Lava Flow Forms

Photo of fresh lava flowing across a field of blocky broken lava.
Pāhoehoe flowing over an older ‘a‘ā flow in 2015.

USGS photo.


Young basaltic lava flows, with surfaces that are either smooth and ropy or rough and clinkery, are dramatic landscapes of dark volcanic rock. Depending on how recently the eruptions that formed them occurred, their surfaces may be barren of vegetation and comprise some of the most inhospitable landscapes on the planet and are among some of the most difficult terrain for people to travel. In addition to forming dramatic and desolate landscapes, young lava flows also have structures and textures that reveal information about their eruptions and the dynamics during emplacement.

Basaltic lava flows come in two major morphological forms that describe the overall structure and textures of their surfaces and other characteristics:

  • Pāhoehoe – Lava flows with smooth, billowy, or ropy surfaces

  • ‘A‘ā – Lava flows with rough, jagged, or clinkery surfaces

Both of these terms are Hawaiian words. The etymology of pāhoehoe shows that it relates to the Hawaiian word “hoe,” meaning to paddle and probably refers to the swirling eddies produced in water by paddle strokes that look similar to the ropy surfaces on pāhoehoe lava flows. ‘A‘ā likely relates to the Hawaiian word for fire.

Pāhoehoe and ‘a‘ā lava flows do not differ in composition in any significant way, but there are important differences between them in terms of temperature, viscosity, and other characteristics.

Characteristics of Pāhoehoe and ʻA‘ā Lava Flows



Surface Description

Smooth, ropy

Rough, clinkery







Eruption Rate






Distance from Vent



The difference in viscosity (resistance to flow) is the most important characteristic of flowing lava that determines whether it will have a pāhoehoe or ‘a‘ā form. Pāhoehoe forms in more fluid (less viscous lavas) relative to ones that form ‘a‘ā. The amount of shear strain (strain produced by pressure), which is frequently related to the speed that a lava travels and the eruption rate, also greatly influences whether a lava is pāhoehoe or ‘a‘ā. Lavas that experience greater amounts of shear strain, such as ones with high discharge rates at the vent, are more likely to be ‘a‘ā than pāhoehoe.

As a lava cools and loses the gas (water, carbon dioxide, etc.) that had been dissolved in it, it becomes more viscous. Sometimes a flow that had a pāhoehoe form near its vent transitions to ‘a‘ā further away.

Photo of a person standing and watching molten lava flow across an older lava rock surface.
An active ‘a‘ā flow over an older pāhoehoe flow with a USGS geologist photographing it during a 2016 eruption of Kīlauea in Hawai’i Volcanoes National Park.

USGS photo.


Pāhoehoe lava flows are not as common as ‘a‘ā flows. Pāhoehoe will generally only form in lavas erupted with a slow rate and that travel at slower speeds on gently-sloping ground.

Pāhoehoe flows may travel as sheets, in lava channels, and in lava tubes. Lava tubes form where the cooled and solidified crust of the lava flow insulates the interior of the flow allowing molten lava to flow within it. Even after the surface has cooled and the flow front has either stopped or slowed, the flow may inflate if lava continues to be erupted at the vent and enter the body of lava. Pāhoehoe flow fronts usually advance as a series of small lobes or toes that breakout from the cooled crust.

Photo of lava ropes close up.
Close up of pāhoehoe ropes showing the incandescent interior.

USGS photo.

Types of Pāhoehoe

Pāhoehoe lava flows have surfaces that are typically ropy, billowing, hummocky, or smooth. Several subtypes of this form have been described, including smooth, ropy, hummocky, shelly, slabby, spiny, toothpaste and entrail.

Ropy Pāhoehoe

Ropy pāhoehoe is the most well-known type of this morphological form since ropy flow surfaces are very photogenetic. Ropy surfaces form as sheer strain accumulates at the surface as lava moving downstream drags the cooling crust along with it.

Photo of ropy lava rock with a series of curved ridges on the surface.
Ropy pāhoehoe in Craters of the Moon National Monument, Idaho.

NPS photo.

Shelly Pāhoehoe

Shelly pāhoehoe forms in gas-rich lava flows where open tubes and blisters develop as the gas exsolves from within it, creating thin crusts (shells) above them at the surface.

Photo of a person standing in a rugged volcanic landscape.
Shelly pāhoehoe in Craters of the Moon National Monument.

Geologic Map Of The Core Visitation Area Of Craters Of The Moon National Monument And Preserve, South-Central Idaho, With Descriptions Of 38 Points Of Geologic Interest.

Slabby Pāhoehoe

Slabby pāhoehoe is made up of jumbled plates or slabs of pāhoehoe crusts that have broken and overturned. Slabby pāhoehoe is transitional to ‘a‘ā as the surface can’t accommodate the rate of sheer without tearing, but is fluid enough to form ropes and other pāhoehoe features.

