Blocky lava

 

Blocky flows are common if the silica content of lavas is higher (composition of basaltic andesite to rhyolite). Blocky lava flows resemble aa lavas. They also have highly irregular surfaces covered with debris, but they contain larger lava blocks with smoother sides and angular edges with common dimensions from few decimeters to several meters. Blocky flows grow higher as they advance and may reach more than 100 meters in height.

Here is a blocky flow (formed in 1712) from La Palma, Canary Islands. Blocks are angular and have much smoother sides than separate pieces of aa lava.

Blocky lava a surface flow of hot, molten lava covered in a carapace of crystalline, angular blocks which tend to be smoothly faceted and may have dimensions up to several metres. The blocks, which have the same composition as the flow interior, are formed by fragmentation of the chilled flow surface as lava continues to move within the flow interior. Blocky lava morphology is usually confined to lavas of high viscosity and intermediate to high silica contents.

Blocky flows may be tens of meters high. Nea Kameni island in the caldera of Santorini, Greece.

A surface flow of hot, molten lava covered in a carapace of crystalline, angular blocks which tend to be smoothly faceted and may have dimensions up to several metres. The blocks, which have the same composition as the flow interior, are formed by fragmentation of the chilled flow surface as lava continues to move within the flow interior. Blocky lava morphology is usually confined to lavas of high viscosity and intermediate to high silica contents.

Andesitic blocks. Nea Kameni, Santorini.

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.

Nea Kameni, Santorini.

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.

Evolution of vesicular lava into blocky lava. (A) Vesicular lava extruding on El Brujo, surrounded by older spines. (B) Photo of the same flow front, 10 months later, with increased brecciation of the flow top. (C) Proximal lava infills a collapse scarp; the lava flow top is made up of large blocks in comparison to longer flows (as in B). See also the blocky lava flow (Rcs) that advanced in the months after the November 2012 collapse, following the initial vesicular lava flow. Note the small block and ash flow descending the dome. 

Block Lava Flows at Geldingadalur Eruption, Iceland