Get the Six Lava Flow Types or Morphologies Explained

There are six lava flow types or morphologies: pahoehoe, aa, blocky lava, pillow lava, sheet flow, and lobate. The first three are subaerial, and the last three are subaqueous (submarine, subglacial, and other subaqueous environments).

These lava flow morphologies result from the pouring or oozing lava from a vent or fissure during an effusive volcanic eruption. It happens mostly in basaltic or low-viscosity lavas but can occur in other types, too. For instance, blocky lava occurs in more viscous, silica-rich varieties.

Factors determining resultant lava flow morphology are discharge rate, flow rates, viscosity, and lava type. Also, topography (ground steepness), where the eruption happened (underwater or on land), and feeding mechanism (tube, channel, or sheet) may affect morphology.

Let us look at these various morphologies. Please note that we will not discuss the types of magma.

Subaerial lava flows

There are three subaerial lava flow types or morphologies, i.e., pahoehoe, aa, and blocky flow. These represent not a discrete but a continuous morphology spectrum.

Most subaerial lava flows are not fast and don’t present a risk to human life, but some are. So far, the fastest subaerial lava flow was the 1997 Mount Nyiragongo eruption in DRC. It was a pahoehoe type that killed 60 to 300 people.

While they don’t often cause loss of human life, they can damage buildings and other infrastructures and render agricultural lands unusable. Usually, these flows bury, overrun, surround, crush, or even burn anything in their path.

1. Pahoehoe

Pāhoehoe (pronounced pah-hoy-hoy) is a lava flow with a smooth, ropy, gently undulating, or hammock continuous glassy skin or surface. They are analogous to pillow lavas in submarine eruptions.

Depending on appearance, there are various varieties. These include shelly, scaly, entrail, reticulate scoria, slabby, elephant hide, and sharkskin (spiny or toothpaste)

Pahoehoe flows are usually less than 15 meters thick, with typical coverage being 1–1000 km². However, they can spread over larger areas, such as those that form flood basalts, which often flow as sheets.

How do they form? Pahoehoes form from calm, effusive eruptions of hot, highly fluid basaltic and carbonatite lavas, usually with less than 55 wt.% silica. Also, they have low discharge and flow rates (10-100 m/h). Vigorous effusive eruptions or those with higher discharge and flow speeds favor the aa lava flow formation.

Pahoehoe advances by forming successive and sometimes overlapping lobes (tongues and toes). These lobes form by breaking from previous ones, allowing incandescent lava to flow. Immediately, their surface forms a congealing skin that restrains flow.

As more molten lava is injected into the lobe constrained by a congealing skin, it inflates. With time, the skin cools, becomes rigid, and cannot stretch further. Any more injected molten lava will break the skin at the margin, creating the next lobe.

An increase in steepness, velocity, and viscosity (by cooling or crystallization) downstream can transform pahoehoe to aa. See the differences between pahoehoe and aa lava.

Lastly, pahoehoes are mostly fed by lava tubes. Therefore, they remain hot for longer and flow over great distances. However, they can also flow in channels or sheet flows.

2. Aa lava flow

‘A’ā (pronounced “ah-ah”) is a type of lava flow whose surface has irregular, spiny, or jagged (rough and sharp) angular rubble or fragments called spinose or clinker.

These clinkers measure 0.01-0.1 meters and may be coriaceous (vesiculated like scoria). Their surface morphology may be platy, cauliflower, or rubbly.  

The cross-section of aa lava has a top with clinkers, a massive core that may form columnar jointing on cooling, and a thin, rubbly bottom. This buried, rubbly bottom forms from clinkers that fall on the flow path.

What is their thickness, extent, and speed? Typical aa lava flows are a few to tens of meters thick with 1-100 km² coverages. Their speeds usually don’t exceed 1km per hour, with speeds of up to 30km per hour known.

Theselava flows form from mostly basaltic to basaltic andesitic lavas. However, they can also from other low-viscosity lavas like sulfur and carbonatites.

Usually, aa lava flows form when shear stress of inner moving molten lava fragments the congealing surface faster than it can heal. This results in a continuous fracture that forms fragmented clinkers.

Lastly, aa lava flows mainly in channels, with a few forming lava tubes. An increase in viscosity due to crystallization or cooling and increased flow rates may cause a transition to blocky lava.

3. Blocky lava flow

Blocky lava flows have surfaces with large jumbles of loose angular to slightly smooth or curved blocks up to a meter in size or larger. These fragmented blocks are more massive and less angular than the aa’s clinkers.

Blocky lavas are the thickest and slowest. Also, they have the least coverage for the same volume discharge of all subaerial lava flows.

Typically, blocky lava measures tens of meters high, with some over 100 meters. They move at a speed of less than a kilometer per day and have coverage of hundreds of meters to a few kilometers.

Their cross-section will have a rubbly top, a massive core, and some buried rubble that fell on the leading edge at the bottom.

How do they form? Block lavas form in more viscous lava flow with more than 55 wt.% silica, i.e., andesitic, dacitic, trachytic, or rhyolitic lavas.

The high viscosity and energy associated with these silicic lavas cause the surface to fragment even before the surface crust solidifies.

Lastly, they move mostly in channels, bulldozing anything in their path. Rarely will they form lava tubes. Also, they may breach channels if fronts halt, forming permanent outflow that extends their coverage.

