This post discussion will compare and contrast pahoehoes and aa subaerial lava flows, highlighting their similarities and differences.
To give you a hint, some similarities include forming from the same eruptions with mafic to intermediate compositions and having Hawaiian names.
On the other hand, differences include surface appearance, formation, viscosity, temperature, discharge rates, thickness, how they advance, feeding, and more.
Pahoehoe and aa description
Pāhoehoe (pronounced pah-hoy-hoy) describes a lava flow type with a continuous smooth, ropy, undulating, hummocky, or billowy glassy surface crust. However, it may have fractures and is comfortable to walk barefoot.
On the other hand, ‘A’ā (pronounced “ah-ah”) is a term that characterizes a lava flow whose surface has loose, irregular, angular to jagged blocks known as clinkers. These contorted, spiny clinkers have sharp edges, making them painful to walk on bare feet.
![USGS[2]](https://commons.wikimedia.org/wiki/File:Aa_large.jpg){kind=link}
Pahoehoe and aa similarities
Some of the aa and pahoehoe similarities include using Hawaiian names and describing types of subaerial lava flows. Also, they are common in Hawaii at Mauna Loa and Kilauea. However, they also occur in places like Mt. Etna (Italy), Mt. Cameroon, Mt. Vesuvius (Italy), etc.
Secondly, both aa and pahoehoe are created during the formation of the surface crust and describe the appearance of the lava flows. Put differently, they both represent the morphology of lava flows.
Thirdly, pahoehoe and aa occur in low-viscosity lava flows, especially basaltic. However, they can also develop from highly fluid lava types like carbonatite and sulfur.
Lastly, aa and pahoehoe can form from lava with the same chemical composition and during the same volcanic eruption, i.e., a transition from pahoehoe to aa can happen. However, the reverse doesn’t occur.
Pahoehoe and aa differences
Differences between these lava flows occur in:
1. Appearance
Under appearance, we will consider surface, cross-section, and vesiculation.
i. Surface appearance
Pahoehoes have continuous, smooth, ropy, hummocky, or gently undulating, usually glassy surfaces. In contrast, aa lava flows form fragmented surfaces with angular, razor-sharp, rough, spiny blocks known as clinkers.
More pahoehoe surficial features are spatter cones, ridges, squeeze-ups, pressure plateaus, tumuli, and lava inflation clefts. Also, there are various varieties, i.e., shelly, scaly, entrail, reticulate scoria, elephant hide or spiny (sharkskin or toothpaste), or slabby pahoehoe, which may have slightly different appearances.
Similarly, aa lava morphologies may vary with some platy, cauliflower, or rubbly. Also, clinkers may be scoriaceous (vesicular-like scoria)
2. Cross-section
The aa cross-section has a thin skin covering an often vesiculated massive lower part, i.e., it shows no zonation. In contrast, aa has a thick rubbly surface, a massive core, and a thin clinker layer separating the middle portion from the ground.
3. Vesiculation
Pahoehoes have smooth-walled subspherical vesicles as the crust traps most of the present volatile. These voids are on the upper part of the massive portion.
On the contrary, in aa, most volatiles are lost by shearing and deformation. However, it, too, may have vesicles, often irregularly shaped and with less glassy surfaces, due to increased crystallization associated with this flow.
2. Formation
Pahoehoes form when congealing crusts form quickly on flowing molten lava lobes (tongues and lobes), or sheet flows restraining further flow. As more molten lava burrows into the lobes or flow sheets, they inflate until they break at the margins. Molten lava will then flow out, creating new successive flows. This process goes on like that resulting in a continuous glassy crust.
On the other hand, the aa lava flows form when shear stress of the inner, fast-moving molten lava fragments the congealing, thick crust faster than the rate of the newly exposed underlying molten lava can heal. Thus, you will have continuously broken or fragmented surfaces forming clinkers.
3. Lava type
Both occur in predominantly basaltic lava but may occur in others like carbonatite and sulfur which have low viscosity. However, aa may form in basaltic andesite with less than 55 wt. % silica while pahoehoe doesn’t occur in basaltic andesites, i.e., it is more restricted to basaltic or others of low viscosity.
4. Discharge rates
Pahoehoe forms from calm effusive eruptions with lower discharge and flow rates on gentle slopes, while aa from violently effusive eruptions with higher flow or discharge rates on steep slopes. According to Martí & Ernst (2005), discharge rates greater than 5–10 m3/s favor aa while those below favor pahoehoe.
5. Temperature and viscosity
Pahoehoe forms from low-viscosity, hotter lava. In contrast, aa forms from relatively less fluid lava at lower temperatures.
6. Speed and advance
Aa moves faster than a pahoehoe and as a single unit, more like a caterpillar-tracked vehicle with a force greater than crust resistance. Also, it powerfully moves after cooling substantially.
