Block or blocky lava flows have a fragmented surface with large (usually up to a meter in size), angular to slightly smooth, or planar rubbles, debris, or blocks. It is one of the three subaerial lava flows, the others being pahoehoe and aa.
This type of lava flow occurs in viscous lava with > 55 wt. % silica, like andesitic, dacitic, rhyolitic, and trachytic. It usually has a small coverage (meters to a few kilometers) and flows slowly in channels.
Learn more about this lava flow, including how it forms, advances, and differs from aa.
What is blocky lava flow?
Blocky lava flows have surfaces with a large jumble of loose angular, slightly smoother to curved blocks up to a meter across. They form from silica-rich, highly viscous lava flows.
Their fragmented or fractured rubbly surface resembles that of aa. However, aa has smaller, irregular, rough, sharp, or jagged rubbles known as clinkers.
Looking at their cross-section, blocky lavas have a rubbly top, a middle core, and a thin rubbly section separating the massive core from the ground. The lowermost section forms when moving lava buries some blocks that fall on the leading edge.
Compared to other subaerial lava flows, block lavas are the thickest. Also, they have steep fronts, tens of meters, some over 100 meters. However, their thickness may be nearly ten times less near their effusive eruption vents.
Usually, the margins of these flows are irregular but only on a few crustal distances, i.e., about 10 meters. This margin appearance is influenced by Internal lava rheological resistance, with local crustal resistance playing a minor role.
Lastly, blocky lavas surfaces may show minor undulations on larger scales, over 100 meters. Thus, you may not notice them quickly.
How does blocky lava flow form?
Blocky lava flows form from silica-rich, highly viscous lavas like andesitic, trachytic, dacitic, or rhyolitic with more than 55% silica content.
On shield mountains, they only occur in late-stage volcanism characterized by highly differentiated, silica-rich lavas. Earlier basaltic lava will form pahoehoe and aa.
How does fracturing occur? Kilburn (2000) states the surface of block lava flows fragments due to the high associated energies and viscosities involved.
Therefore, they will break their surface even before cooling significantly. i.e., it will occur to partly to wholly congealed upper crust. This indicates that the fracturing is independent of the level of surface cooling.
Put differently, the strength of lava movement beneath or interior rheology influences the fragmentation of the surface, not crust formation.
This contrasts what we see in aa where fragmented surfaces form when the rate of fracturing exceeds that at which exposed molten lava heals. If it doesn’t, you will have a pahoehoe.
We can conclude that crustal resistance doesn’t influence the formation of the aa and blocky rubbles, unlike pahoehoe.
However, aa requires a higher shearing than healing rate. In contrast, internal forces influence fracturing in blocky lava flows.
Besides the characteristic surface, blocky lavas may form columnar jointing in its massive core. However, this jointing is typical in basaltic lava flows and flood basalts. Also, columnar jointing may develop in lava domes.
How it flows or advance
A blocky lava flow advances as a single unit, bulldozing any obstacles it meets. Rarely will one portion be significantly ahead more than its neighboring part, as seen in the pahoehoes.
These flows advance because the internal forces overwhelm crustal resistance. This flow behavior makes them different from pahoehoes, where crusts restrain flow in tongues and sheets.
Compared to aa and pahoehoe, blocky lava flows at the slowest speeds due to high viscosity. Lopes (2005) notes their velocity is less than a kilometer per day, i.e., < 42m/hr.
Furthermore, blocky flows don’t flow far, with typical coverage of a few hundred meters to a few kilometers.
One peculiar thing about block lavas is that the high viscosity slows crystallization during the flow and prevents surface fragments from entering the interior lava, something seen in aa lava.
Lastly, block lavas flow mainly in channels, either naturally existing or one they form with high chilled banks or levees. Nonetheless, some may have lava tubes.
1. Channel formation and breaching
Channels form when the initially broader flow at eruption sites concentrates into narrower passages downhill. During this flow, lava fronts will thicken up to 10 times the size at the start of emplacement.
Also, they will slowly decelerate and halt when no more lava comes from upstream. This happens when there is no more lava effusion.
However, molten lava will fill the channel upstream if effusion continues after the channel front has halted. It may breach the banks or levees as it thickens, creating a new outlet.
