Pahoehoe or ropy lava flow is one of the subaerial lava flows, characterized by a smooth, ropy, gently undulating, or hummocky continuous chilled surface or skin that may occasionally have fracture. The other subaerial lava flows are ‘a’ā (pronounced “ah-ah”) and blocky lava.
The term pāhoehoe (pronounced as pah-hoy-hoy or pa hoey-hoey) is a Hawaiian. Hoe means to paddle, and it describes the swirling pattern made as one paddle in the water. Pa, on the other hand, means flat. Therefore, this word translates to flat swirly-swirly.
Clarence E. Dutton was the first to use the term pahoehoe in 1882. He used it to describe the shiny, swirly, ropey, or bulbous characteristic of hot, low-viscosity lava flow, especially basaltic common in Hawaii. However, the term is now globally accepted.
This post has an in-depth discussion about the pahoehoe. This includes its meaning, characteristics, formation, varieties, and features. We will also look at where it is found, transition to aa, and more.
However, if you are looking for a comparison with aa, see pahoehoe vs aa.
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What is a pahoehoe?
Pahoehoe lava flow is characterized by a smooth, ropy, billowy, gently undulating, or hummocky continuous surface or skin, usually glassy. However, sometimes, the surface can have fractures.
A ropy texture describes the interweaved or twisted rope-like appearance and small-scale wrinkling. It forms from the dragging, folding, and rumbling of threads, filaments, or creases of the smooth, flat, still plastic crust by lava rapidly flowing beneath it.
However, this ropy morphology varies. For instance, it may appear as twisted rope braids, finger strings, or coarse wrinkles with millimeters to centimeters scale.
Furthermore, the dragging of the ropy beneath the underlying lava forms pressure ridges about ∼5–50 cm apart, known as pahoehoe festoon ridges or patterns. Also, corrugated ridges in an inch scale may form concave or perpendicular to the flow direction.
When fresh, the pahoehoe surfaces are shiny and may have a metallic glaze iridescence due to oxidation. However, weathering will make it duller. Also, burial, erosion, and later exposure can make pahoehoe appear like pillow lavas, their submarine version.
In this lava flow, gas vesicles are ubiquitous. Some may have >20% voids. These bubbles form near the congealing skin which prevents their escape. These bubbles are usually large since they merge as they rise and have smooth walls.
Lastly, ropy textures usually occur in fluid lava such as those that form pahoehoes. However, thicker lavas that form andesite or rocks can also have ropy surfaces. However, the scales and wavelengths are 10s of meters, making it not visible from the ground.
How does pahoehoe form and advance?
Pahoehoe lava flows form in highly fluid, hot (high temp.) especially basaltic during a calm effusive eruption with low discharge and flow rates. Usually, speeds of 10-100 m/h, favor the formation of pahoehoe.
However, it may occur in basaltic andesites with less than 55 wt.% silica content. Also, it is common in less viscous lava like carbonatites (natrocarbonatite) or sulfur flows.
In contrast, vigorous or high discharge rates favor aa flows. This happens because degassing and stirring promote crystallization and increase viscosity.
How do pahoehoes form? Pahoehoes form by breaking previous lobes, oozing incandescent, and forming new thin tongues, toes, or sheets. Immediately, their surface will chill, creating a thin, glassy crust or skin on the surface and fronts.
This congealed crust will restrain lava advance. Also, it slows cooling and prevents the escape of exsolved gases, keeping lava hot for a long time.
Injection of more lava into the lobes will inflate, stretch, and expand the tongue or lobe tips. Soon, the skin becomes too rigid to extend due to continued cooling, and molten lava breaks, forming a new tongue, toe, or sheet.
Usually, pahoehoe lava flow advances by forming thousands of these intricate successive, sometimes overlapping protrusions of small lobes (tongues and toes), one after another, at their margins.
Tongues may flow over 100 meters long and across. Some develop feeding channels or tubes before crusting and extruding pahoehoe toes. However, these tongues are 10-1000 times narrower than the width of the whole flow.
On the other hand, pahoehoe toes are a few meters long or less, 30-50 cm wide, and less than 30 cm thick.
However, large, smooth, flat, sheet-like lobes form in case of high effusion rates. Also, a rolling motion may occur in steep slopes, carrying the crust forward, down, or under the advancing flow.
Pahoehoe feeding
Most Pahoehoe lava flows are fed by lava tubes. However, these flows may be channel and sheet-fed.
1. Lava tubes
Lava tubes are well-established, intricate tunnel-like conduits or tube networks flowing beneath a solidified crust or skin. These channels or streams will distribute lava in multiple fingers and resemble an underground river system.
They run from vents to leading flow edges or fronts and offer insulation. Thus, lava can flow to great distances without losing heat or fluidity. The only place where the hottest lava (incandescent) will be exposed is a local breakout, as it forms a new lobe (tongue or toe).
2. Sheet flows
Sheet-fed pahoehoes flow as a single, laterally extensive body or blanket of lava. Such flows are common in flood basalts or submarine lava outflows. Sheet flows are eruptions with a high discharge rate or on gently sloping terrain. The fast extrusion of lava allows individual lobes to merge into a single flow field.