In contrast, pahoehoes are slower because they move by forming thousands of successive thin sheets, tongues, and toes whose skins restrain interior molten lava movement. Advance only happens when the molten lava fills the lobes and breaks the skin at the tips or margin to form successive tongues, toes, or lobes.
7. Thickness
Aa lava flows are thicker with steeper fronts than pahoehoe, often up to or > 20 meters. On the contrary, most pahoehoe forms thin flows but may grow thicker, typically <15 m with more injection of lava.
8. Coverage
Pahoehoe will flow over a larger area for the same volume of lava erupted with a typical 1-1000 km2 coverage due to the insulation and efficient lava distribution downstream via lava tubes.
On the other hand, aa will have a smaller coverage area for the same volume of lava erupted with a typical extent of 1-100km2 notes Martí & Ernst (2005).
9. Feeding
Pahoehoes are fed mainly by lava tubes, less often channel or sheet-fed. By contrast, aa is primarily fed by open channels, with some long-lived becoming tubes if they develop a roof.
The aa lava tubes form on a well-established channel that takes several weeks. Such will happen in long-lived lava flows and forms when most emplacement has occurred. Thus, they don’t contribute much to extending the flow field in aa as in pahoehoes.
10. Cooling and crystallization rate
Pahoehoe cools and crystalizes more slowly since it has a continuous glassy crust that insulates molten lava and prevents hot gas from escaping. Also, the flow happens mostly in tubes.
On the other hand, aa lava flow cools and crystallizes faster because broken clinkers in the surface expose the incandescent core and allow hot gases to escape, triggering crystallization. Also, it flows mainly in channels that don’t have insulation, like lava tubes in pahoehoe.
Furthermore, aa cooling rate and solidification are higher than pahoehoe due to the entrainment of cold surface pieces into interior hot lava in aa lava flows (not due to natural turbulence). However, in pahoehoe, entrainment of surface crust rarely occurs, meaning cooling and solidification will be slower.
11. Transition
Pahoehoe may transition to aa downstream or as it moves away from the vent. It occurs due to changes in flow rates or increases in viscosity. However, aa doesn’t transition to pahoehoe.
12. Walking experience
Since they have a smooth surface, pahoehoes are easier to walk on with bare feet. However, it would help if you were wary of shelly pahoehoes as their surface may crumble. In contrast, walking on aa bare feet is a harrowing experience and you may end up with lacerations.
Pahoehoe vs. aa: Differences summary
We have looked at the similarities and differences between aa and lava flow. Here is a summary of pahoehoe vs. aa showing differences.
| Attribute | Pahoehoe | aa |
|---|---|---|
| Appearance | Pahoehoes have a thin, smooth continuous crust, often ropey, hummocky, or gently undulating. They may have tumuli, ridges, squeeze-ups, pressure plateaus, etc. However, there are many varieties with varying appearances. | They have thicker, rubbly surfaces with loose, irregular, contorted, ragged (rough and sharp) angular blocks, known as clinkers that may be scoriaceous. However, their morphology may vary for platy, rubbly, or cauliflower varieties. |
| Zonation | Its cross-section doesn’t show zonation. It has a glassy crust covering a massive portion. | Aa shows zonation with a rubbly top and bottom and a massive center. |
| Formation | It forms from successive lobes and sheet flow with a crust that restrains flow until the injection of more molten lava causes inflation and breaks at margins to form another succession. | The high shearing stress of flowing inner molten lava fragments, the thicker congealing surface forming rough, angular clinkers or rubbles. |
| Lava | It is mostly highly fluid basaltic but can occur in low-viscosity carbonatite and sulfur flows. | It can occur in relatively more viscous basaltic to basaltic andesite flows and in fluid sulfur and carbonate flows. |
| Advance | Via the formation of thousands of successive lobes (tongues and fingers) and flow sheets | Rumbling as a single unit like a caterpillar-tracked vehicle with no part significantly ahead |
| Discharge rate | Lower | Higher |
| Flow rate | Slower, and will insignificant flow after cooling | Faster and flows strongly even after significant cooling |
| Temperature | Hotter | Lower temperature |
| Viscosity | Lower | Higher |
| Feeding | Lava tubes | Channels |
| Coverage | Higher for the same lava volume, usually in the order of 1-1000km2 | Lesser for the same lava volume, usually in the order of 1-100km2 |
| Cooling and crystallization | Cools slower and crystallizes less since the skin slows cooling and traps hot gases | Cools faster and crystallizes more since the rubbly surface doesn’t offer insulation or trap hot gases |
| Walking experience | Comfortable even with bare feet | Painful and miserable with bare feet |
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.
- Marti, J., & Ernst, G. (2010). Volcanoes and the environment (1st ed.). Lighning Source, UK, Ltd.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
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- 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
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.