Lava Flow Forms

Pāhoehoe flowing over an older ‘a‘ā flow in 2015. USGS photo.

Introduction

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

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.

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

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.

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.

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.

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.

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.

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.

Pāhoehoe toes on an active lava flow from Kīlauea Volcano in Hawai’i Volcanoes National Park in 2012. USGS photo.

ʻA‘ā

‘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.

‘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. USGS photo.

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

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

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

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

Capulin Volcano National Monument (CAVO), New Mexico—[CAVO Geodiversity Atlas] [CAVO ParkHome ] [CAVO npshistory.com]

Crater Lake National Park (CRLA), Oregon—[CRLA Geodiversity Atlas] [CRLA ParkHome] [CRLA npshistory.com]

Craters of the Moon National Monument (CRMO), Idaho—[CRMO Geodiversity Atlas] [CRMO ParkHome] [CRMO npshistory.com]

Death Valley National Park (DEVA), California & Nevada—[DEVA Geodiversity Atlas] [DEVA ParkHome] [DEVA npshistory.com]

Devils Postpile National Monument (DEPO), California—[DEPO Geodiversity Atlas] [DEPO ParkHome] [DEPO npshistory.com]

El Malpais National Monument (ELMA), New Mexico—[ELMA Geodiversity Atlas] [ELMA ParkHome] [ELMA npshistory.com]

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

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

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

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

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

Haleakala National Park (HALE), Hawaii—[HALE Geodiversity Atlas] [HALE ParkHome] [HALE npshistory.com]

Hawai’i Volcanoes National Park (HAVO), Hawai’i—[HAVO Geodiversity Atlas] [HAVO ParkHome] [HAVO npshistory.com]

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

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

Kaloko-Honokohau National Historical Park (KAHO), Hawai'i—[KAHO Geodiversity Atlas] [KAHO ParkHome] [KAHO npshistory.com]

Katmai National Park (KATM), Alaska—[KATM Geodiversity Atlas] [KATM ParkHome] [KATM npshistory.com]

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

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

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

Lassen Volcanic National Park (LAVO), California—[LAVO Geodiversity Atlas] [LAVO ParkHome] [LAVO npshistory.com]

Lava Beds National Monument (LABE), California—[LABE Geodiversity Atlas] [LABE ParkHome] [LABE npshistory.com]

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

Mojave National Preserve (MOJA), California—[MOJA Geodiversity Atlas] [MOJA ParkHome] [MOJA npshistory.com]

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

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

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

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

Pu'ukohola Heiau National Historic Site (PUHE), Hawai'i—[PUHE Geodiversity Atlas] [PUHE ParkHome] [PUHE npshistory.com]

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

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

Sunset Crater Volcano National Monument (SUCR), Arizona—[SUCR Geodiversity Atlas] [SUCR ParkHome] [SUCR npshistory.com]

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

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

Yellowstone National Park (YELL), Wyoming—[YELL Geodiversity Atlas] [YELL ParkHome] [YELL npshistory.com]

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

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

Related Links

NPS—Rocks and Minerals: Igneous Rocks

NPS—Geology

Volcanoes, Craters, and Lava Flows

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