‘A’ā (pronounced “ah-ah”) is a Hawaiian term that describes a lava flow with jagged (sharp and rough) rubbly debris called clinkers, now a geological term accepted globally. However, this word may also mean fire, burn, glow, or blaze in Hawaiian.
Lopes (2005) notes that the word translated to ouch-like or a painful exclamation one makes when walking on aa lava flow bare feet. It isn’t pleasant. If you are planning to go hiking where there are aa lava emplacements, wear shoes, long pants, and gloves to avoid injuries if you fall. Agood pair of boots will only last a few weeks.
Discover more about aa lava flow, including what they are and their morphology. We will also discuss how they advance, speeds, formation, and examples of where they are found.
What is an aa lava flow?
Aa is a subaerial lava flow whose surface is characterized by sharp and rough or jagged irregular blocks/rubbles known as clinkers or spinose. These loose, broken, contorted, spiny, or angular fragments or clinkers have sharp edges, making them dangerous to walk on without shoes.
Clinkers are usually centimeter to decimeter dimensions (0.01 to 0.10 meters). Sometimes, their surface may appear scoria-like due to numerous vesicles with sharp spines created by escaping gas formed before congealing lava breaks.
According to Martí & Ernst (2005) aa lava flow units are typically a few tens of meters thick with 1 and 100 km2 coverage areas. Significant flows will have 1–100 million cubic meters of lava.
Did you know that aa lava flows are the most common, followed by pahoehoe, then blocky flows? Now you know.
Let us talk about morphology and zonation.
2. Aa lava flow morphology
The morphology of lava is a description of surface texture or form. Murcia et al. (2014) say aa morphology may be cauliflower, platy, or rubbly. These words describe the appearance of the surface and its characteristics.
Cauliflower has agglutinated bulbous clinkers attached to dense underneath, resembling a cauliflower. They may be elongated, spherical, or irregular and have rough surfaces. Their crusts are vesicular, while their cores are dense.
On the other hand, platy aa lava flows have scorious thin slabs that may be slightly agglutinated together. Also, you may find a few smaller scoriaceous fragments in these flows.
Lastly, rubbly aa has loose angular fragments above a massive dense interior. These fragmental pieces have various shapes, less often agglutinated to form accretionary lava balls.
2. Zonation
Aa lava flows show zonation in their cross-section. They have an upper portion with clinkers, a massive interior, and a thin rubble zone at the bottom separating a massive middle core and ground. The underneath rubbly layer forms when some clickers fall on fronts and are buried by the advancing lava.
Usually, the blocky core may also be vesicular due to the exsolution of gases. Also, it features columnar jointing or basalt columns, often polygonal prisms due to contraction when it finally cools.
How does aa lava advance?
Aa advances as a single unit, rumbling and bulldozing anything on their path. Rarely does it have a section significantly ahead of its neighbor, a behavior seen in pahoehoe tongues. More crystalline flow areas, such as on the levées that occur on margins of channels, may show non-Newtonian rheology flow.
Usually, aa advances mainly in open channels and less often in lava tubes. The latter forms in a later eruption stage in long-lived streams.
1. Lava channels
Usually, aa and blocky lavas flow in formed or preexisting channels. Those formed occur when the flow stops widening and instead concentrates downhill. Such channels will have levees or banks with chilled higher margins.
Britannica notes that these channels are usually 25-50 feet (8-15 meters) wide and occur near centers of vast, slower-moving broad flow fields.
Typically, these aa lava flow channels will have a thicker front (up to 10 times the initial flow) due to its solidification that causes deceleration. Therefore, you expect a faster lava accumulation downstream.
Also, the advance and thickening will continue until no more new lava is fed. In such a case, fronts will slow until it halts as the remaining lava drains.
However, if effusion continues after the thickening and halting of fronts, lava will accumulate backward towards the vent. Eventually, it may overflow or breach the channel margin.
Some breaches, especially the smaller ones, may heal, others forming new permanent outflow. This can happen severally, creating interconnected flows.
According to Kilburn (2000), breaching occurs in the upper half of the flow’s length. Although new flows travel beyond the older channel, they rarely change the total flow length by over a half. Instead, they will propagate lava over a larger surface area than make it longer.
2. Lava tubes
Sometimes, channels may form overhanging crust above them, forming lava tubes. It occurs due to thickening at the front. This thickening can cause superposed overflows that will build channel margins upward.
