture and dark? → Pāhoehoe

• Pointy, jagged, looks like an elf church, dark? → 'A‘ā

• Smooth, vertical columns, hexagonal shape → Columnar basalt, stuðlaberg

• Light- or multi-colored, thick, banded? → Rhyolite

• Glassy, smooth, black? → Obsidian

• Light and frothy, with holes? → Pumice

• Dark, rounded blobs, appearance like a pile of boulders? → Pillow lava

• Brown, fragmented ridges, sometimes with sandy layers? → Hyaloclastite

  
  

Kleifarvatn lake on the Reykjanes Peninsula, where subglacial eruptions have formed glassy, fragmented hyaloclastite landscapes along the shoreline. Photo by Jessica Poteet.

Silica content

Once you know what to look for, lava fields start to feel like a geological scavenger hunt. A footpath carved into ropy pāhoehoe, a mossy field of ʻaʻā, a piece of glassy obsidian on a hiking trail, a cliff face of columnar basalt: it is all out there waiting to be spotted. You might even find yourself thinking, “gotta catch ’em all,” like the Pokemon game, as you move across the landscape, ticking off each texture and form as another clue to how an ancient eruption unfolded.

The final thoughts on lava types

Lava is never just lava.

As you now know, it is the final expression of a journey that begins deep underground. A story that began when chemistry, temperature, pressure, gas, and tectonic setting all worked to shape how molten rock behaved once it reached the surface.

Pillow lava formations at Nesjavellir, created when molten lava erupted underwater and rapidly cooled into rounded, bulbous shapes.

Learning to recognize lava types is like learning a new language – one that lets you read the history of an eruption written across the landscape. At the Lava Show, that language comes alive. You don’t just hear about flow behavior, you see it happen in real time. And in that moment, the science of lava becomes something more: a direct connection to the dynamic forces still shaping our planet today.

The Lava Academy Podcast

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This article is written by geologist Jessica Poteet. Listen to the interview with her on the Lava Academy Podcast.