Photo of lava flow with molten lava under cooler slabs of rock.
Slabby pāhoehoe erupted from Kīlauea Volcano in Hawai’i Volcanoes National Park in 2014.

USGS photo.

Spiny Pāhoehoe

Spiny pāhoehoe is also transitional to ‘a‘ā. Spiny pāhoehoe forms from relatively viscous lavas that contain gas bubbles that create spines.

Photo of a lobe of molten lava with a spiny surface.
Spiny pāhoehoe lava erupted from Kīlauea in Hawai’i Volcanoes National Park.

USGS photo.

Pāhoehoe Toes

The small buds that breakout on the edges of an advancing flow as it moves are known as pāhoehoe toes. Pāhoehoe toes are usually small, up to a few feet (m) wide. Their surfaces quickly cool upon exposure to the atmosphere, but the toes can continue to inflate with the advancement of the flow. Individual toes may coalesce together to form a broader flow front.

Photo of pāhoehoe toes on an active lava flow.
Pāhoehoe toes on an active lava flow from Kīlauea Volcano in Hawai’i Volcanoes National Park in 2012.

USGS photo.


‘A‘ā lava flows have irregular rough surfaces made of jagged, spiny and rough clasts of lava called clinkers. These lavas have surfaces that are either too viscous or are flowing too rapidly to flow plasticly like pāhoehoe. Instead they are ripped apart by shear strain forming a breccia at the top of the flow.

The interior of an ‘a‘ā flow is massive and dense with clinkers only being present near the surface. Clinkers may fall in front of an advancing flow that will then bury them as it moves, leading to rubbly zones at the top and bottom of flows. ‘A‘ā lavas travel either as sheets or in lava channels between levees, and move at rates that are more rapid than pāhoehoe.

‘A‘ā is the most common type of lava flow and is more common than pāhoehoe.

Photo of a blocky lava flow advancing into a grassy meadow.
‘A‘ā lava flow in 2018 during the lower East Rift Zone eruption of Kīlauea.

Hawaiian Volcano Observatory photo.

Close up view of ‘a‘ā showing the incandescent interior as well as the cooled rubbly clinker.
Close up view of ‘a‘ā showing the incandescent interior as well as the cooled rubbly clinker.

USGS photo.

Photo of a volcanic landscape covered with rough and rubbly lava rock.
The Bonita Lava Flow, which mostly consists of ‘a‘ā, which was erupted from Sunset Crater Volcano in Arizona.

John St. James photo on Flickr.

Parks with Basaltic Lava Flows

  1. Bandelier National Monument (BAND), New Mexico—[BAND Geodiversity Atlas] [BAND Park Home] [BAND]

  2. Bering Land Bridge National Preserve (BELA), Alaska—[BELA Geodiversity Atlas] [BELA Park Home] [BELA]

  3. Big Bend National Park (BIBE), Texas—[BIBE Geodiversity Atlas] [BIBE Park Home] [BIBE]

  4. Capulin Volcano National Monument (CAVO), New Mexico—[CAVO Geodiversity Atlas] [CAVO Park Home ] [CAVO]

  5. Crater Lake National Park (CRLA), Oregon—[CRLA Geodiversity Atlas] [CRLA Park Home] [CRLA]

  6. Craters of the Moon National Monument (CRMO), Idaho—[CRMO Geodiversity Atlas] [CRMO Park Home] [CRMO]

  7. Death Valley National Park (DEVA), California & Nevada—[DEVA Geodiversity Atlas] [DEVA Park Home] [DEVA]

  8. Devils Postpile National Monument (DEPO), California—[DEPO Geodiversity Atlas] [DEPO Park Home] [DEPO]

  9. El Malpais National Monument (ELMA), New Mexico—[ELMA Geodiversity Atlas] [ELMA Park Home] [ELMA]

  10. Fort Vancouver National Historic Site (FOVA), Washington—[FOVA Geodiversity Atlas] [FOVA Park Home] [FOVA]

  11. Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA]