Sometimes, some favorable conditions may result in the margins growing inward and eventually forming a roof crust over the channel. This crust will offer insulation to flowing molten lava.
However, unlike in pahoehoes, where they act as the main conduit for channeling molten lava, in aa they don’t. Instead, they form at a later stage after the formation of well-established channels with long-lived flows. Thus, they hardly contribute to much emplacement as it is with pahoehoes.
Lastly, lava may drain, leaving hollow tubes, some large enough to crawl or walk through.
Typical speeds
Many factors affect aa lava flow speeds, including topology. Thus, it is not easy to quantify it. Also, speeds tend to be faster at the start of the eruption, with some occasionally covering 10-30 km within the first 24 hours.
Typical aa lava flow speeds don’t exceed 1 km per hour. However, steeper places may have 10 km per hour or six mph speeds. Speeds exceeding 30 km per hour, or 19 mph, are known but are rare.
How does aa lava flow form?
AA lava forms from high volumetric and flow rate effusive eruption of lava with relatively low viscosity. It is common in basaltic to basaltic andesite with < 55 wt. % silica. However, it may also form in less viscous flows like carbonatites or sulfur.
The rubbly or clinkery surface forms when the shear stress from inner, fast-moving molten lava fragments the thick, congealing surface crust faster than the newly exposed incandescent surface can heal. Thus, the crust will persistently break, forming clinkers.
Shear strain rate is a measure of differential motion occurring between two layers. When it is high, expect fragmentation and if low, the torn surface may heal quickly enough to form a smooth, continuous crust or a pahoehoe.
Transitioning
According to Winter (2014), most basaltic lava flow starts as a pahoehoe and transitions into a pahoehoe further away from the vent, i.e., downstream. However, aa doesn’t revert into pahoehoes. Also, a flow can remain with the same morphology in the entire emplacement.
Some of the reasons for the change in morphology from the pahoehoe’s smooth, continuous, ropy, or hummocky surface to aa include an increase in 1) viscosity 2) shear stress rate, and 3) discharge rate.
Viscosity may increase due to crystallization, degassing, cooling, and other causes while change in steepness can cause an increase in velocity.
Also, note that there is a threshold at which this transition occurs. Martí & Ernst (2005) place the volumetric flow rate > 5–10 m3/s) to result in a transition of pahoehoe to aa. However, this value may vary depending on other conditions. Also, other factors come into play.
Once the transitioning threshold has been reached, the resultant energy and viscosity will break the surface independent of surface cooling.
We did a detailed discussion on transition under pahoehoes. Also, see a pahoehoe vs. aa for similarities and differences between these two if you want to know more.
Where is aa lava flow found?
Aa lava flow commonly occurs at Mauna Loa and Kilauea volcanoes in Hawaii, USA, where it gets its name.
However, it can and has occurred in other places. Notable ones are Mt. Etna (1971, 1991-1993 flows) and Vesuvius in Italy, Paricutin Volcano (1943) in Mexico), Piton de la Fournaise in Réunion, and Askja (1961) in Iceland. There are many other examples.
Finally, all effusive basaltic to basalt andesite eruptions often have the aa flow.
Hazards
Aa lava flows move slowly. Thus, they rarely cause human death as people can quickly move away. However, they will crush, overran, bury, burn, or bulldoze over anything on their path, including natural and artificial structures like houses, roads, and bridges. Also, they may render otherwise productive agricultural land barren.
For instance, the Kīlauea eruption destroyed Kaimū and Kalapana towns in 1990 and Kapoho in 2018. In 1988, it buried an abandoned school bus near Kaimū.
Another case is the Parícutin Volcano (Mexico), which had a ten-foot-thick cinder cone base in June 1944. It buried much of the San Juan Parangaricutiro village, leaving some remnants of the church notes Kusky & Cullen (2005).
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.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Marti, J., & Ernst, G. (2010). Volcanoes and the environment (1st ed.). Lighning Source, UK, Ltd.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Lopes, R. (2005) The Volcano Adventure Guide. 1st ed. Cambridge: Cambridge Univ. Press.
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Murcia, H., Németh, K., Moufti, M. R., Lindsay, J. M., El-Masry, N., Cronin, S. J., Qaddah, A., & Smith, I. E. M. (2014). Late Holocene lava flow morphotypes of Northern Harrat Rahat, Kingdom of Saudi Arabia: Implications for the description of Continental Lava Fields. Journal of Asian Earth Sciences, 84, 131–145. https://doi.org/10.1016/j.jseaes.2013.10.002