3. Channel flows
Channels flow occur between levees of chilled lava formed due to the topography of the place or natural channels like river valleys. Levee edges will be higher if discharge rates decrease. However, overtopping may occur if the discharge rate increases or it meets obstructions.
Thickness and extent
Most pahoehoes will have thin, low-profile sheets, less than a meter or two. However, inflation due to the injection of more lava makes them massive, i.e., 6.5-50 feet (2-15 meters)
According to Winter (2014), most of these flows start as thin lava, about 20-30 cm thick. However, injecting more lava soon after a congealing skin form will cause inflation to be as high as 18 meters.
This inflation creates a complex surface pattern. Inflation has been observed in Oregon, Hawaii, the Juan de Fuca Ridge, and Columbia River Basalts.
This isn’t far from what Martí & Ernst (2005) say. They note that pahoehoes are usually < 15 meters thick due to more lava injection and with a 1–1000 km2 coverage.
However, some spread tens of kilometers, like the Big Island of Hawaii. Others spread over 100 km, seen in Queensland, Australia.
According to Kilburn (2000), recent studies show Columbia River flood basalt that spread over hundreds of kilometers was an enormous pahoehoe, not aa.
Such expansive flows will have tube systems that may be 10s of meters across and 10-20 meters high. Some extend several tens of kilometers; others go up to 100km.
What limits flow?
Magma supply limits pahoehoe emplacement, i.e., it will only advance if given fluid internal lava. Also, spreading occurs only when internal energy exceeds crustal resistance.
Therefore, some tongues and toes stagnate or cease if they don’t have enough lava supply or driving force cannot break lobes to create new ones. Others continue, especially those connected to feeding tubes.
However, a large force will fragment the crust and change flow morphology to aa.
Varieties of pahoehoes
Some surficial features classify it into various varieties which include:
- Shelly pahoehoe: It has thin crusts with numerous burst bubbles, cavities, blisters, and small tubes forming a frothy-like texture. Shelly pahoehoes often occur near the vent from gas-charged hot lava erupting with little or no fountaining. They feel like large eggshells when walking on them and will often buckle, thus making it tiring and painful to walk over.
- Scaly pahoehoe: It resembles a shingled roof or scales of a fish and usually has overlapped lobes 2-12 inches thick.
- Entrail pahoehoe: this variety has small swells and lobes piled into a mass resembling entrails.
- Reticulate scoria: The numerous burst vesicles or bubbles may result in a flow thread-lace or reticulate scoria characterized by large holes and delicate, three-dimensional glassy threads.
- Slabby pahoehoe: This variety has broken, closely spaced, jumble-arranged slabs of flow crusts. It forms when flow rates are so high to allow the formation of a continuous crust, i.e., it cannot contain resultant shear strain. The 1984 Mauna Loa eruption had slabby pahoehoe 3.5km from the vent.
- Elephant-hide pahoehoe: It has a surface with tumuli, pressure ridges, and swells, making its texture like an elephant’s skin.
- Spiny, toothpaste, or sharkskin pahoehoe: This variety has numerous centimeter scale spicules or spines. It forms from viscous, crystalline lava under low shearing or flow rates, preventing a glassy surface from forming.
Note that some varieties, like the slabby and sharkskin pahoehoes, represent a transition to aa lava flow.
Additional pahoehoe features
According to Kusky (2005), pahoehoes may have irregular surface features, including linear ridges, spatter cones, lava blisters or tumuli, squeeze-ups, lava-inflation clefts, and pressure plateaus. Such occurs when lava beneath the crust pushes its way up.
Tumuli or blisters may extend a few to hundreds of meters. These are small, low-laying, elliptical swollen, dome-shaped structures or mounds formed when lava pushes the crust upwards. Sometimes, it may break the skin from blobs if viscous.
Also, explosion tubes, thin pipes, or pipe amygdules may form when lava covers water, causing an explosion that creates a conduit to the surface.
Furthermore, there may be molds of anything burnt (structures or trees) and lava tubes or tunnels.
A significant accumulation of pahoehoe flow lava may form a scutulum. These are gently sloping shield volcanoes with slopes ranging from 1-7 degrees. An example is Skjaldbreid in Iceland, which has a diameter of 8 kilometers and rises about 500m from its base.
Lastly, when thick pahoehoe flow cools, they may form basalt columns. Such form is due to contraction, resulting in polygonal fissures perpendicular to the cooling surface. However, in lava tubes, columns form radially upon each other.
Where are pahoehoes found?
Pahoehoe lava flows occur in Mauna Loa and Kilauea Hawaiian basaltic volcanos. Also, they are known in Mount Etna and Vesuvius in Italy and Surtsey in Iceland.
Sometimes, they may occur intermittently for decades. Examples are Etna (1614-1624) and Pu’u O’o, Kilauea since 1983.
