Inflation Structures, Lava-Rise Plateaus & Inflation Pits

Aerial photo of a landscape covered with lava rock that has rounded pits and cracks.
An inflated portion of the McCartys Flow in El Malpais National Monument (New Mexico) showing inflation pits and inflation clefts.

Photo courtesy of Larry Crumpler.


Images of moving basaltic lava flows are among the most dramatic of those that show active Earth processes. These images may reveal incandescent interiors, new buds (toes) of lava breaking out at flow fronts, or rivers of lava flowing in surface channels or through skylights that peer into otherwise enclosed lava tubes. But some of the most important processes that occur during the emplacement of basaltic lava flows happen within the flows themselves, sometimes even after a flow front has stopped advancing or has slowed substantially.

Inflation is the process that occurs when lava continues to be supplied within a solidified crust of a basaltic lava flow, causing the flow surface to be lifted upward. Inflation can cause lava flows to substantially thicken and create other features such as tumuli, inflation pits, and inflation clefts to form.

Inflation mostly occurs only in pāhoehoe flows that were emplaced as sheets. Sheet flows may form in areas with gentle slopes. Flows inflate when lava supply exceeds its ability to spread laterally due to cooling and solidification of its surface and along its margins.

Inflation Structures


Tumuli (pl.) are small dome-shaped structures on flow surfaces that are usually less than 33 feet (10 m) high. They are formed when pressure within an inflating lava flow buckles the lava solidified crust. Molten lava may sometimes be squeezed out through cracks.

Photo of a dome shaped lava with dark rock forming a crust and molten lava leaking out from cracks at its base.
Tumulus with a squeeze up (small extrusion of viscous lava) during an eruption from Kīlauea in 2003.

USGS photo.

Inflation Pits

Inflation pits (also known as lava-rise pits) are uninflated zones of an inflated flow where the solid upper crust and hardened bottom surface were not separated by further injection of liquid lava. They form when a lava encountered a topographic obstacle so that the flow is thinner above it. The thinner area solidified more quickly so that it was unable to inflate while the surface of the thicker lava flow rose all around it. Inflation pits are not produced by collapse of lava tubes.

Photo of landscape covered with lava rock that has pits and cracks.
An elongated inflation pit on the McCartys Flow in El Malpais National Monument.

Photo courtesy of Larry Crumpler.

Inflation Cleft

Inflation clefts are large factures formed during inflation. They are frequently found on the edges of inflation pits and near flow margins where the interior of the flow has uplifted relative to the thinner chilled edges of the flow that weren’t inflated.

photo of a volcanic landscape covered in dark lava rock.
Clefts in the inflated McCartys Flow in El Malpais National Monument, New Mexico.

NPS photo by Maci MacPherson.

Rubbly Pahoehoe

Rubbly pāhoehoe forms when inflation causes the crust of a stagnant flow to break into pieces.

photo of land surface covered with large broken slabs of volcanic rock
Rubbly pāhoehoe on the surface of the McCartys lava flow in El Malpais National Monument.

Photo courtesy of Larry Crumpler.

National Park Inflation Structures

At least five units of the National Park System contain inflation structures such as lava-rise plateaus and inflation pits, including:

Bering Land Bridge National Preserve

The Lost Jim lava flow in the Imuruk Lake Volcanic Field in Bering Land Bridge National Preserve was erupted between 1,000 and 2,000 years ago. Inflation of this lava flow caused features such as tumuli and inflation pits to form.

photo of a rolling volcanic landscape
Hummocky terrain and inflation pits on the Lost Jim lava flow.

USGS AVO photo by Tim Orr.

Craters of the Moon National Monument

Inflation features including tumuli and inflation pits are present on several lava flows in Craters of the Moon National Monument.

photo of a rocky slope
Inflation pit on the surface of Grassy Flow in Craters of the Moon NM. This pit shows 75 feet (23 m) of inflation.

Niles JH and Others. 2011. 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.

El Malpais National Monument

The youngest lava flow in the Zuni-Bandera Volcanic Field, the McCartys Flow, has been the subject of significant research into inflation processes and is also used as an Earth analogue to better understand lava flows on the moon and Mars.

photo of rocky land surface with cracks and rounded pits
Inflation pits on the McCarty Flow in El Malpais National Monument.

Photo courtesy of Larry Crumpler.

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Hawaii Volcanoes National Park

Many of the lava flows erupted from Kīlauea Volcano experience inflation, particularly after their flow fronts stop advancing. Inflation can be observed when the center portions of flows are higher and thicker than the front edges, and when tumuli and other structures are present.

photo of hot lava deposit on a grassy field
Inflating lava flow erupted from Kīlauea. The horizontal incandescent cracks show that the flow is undergoing inflation causing the surface of the flow to rise due to continued lava supply.

USGS HVO photo.

Lava Beds National Monument

The rugged terrain of the basalt of Mammoth Crater lava flow created a natural fortress known as Captain Jacks Stronghold in the Modoc War of 1872–1873. Tumuli and inflation pits in Captain Jacks Stronghold helped provide shelter for the Modoc warriors.

photo of lava rock outcrop and grassy mesa
A tumulus in Caption Jacks Stronghold.

USGS photo by Julie M. Donnelly-Nolan.

National Park Sites Containing Inflation Structures

Related Links

Craters Of The Moon National Monument & Preserve, El Malpais National Monument, Hawaiʻi Volcanoes National Park, Lava Beds National Monument

Last updated: April 18, 2023