Block Flows

photo of dark colored blocky rocks
Block Lava. Fantastic Lava Beds in Lassen Volcanic National Park.

Photo by Frank Schulenburg. Creative Commons Attribution-Share Alike 4.0 International license. Image available at


Block lava flows have surfaces that consist of large angular blocks of lava. Blocky lava flows usually have andesitic or basaltic-andesitic compositions and are more viscous than basaltic lava flows that form pāhoehoe or ʻa‘ā. The blocks in block lavas are smoother than the rough clinkery ones in ʻa‘ā. The blocky tops of these flows generally grade into a massive unbroken lava interior, with a brecciated layer at the base of the flow. Block lavas are also typically glassy.

Compared to basaltic lava flows, block lavas are thicker and have steep flow fronts. They also tend to not travel as far or as fast as basaltic flows.

photo of the edge of a blocky lava flow
The steep flow front on the Fantastic Lava Beds block lava flow in Lassen Volcanic NP.

NPS photo.

National Parks with Blocky Lava Flows

At least 3 units of the National Park System contain block lava flows.

Katmai National Park

Four block lava flows were erupted from Trident Volcano between 1953 and the early 1960s. The flows traveled between 1.5 and 2.5 miles (2.5 and 4 km) and are between about 80 and 200 ft (25 and 60 m) thick.

photo of a mountain valley with the near slope covered with blocky lava.
Block lava flows erupted at Trident Volcano between 1953 and the 1960s.

Photograph by T. Miller, USGS, 1974.

Lassen Volcanic National Park

The Fantastic Lava Beds, a series of two lava flows erupted from Cinder Cone in Lassen Volcanic NP, are block lavas. The eruption of Cinder Cone probably lasted a few months and occurred sometime between 1630 and 1670 CE (common era) based on tree ring data from the remains of an aspen tree found between blocks in the Fantastic Lava Beds flow. The Fantastic Lava Beds were erupted late in the Cinder Cone eruptive cycle as their surfaces are free of ash. These flows are basaltic to basaltic andesite in composition.

Other andesitic block lava flows are found in Lassen Volcanic NP.

photo of a large lava field surrounded by forested hills
The Fantastic Lava Beds.

Photo by Daniel Mayer. Creative Commons Attribution-Share Alike 1.0 Generic license.

photo of a large lava field next to a forest and a lake
Flow fronts on the Fantastic Lava Beds are typically between 65 to 100 ft (20 to 30 m) high.

USGS photo.

Lava Beds National Monument

The approximately 65,000-year-old Schonchin Flow in Lava Beds NM is an andesitic (57.2 weight percent SiO2) blocky lava flow with steep flow fronts and a rough surface morphology.

National Park Sites Containing Blocky Lava Flows

Katmai National Park & Preserve, Lassen Volcanic National Park, Lava Beds National Monument

Last updated: April 18, 2023