Pahoehoe to aa transition
A pahoehoe may maintain its morphology throughout an eruption. However, most of these basaltic eruptions begin as pahoehoe and transform to aa downstream, characterized by a rough, sharp, rubbly, or clinkery surface. This transition is unidirectional, i.e., it occurs in one direction and doesn’t reverse.
Similarly, aa may also change to blocky lava flows in basaltic andesites. Therefore, you should view pahoehoe, aa, and blocky lava flow morphologies as a continuous spectrum, not independent flows.
Martí & Ernst (2005) state that a transition threshold zone (TTZ) separates pahoehoe to aa flow behavior or morphology. Of course, the composition doesn’t change. However, variations in viscosity, discharge, and flow rates may occur.
The TTZ has a critical value or energy flux. It controls whether lava will form a continuous crust or break and depends on how quickly cracks form and heal by chilling the newly exposed lava.
Exceeding this critical value means cracking is faster than healing, and a transformation from smooth to fragmental surface happens. However, if not exceeded, the surface tearing is slower than healing, and there will be continuous crust or skin.
Some of the factors that may cause a transition from pahoehoe to aa lava flow include the following:
1. Increase in viscosity
As lava flows away from the vent, it may undergo heat loss, oxidation, degassing, or loss of volatiles, increasing viscosity. An increase in viscosity reduces velocity and results in the formation of aa. The skin cannot heal as fast as it breaks since the lava is thicker.
2. Increased flow rate
Increasing slope or falling to an escarpment may increase strain (deformation) and shear stress rates favoring aa formation. This happens as the energy supplies per unit volume to deform the congealing crust increases and is from gravity, i.e., gravitation energy flux.
Also, an increase in flow rate such as plunging in an escapement may promote degassing, nucleation, and crystallization and increase viscosity.
3. Rate of discharge
According to Martí & Ernst (2005), discharge rates of > 5–10 m3/s favor aa while those lower favor pahoehoe. A higher discharge rate means a higher flow rate.
Deceptive change of aa to pahoehoe
Kilburn (2000) discusses some instances that may deceptively appear as if aa lava flow morphology changes to pahoehoe, which doesn’t occur.
i. Change in effusion rate
A good example is towards the end of an effusive eruption. The late-stage lava emerges slowly, favoring a crust formation and pahoehoe morphology, while earlier faster emerging favored aa.
Here, no transition happened. Only conditions at the vent changed, and it happens due to the independent flow, not a change from aa on already erupted lava with aa characteristic surface. An example is in Mauna Loa, Hawaii.
ii. Escape of internal less crystallized lava
If hot internal, less crystallized lava escapes from aa channel, such as overflows or breaches the margin and advances slowly, it may form a pahoehoe flow.
Again, this isn’t an evolution. The hot internal escaping lava deep inside the parent flow has a different crystallization history not associated with near-surface layers forming the earlier crust.
Been careful when walking on pahoehoes
You can walk on ordinary pahoehoes. They are comfortable, unlike aa, and generally safe. However, Lopes (2005) warns of shelly pahoehoes. The thin shells over voids may crumble, causing you to drop, usually a few feet deep.
Also, wear rugged shoes and clothes to avoid cuts when walking on a shelly pahoehoe. Avoiding them when possible or walking near edges of associated flow channels is a safer option. You will identify them by the crunchy noise under your shoes as you walk on them or by visibly broken lava crusts.
A more severe risk of shelly Lava will be if there is still some molten magma, as it can cause severe burns. Even ordinary pahoehoe can have an unstable surface crust that may collapse. Therefore, always be careful when walking on a recent emplacement.
Lastly, be wary of snakes that may camouflage on the ropy texture. One such black and very poisonous snake is the Hawaiian ahu.
What are some of the hazards of pahoehoes?
Some of the hazards of pahoehoe and other lava flows, i.e., aa and blocky, lie on their path. They can surround, overrun, damage, burn, bury, or crush natural and artificial structures. Also, they will render land parcels unproductive agriculturally, partially to totally.
Although pahoehoes are slow and unlikely to cause life loss, it happened in Nyiragongo volcano in DRC in 1977. It involved draining the summit crater lake due to a fissure on the volcano’s flank.
A highly fluid magma with a silica content of about 42% emerged from the crack, with parts moving at 100 kilometers per hour. It engulfed villages on its flank, killing between 60 and 300 people before they could escape.
Mauna Loa, Hawaii, has also had voluminous and fast-flowing lava reaching 10s of km per hour, making it potentially dangerous to human and animal life. However, usually, flows are only a few kilometers per hour, with flow and spreading in a flat terrain much slower.
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.
- Lopes, R. (2005) The Volcano Adventure Guide. 1st ed. Cambridge: Cambridge Univ. Press.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Klein, C., & Philpotts, A. R. (2017). Earth materials: Introduction to mineralogy and petrology(2nd ed.). Cambridge University Press.
- Reading, H. G. (1996). Sedimentary environments processes, facies, and stratigraphy (3rd ed.). Blackwell.
- Marti, J., & Ernst, G. (2010a). Volcanoes and the environment (1st ed.). Lighning Source, UK, Ltd.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.