Usually, surficial blocks may plug smaller outlet channels. However, larger ones may start a new permanent outflow. This process can repeat itself, and you will end up with several.
Most outflow happens on the upper half of the original channel. Also, they hardly develop more than half longer than the older channel. Thus, they will not change length much. However, they will propagate lava over a larger area.
2. Lava tubes
Repeated superposed channel overflows may build margin walls upwards and sometimes inward when conditions are favorable. These channels may form a roof as the narrow adjacent sections congeal, forming a crust.
Lava tubes in blocky lavas form only in long-lived flows after forming well-established channels. Hence, they don’t contribute much to emplacing like in the pahoehoe.
Lastly, if channels drain lava towards the end of the eruption, they may form tunnels several kilometers long, some large enough to crawl or walk through.
Aa transition to block lava flow
Increased viscosity downstream may result in basaltic andesite aa transition to blocky. This transition is unidirectional. It occurs when the interior lava of aa becomes too crystalline so that fracturing will occur independent of surface cooling.
Blocky lava flow examples
Examples of eruptions associated with blocky lava flow in the US include Lassen Volcanic National Park, Medicine Lake Highland, and Lava Beds National Monument in California, Katmai National Park in Alaska, Newberry Volcano in Oregon, and Mt. St. Hellen in Washington.
Elsewhere, they have occurred in Arenal volcano (Costa Rica), Nea Kameni (Santorini, Greece), Colima volcano (Mexico), Yatsugatake (Japan), Mt. Asama (Japan), etc.
Essentially, any lava flow with silica content over 55% will be blocky.
Hazards
Block lava flow at extremely low speeds, presenting little risk to human life. However, they can crush, bury, damage, or even burn anything on their path, including infrastructural or natural.
However, being viscous, these flows are often preceded by explosive volcanic eruptions that may be disastrous. They eject gas, ash, and lava fragments (lapilli, blocks, and bombs)
Blocky lava flow vs. aa
We did a pahoehoe vs aa lava flow. It is time to compare and contrast blocky and aa flows since they share similarities and differences.
For instance, both these flows have fractured surfaces, i.e., rubbly. Also, both form channels and aa can transition into block lava at certain favorable conditions. These represent some of their similarities.
Here is a summary of their differences:
| Attribute | Blocky lava | Aa |
|---|---|---|
| Surface appearance | It has larger, angular blocks that are slightly smoother and less irregular, with some having polyhedral shapes. | Aa produces smaller, jagged (rough and sharp), spiny, and irregular debris, known as clinkers or spinose. |
| Fronts | Crumble at a much earlier stage of emplacement due to their higher viscosity and more energy associated with this kind of flow. | It shows more character starting as fluid sheets to fragmented near solid mass throughout their thickness with a combination of fracture and fluid flow in-between. |
| Viscosity | High | Low |
| Lava | Silica-rich i.e., > 55wt. % like andesitic, dacite, rhyolitic to trachytic | Silica-poor basalt, basaltic andesite, and fluid flow like carbonatite or sulfur. |
| Thickness | Thicker, tens of meters going up to 100 m | Thinner, often up to 20 meters, but can go beyond |
| Speeds | Slower. Emplacement of 1-100 million cubic meters will take months to complete. | Faster, 1-100 million cubic meters emplacement takes days to complete. |
| Coverage | Smaller, 100s of meters to a few kilometer-scale | Larger, in tens of kilometer-scale |
| Breaching and formation of tubes | It occurs less often since it is more viscous, with a lower difference between channel margin and molten lava. | More often, the higher contrast in viscosity of margins and internal fluid favors the formation of lava tubes and breaching. |
| Rubble entrainment | It doesn’t occur due to the high viscosity of the lava. | Occurs, resulting in faster crystallization and cooling |
References
- Kilburn, C. R. J. (2000). lava flows and flow fields. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. pp. 291-305) San Diego: Academic Press.
- Lopes, R. (2005) The Volcano Adventure Guide. 1st ed. Cambridge: Cambridge Univ. Press.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Marti, J., & Ernst, G. (2010). Volcanoes and the environment (1st ed.). Lighning Source, UK, Ltd.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.