A sheet flow is a type of submarine lava flow. It has a broad lateral extent relative to its thickness, making it look more like a blanket of lava. Its surface morphology is glassy and may be smooth to deformed (ropy, lineated, folded, jumbled, swirly, etc.).
How do they form? Sheet flows form during the effusive eruption of highly fluid lava with a high discharge rate, mainly in subaqueous conditions. The seawater quickly chills the surface, creating a glassy crust as molten lava is still flowing beneath the crust.
Besides this type, pillow lava (most common and analogous to pahoehoe), and lobate are the other subaqueous lava flows.
Note: Slope wash, sheet flow, or wash may refer to surface water runoff spread in broad, thin, even layers, i.e., don’t concentrate in a channel. We will not be talking about this kind.
What is a sheet flow?
A sheet flow describes submarine eruptions (and a few subaerial) with vast or extensive, laterally lava flow relative to its thickness. It forms from a high discharge rate fissure extrusion of highly fluid lava. Such lava often ponds and fills the low topographic area.
Batiza & White (2000) notes that sheet flows have surfaces resembling pahoehoes. Why? Because their surfaces may be smooth, swirly, ropy, jumbled, folded, or have coils and lineation.
These surficial textures form from the deformation of the lava crust during eruption when it is still plastic. Local flow conditions may also influence their formation.
For instance, lineated ones have a nearly flat glassy surface with furrows or grooves parallel to the flow direction. They result from crust scraping followed by quick chilling as molten lava flows.
On the other hand, the ropy morphology forms when fast-moving molten lava drags the thin congealing crust. It is usually bow or arc-shaped transverse to the flow direction.
Others like coils, whorls, or spiraling morphologies can result in shear zones between two or more parts of lava flowing at different speeds.
Also, some kind, known as hackly flow, will have irregular surfaces with jagged blocks up to a meter in size, resembling the aa lava flow.
Besides the above surface morphologies, inflation may result in tumuli and other features. Also, these massive lava flows may have columnar jointing and vesicles forming different patterns and can occur with hyaloclastites and pillows breccia.
Lastly, according to Carey (2010), sheet flow has a thicker glass rind than basaltic pillow lava, with typical dimensions of 5-15 cm. This rind forms from rapid quenching of the surface by seawater
How does sheet flow lava form?
Sheet flow lava forms from highly fluid, usually basaltic effusive eruptions with high discharge rates. However, Oahu in Hawaii has abnormal and voluminous deep submarine silica-undersaturated basanites, alkali basalt, and nephelinite sheet flows.
No one has ever seen sheet lavas form. However, considering sheet lava morphology, we may have a hint at how they form.
Many volcanic fields in a submarine environment will form pillow, sheet, and lobate flows from the lava of the same chemical composition. Thus, the magmatic compositions don’t influence their formation.
What influences the different or contrasting flow morphologies formed are discharge rate (supply rate), preexisting topography, viscosity, and flow conditions. For instance, a decrease in discharge rate will transform sheet lava to lobate and, eventually, pillow lavas.
Therefore, it is likely that sheet flow forms during high discharge rates of highly fluid lava from fissures. This high flow rate makes the lava spread over a wider area beneath the chilled glassy crust and not in constrained channels, as seen in pahoehoes.
Also, higher discharge flow rates may result in quick coalescing of advancing lobes forming a single flow unit or sheet. It is more like high discharge rates in surface-fed pahoehoe, i.e., not constrained channels.
Furthermore, the formation of large sheet flows, not shield volcanoes, indicates unusually high eruption volumes within a short time. Such happens with a high discharge rate.
Besides forming sheet flows, a high flow rate causes ponding of low-laying areas (topographic depression). Sometimes, their quenched surface may collapse to form pits as magma breaks out at the pond margin or drains back to the feeding vents. Some of these pit margins may have horizontal terraces in discontinuous drainage. Such terraces will be a record of changes.
Also, unlike pillow lava, which solidifies in place, lava continues to flow beneath a chilled crust, i.e., the molten interior continues flowing down the slope or back into the fissure, resulting in a partially hollow interior. Sometimes, the roof of these flows may collapse from their weight, showing lava pillars.
Lava pillars are hollow, pipe-like channels or conduits occurring from the bottom to the top of the sheet lava. They form when water trapped under the sheet of lava is ejected.