Submarine lava flows

Non-explosive or effusive submarine and subglacial eruptions will form pillow lava, lobate lava, and sheet flow morphology. However, some sheet flows are subaerial.

Sheet flow, lobate lavas, and pillow flows form favorably with decreasing discharge rates. Nevertheless, pillow lavas are the most common and represent the largest volcanic rock deposits on the surface of Earth.

Usually, pressure from submarine water or ice cap prevents explosive eruption with seawater or subglacial water absorbing any gases released during eruptions. Nonetheless, many factors control explosive and effusive eruptions. 

Let us look at these subaqueous lava morphologies that result from non-explosive, mostly submarine eruptions.

1. Pillow lavas

Pillow lavas are nearly spherical, tubular, or pillow-shaped mounds or structures associated with effusive submarine eruptions with low discharge rates. However, they can also form under glaciers or if lava enters rivers, lakes, or oceans, like in Hawaii.

These pillows are often 0.1-1m in size or larger and form irregular piles, intertwined, stacked, or interconnected rock masses.

Their surface is smooth, corrugated, or striated with a glassy rind < 1 inch thick. Some may have toe-like protrusions.

On the other hand, the interiors of pillows are mostly crystalline due to slower cooling rates. These interiors may show banding fabric and have concentric vesicles and radial fractures. Also, most are entirely solid, but some may have hollows or shelves.  

Pillow lavas are composed of mainly basaltic lava. However, komatiitic, picritic, basaltic andesite, andesitic, dacitic, and rhyolitic pillow lavas are known but rare. Usually, highly silicic lava will instead fracture in subaqueous or subglacial conditions, forming volcanic breccia.

How do they form? Pillow lavas form when lava from fissures chills to create a plasticky skin with a molten interior upon encountering water. The injection of more lava into the lobe makes it inflate. At a certain point, the skin will be rigid enough not to expand. Instead, it will break extruding incandescent molten lava to form the next pillow.

Where do they occur? Pillow lavas are found on the seafloor (below sediments above sheeted dikes and gabbro) and mid-ocean ridge (MOR). In MOR, they are common in slow and moderate-spreading ridges and less often in fast-spreading. Also, they occur in seamounts, ophiolites, and greenstone belts.

Lastly, their occurrence is evidence of submarine volcanism, and if on land, the place was once under the ocean. Also, their structure with a convex top and concave bottom with cusps can help identify the upper part in tectonically tilted or folded terrain.

2. Sheet flows

Sheet flows are mostly submarine lava morphology with a few subaerial that spread as an extensive thin to thick lava sheet. They form from effusive eruptions characterized by high discharge rates and often ponds or fills low topographic areas.

Sheet flows have a relatively thin glassy rind (5-15cm) whose surface may be smooth, ropy, lineated, grooved, folded, whorl-shaped, or jumbled. Thus, they resemble a pahoehoe. But they are sheet-fed, not lava tubes.

Lastly, sheet lava occurs mainly in fast and intermediate-spreading mid-ocean ridges and less often in slow-spreading ridges. Also, they occur in seamounts. On land, they can occur in continental basalt fields and some pahoehoes.

3. Lobate lava flow

Lobate lavas have elongated, nearly flat lobes with smooth glassy rinds. Most are abundant in drain-back shelves, with some entirely hollow, like shelly pahoehoe.

Their surface has local depression where chilled, but still, plastic individual lobes met and merged without stretching as the beneath fluid lava advanced.

Usually, lobate lavas advance by forming successive lobes linked with lava tubes. However, densely concentrated lobes from higher lava flow rates may have coalesced interiors like sheet flows.

The strong association of lobate lava with filled or inflated and later drained lava pools leaving depressions and collapsed lobate flow rubble, supports a lava tube feeding or spillover from overfills of lava pond. The lobate at pool margins will give lava level before draining.

Most lobate lavas have lava pillars, some supporting lava carapaces whose interiors drained or subsided. These pillars allow trapped seawater beneath the lava to escape rapidly during the fast ponding or slowly as it inflates.

Lava pillars may be tens of meters with a glassy interior. Their surface is rugged with truncated shelves, recording episodic lava pool drainage around the pillars. 

Usually, lobate lavas occur in mid-ocean ridges, especially on domed or flat cross-sections of fast-spreading ones, notes Batiza & White (2000). Also, lateral breakouts on sheet flow edges can form lobate lavas.

Geologists recently identified lobate lava in the seafloor, postdating descriptions of other submarine lava flows. Thus, some structures described as flatted pillow lava from Abitibi greenstone belts may be lobate lavas. 

What do they look like? The shape and structure of lobate lava resemble pillow lava but are often hollow and flattened. Also, their surfaces are smooth, not striated, grooved, or lineated, something not easy to notice on ancient deposits in ophiolites or ocean basins.

Lastly, some authors consider lobate lava as a variant of pillow flow with higher extrusion rates.

References

• Batiza, R. & White, J. D. L. (2000). Submarine lavas and hyaloclastite. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. pp. 364-370) San Diego: Academic Press.
• 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.
• Carey, S. N. (2010). Understanding the physical behavior of volcanoes. In Marti, J., & Ernst, G. (Eds). Volcanoes and the environment (1st ed. pp. 1-43). Lighning Source, UK, Ltd.
• 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.
• 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.