  12. Grand Canyon-Parashant National Monument (PARA), Arizona—[PARA Geodiversity Atlas] [PARA Park Home] [PARA]

  13. Grand Teton National Park (GRTE), Wyoming—[GRTE Geodiversity Atlas] [GRTE Park Home] [GRTE]

  14. Hagerman Fossil Beds National Monument (HAFO), Idaho—[HAFO Geodiversity Atlas] [HAFO Park Home] [HAFO]

  15. Haleakala National Park (HALE), Hawaii—[HALE Geodiversity Atlas] [HALE Park Home] [HALE]

  16. Hawai’i Volcanoes National Park (HAVO), Hawai’i—[HAVO Geodiversity Atlas] [HAVO Park Home] [HAVO]

  17. Isle Royale National Park (ISRO), Michigan—[ISRO Geodiversity Atlas] [ISRO Park Home] [ISRO]

  18. Kalaupapa National Historical Park (KALA), Hawai’i—[KALA Geodiversity Atlas] [KALA Park Home] [KALA]

  19. Kaloko-Honokohau National Historical Park (KAHO), Hawai'i—[KAHO Geodiversity Atlas] [KAHO Park Home] [KAHO]

  20. Katmai National Park (KATM), Alaska—[KATM Geodiversity Atlas] [KATM Park Home] [KATM]

  21. Keweenaw National Historic Park (KEWE), Michigan—[KEWE Geodiversity Atlas] [KEWE Park Home] [KEWE]

  22. Lake Mead National Recreation Area (LAKE), Arizona and Nevada—[LAKE Geodiversity Atlas] [LAKE Park Home] [LAKE]

  23. Lake Roosevelt National Recreation Area (LARO), Washington—[LARO Geodiversity Atlas] [LARO Park Home] [LARO]

  24. Lassen Volcanic National Park (LAVO), California—[LAVO Geodiversity Atlas] [LAVO Park Home] [LAVO]

  25. Lava Beds National Monument (LABE), California—[LABE Geodiversity Atlas] [LABE Park Home] [LABE]

  26. Lewis and Clark National Historical Park (LEWI), Oregon and Washington—[Volcanoes of the Lewis & Clark Trail]

  27. Mojave National Preserve (MOJA), California—[MOJA Geodiversity Atlas] [MOJA Park Home] [MOJA]

  28. National Park of American Samoa (NPSA), American Samoa—[NPSA Geodiversity Atlas] [NPSA Park Home] [NPSA]

  29. Petrified Forest National Park (PEFO), Arizona—[PEFO Geodiversity Atlas] [PEFO Park Home] [PEFO]

  30. Petroglyph National Monument (PETR), New Mexico—[PETR Geodiversity Atlas] [PETR Park Home] [PETR]

  31. Pu'uhonau o Honaunau National Historic Park (PUHO), Hawai'i—[PUHO Geodiversity Atlas] [PUHO Park Home] [PUHO]

  32. Pu'ukohola Heiau National Historic Site (PUHE), Hawai'i—[PUHE Geodiversity Atlas] [PUHE Park Home] [PUHE]

  33. Rocky Mountain National Park (ROMO), Colorado—[ROMO Geodiversity Atlas] [ROMO Park Home] [ROMO]

  34. Shenandoah National Park (SHEN), Virginia—[SHEN Geodiversity Atlas] [SHEN Park Home] [SHEN]

  35. Sunset Crater Volcano National Monument (SUCR), Arizona—[SUCR Geodiversity Atlas] [SUCR Park Home] [SUCR]

  36. Voyageurs National Park (VOYA), Minnesota—[VOYA Geodiversity Atlas] [VOYA Park Home] [VOYA]

  37. Wupatki National Monument (WUPA), Arizona—[WUPA Geodiversity Atlas] [WUPA Park Home] [WUPA]

  38. Yellowstone National Park (YELL), Wyoming—[YELL Geodiversity Atlas] [YELL Park Home] [YELL]

  39. Yosemite National Park (YOSE), California—[YOSE Geodiversity Atlas] [YOSE Park Home] [YOSE]

  40. Zion National Park (ZION), Utah—[ZION Geodiversity Atlas] [ZION Park Home] [ZION]

Bandelier National Monument, Bering Land Bridge National Preserve, Big Bend National Park, Capulin Volcano National Monument, Crater Lake National Park, Craters Of The Moon National Monument & Preserve, Death Valley National Park, Devils Postpile National Monument, El Malpais National Monument, Fort Vancouver National Historic Site, Grand Canyon National Park, Grand Canyon-Parashant National Monument, Grand Teton National Park, Hagerman Fossil Beds National Monument, Haleakalā National Park, Hawaiʻi Volcanoes National Park, Isle Royale National Park, Kalaupapa National Historical Park, Kaloko-Honokōhau National Historical Park, Katmai National Park & Preserve, Keweenaw National Historical Park, Lake Mead National Recreation Area, Lake Roosevelt National Recreation Area, Lassen Volcanic National Park, Lava Beds National Monument, Lewis and Clark National Historical Park, Mojave National Preserve, National Park of American Samoa, Petrified Forest National Park, Petroglyph National Monument, Puʻuhonua o Hōnaunau National Historical Park, Puʻukoholā Heiau National Historic Site, Rocky Mountain National Park, Shenandoah National Park, Sunset Crater Volcano National Monument, Voyageurs National Park, Wupatki National Monument, Yellowstone National Park, Yosemite National Park, Zion National Park more »

Last updated: July 9, 2024