Lastly, as the magma supply diminishes or you move from topographic high, discharge rates decrease, and you will have more of the pillow lava flow.
Where does sheet flow occur?
Sheet flow lavas occur mainly in fast-spreading ridges, with the intermediate ones having both pillow lava and these sheets. Slow-spreading ones predominantly have pillow lavas. These varying morphologies indicate discharge rates vary in these spreading environments.
Also, they can occur in some seamounts, and some are associated with collapse pits in summit calderas.
That is not all. Ophiolites may have sheet flows, small amounts of pillow lava, and other ancient submarine volcanic rock deposits. An example is Troodos in Cyprus.
Note that the abundance of sheet flow lavas also varies along ridge axis segments. For instance, the topographic highs of first-order ridge segments will have more. These segments have a high magma supply, with eruptions getting voluminous fluid magma from chambers below the axis.
Let us look more into the various places they occur.
1. Fast-spreading ridges
Because of the high discharge rates, fast-spreading ridges like the East Pacific Rise will form more sheet lavas. Their surface may be flat, thin to ropy, with jumbled varieties showing chaotic folding and deformation.
Also, Axial Summit Troughs in fast-spreading ridges will have sheet flows. These troughs have much magma supply from shallow magma reservoirs.
2. Intermediate spreading ridges
Intermediate spreading ridges like Juan de Fuca, Gorda Ridges, and Galapagos Ridges will have pillow and sheet lava. Discharge rates here are moderate, favoring the formation of both these two.
3. Slow-spreading ridges
Slow-spreading ridges like the Mid-Atlantic Ridge (MAR) don’t have a well-developed melt lens. Thus, they have a limited amount of magma available.
This results in low discharge rates producing mainly pillowed and bulbous lavas on the flow of the rift axis. However, they can have a smaller volume of sheet lava.
4. Seamounts
Related to mid-ocean ridges are seamounts. Seamounts may also have sheet lava, especially on steep slopes, and central plateaus or calderas.
An example is Reykjanes Ridge in the South of Iceland on the Mid-Atlantic Ridge (MAR). It has seamounts with smooth shapes and abundant sheet lava flow, with those hummocky having pillow lava.
Also, La Palma seamount has a small amount, < 5% sheet flow and 76% pillow lava.
5. Other places
Besides, subaqueous conditions, sheet lava flows may occur in subaerial flood basalt fields or when pahoehoe coalesces. Such will occur in:
a). Continental basalt fields
Continental flood basalts associated with a very high discharge rate of highly fluid lava may result in formation sheet lava. The high extrusion rates of primarily tholeiitic basalt may form sheet lavas that are tens of meters thick and may cover thousands of kilometers.
For example, the Columbia Plateau has sheet flows estimated from 700 km3 of lava eruption spreading up to 600 meters.
b). Pahoehoes
Some pahoehoe may form sheet flows in gently sloping areas with high lava discharge rates. These high discharge rates will make pahoehoe lobes merge into a single flowing unit without constraining tubes with more injection of lava.
However, pahoehoes will have different characteristics than those formed in submarine environments.
Cow’s heart- A rare sheet flow-like lava
Cow’s heart is an informal name for a rare submarine lava flow that resembles sheet lava flow. It has thin (< 1-3 cm), intricately folded tubular septa or sheets. However, it lacks the glassy crust.
Also, many cow-heart masses have thin septa, nearly spherical or curved folds. In some places, it may have weakly welded, non-glassy basalt fragments.
The non-glassy surface indicates a relatively slow cooling. Thus, they likely erupted in places without cold water, such as the ceiling spaces of partly drained flows.
Lastly, they occur in dredges from ridges near seamounts. An example is in the East Pacific Rise, which has a shape that resembles the Teahitia seamount (Society Island hotspot) submarine hornitos.
References
- Carey, S. N. (2010). Understanding the physical behavior of volcanoes. In Marti, J., & Ernst, G. (Eds). Volcanoes and the environment (1st ed. pp. 29-32). Lighning Source, UK, Ltd.
- Batiza, R. & White, J. D. L. (2000). Submarine lavas and hyaloclastite. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. pp. 369-370) San Diego: Academic Press.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.