Lava morphology

Lava flow is a surficial outpouring of molten rocks. The same name is also given to already solidified rock bodies that formed as molten or semi-molten flows of rocky material. Lava flows are the most common volcanic feature on Earth. They cover roughly 70% of the Earth and are also very common on other terrestrial planets, covering 90% of Venus and 50% of Mars.

 

1. Red hot basaltic lava flow. Hawaii. 2. Blocky lava. La Palma. 3. Slowly solidifying pahoehoe lava flow. Hawaii. 4. Pillow lava. Iceland. 5. Transition from smooth pahoehoe to rubbly aa. Hawaii. 6. Columnar lava. Northern Ireland.

Lava flows are very common features on planet Earth although the vast majority of them are hidden from us in the deep ocean basins. The lava type associated with submarine volcanism - pillow lava, is therefore underrepresented where ordinary people have a chance to see it. The most common way to divide lava flows into distinct types is following : Pahoehoe lava flow, Aa lava flow, Blocky lava flow, and also Pillow lava flow. Sometimes Turbulent lava flow is also added, but the latter is only of theoretical interest to scientist because we will not see that type of lava flow in the nature. Turbulent lava flows may have been present billions of years ago when the interior of the Earth and consequently lava flows as well were significantly hotter and the composition of lava was less siliceous. That enabled the lava to flow more easily and turbulently. The most common subaerial lava flows today are pahoehoe, aa, and blocky lavas.

Lava surface is cooling very rapidly. The temperature of glowing lava is at least 475 °C. Bright yellow is hotter (over 1000 °C) and orange cooler (800…900 °C). Dull red colors indicates a temperature in the range of 600…700  °C. Lava surface may cool from bright yellow to dull red within minutes. Pu’u O’o vent, Kilauea volcano.

Volumetrically, most lava is of basaltic composition. Basaltic melts have overall lower gas contents and are more fluid than their andesitic-to-rhyolitic counterparts. Their higher fluidity (lower viscosity) is a product of their lower SiO2 (silica) contents. When gases exsolve from basaltic melts they are allowed to rise unimpeded through the fluid magma without a significant build up of gas pressure. This results in relatively calm, nonexplosive eruptions, and a preponderance of lava. In contrast, when gases exsolve from felsic magmas, their upward mobility is impeded by the high viscosity of the melt. This results in the buildup of gas pressure, which generates explosive eruptions associated with a preponderance of pyroclastic ejecta. The low viscosity of basaltic lavas allows them to be extruded over great distances, often producing high-volume lava flows with low aspect ratios (ratio of thickness to area). Under the right conditions, de-gassed felsic magmas can also erupt lava in a nonviolent manner. However, felsic lavas tend to be much thicker than basaltic lavas and have much higher aspect ratios.

The volume of magma generated over a given amount of time is known as the effusion rate. The effusion rates for historical eruptions of basaltic lava are highly variable, from 0.5 to 5000 m3/sec. Historic flows from Mt. Etna average about 0.5 m3/sec, whereas the fissure-generated flows associated with the Icelandic Laki eruption in 1783 were released at a rate of ~5000 m3/sec. The effusion rates for andesite and dacite are much lower (~10 to 0.05 m3/sec) due to their higher viscosities.

Red hot lava in action. Here, on the Kilauea shield volcano, two lava springs have broken through solidified lava crust on top of a lava tube; each is about one meter wide. Note the development of ropes in the forground. Photo curtesy of Hetu Sheth, August, 2002. It is postulated that lavas are non-Newtonian liquids with a yield stress and that it is the yield stress which determines flow dimensions. An appropriate theory was developed for the unconfined flow of ideal Bingham liquids on inclined planes. The occurrence of structures similar to levées on lava flows was predicted. The theory was verified by laboratory measurements on flows of suspensions of kaolin. These flows showed similarities to lava flows. Data from lava flows was also found to be in general agreement with the theory which was then used to interpret the shapes of two lunar lava flows. It was possible to estimate yield stresses and flow rates for these lavas.

The morphology of lava describes its surface form or texture. Deep-water submarine lavas can be categorized as sheet flows or pillow flows. Sheet flows are always broad, relatively flat, and fill in low areas in the landscape. This is because they are very fluid when erupted, as reflected in their various morphologies: "ropy", "lineated", "lobate", or "jumbled"(if their surface becomes disrupted). Pillow flows, on the other hand, are comprised of individual bulbous lobes of lava that are piled one upon the next, and tend to form large haystack-shaped mounds or ridges. Different parts of a single flow can have different lava morphologies.

When it is first erupted molten lava is very mobile. But in the frigid deep ocean its surface is cooled very quickly and it starts to form a solid crust. Pillow lavas usually solidify completely in place. However, beneath the frozen carapace of sheet flows, molten lava often continues to move freely for hours after eruption before finally solidifying. Because of this, sheet flows tend to be at least partially hollow, where the molten interior of the flow drained back into the eruptive fissure or continued downslope before freezing. Where they are hollow, flow roofs often collapse under their own weight, sometimes revealing lava pillars locally supporting a portion of the uncollapsed carapace.

The morphology of lava flows is controlled by eruption rate, composition, cooling rate, and topography. Lava flows are used to understand how volcanoes, volcanic fields, and igneous provinces formed and evolved. This is particularly important for other planets where compositional data is limited and historical context is nonexistent. Numerical modeling of lava flows remains challenging, but has been aided by laboratory analog experiments. Experiments using polyethylene glycol (PEG) 600 wax have been performed to understand lava flow emplacement. These experiments established psi (hereafter denoted by Ψ), a dimensionless parameter that relates crust formation and advection timescales of a viscous gravity current. Four primary flow morphologies corresponding to discreet Ψ ranges were observed. Gregg and Fink  also investigated flows on slopes and found that steeper slopes increase the effective effusion rate producing predicted morphologies at lower Ψ values. Additional work is needed to constrain the Ψ parameter space, evaluate the predictive capability of Ψ, and determine if the preserved flow morphology can be used to indicate the initial flow conditions. We performed 514 experiments to address the following controls on lava flow morphology: slope (n = 282), unsteadiness/pulsations (n = 58), slope & unsteadiness/pulsations (n = 174), distal processes, and emplacement vs. post-emplacement morphologies. Our slope experiments reveal a similar trend to Gregg and Fink [2000] with the caveat that very high and very low local & source eruption rates can reduce the apparent predictive capability of Ψ. Predicted Ψ morphologies were often produced halfway through the eruption. Our pulse experiments are expected to produce morphologies unique to each eruption rate and promote tube formation and compound flows. Post-emplacement morphologies are modified by a variety of factors (e.g. solidification, deflation), which may not preserve the initial morphology produced during an eruption. Relating this morphology to the eruption conditions is pertinent to understanding the evolution of planetary surfaces.

During eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Réunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2-3 times smaller and their length being 10-15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows.