Rheology is relevant for any field that involves fluids and soft solids, including geology. Volcanologists study the rheology of magma to understand how a volcano erupts and the extent of the lava flow. The examination of volcanic flows applies rheological concepts familiar to scientists in industrial labs while also bringing insights valid for common colloids and suspensions.
Magma is Viscoelastic
For the non-geologist, magma (underground molten rock) and lava (above-ground molten rock) may be perceived as fiery, deadly sludge that oozes forth from volcanoes with little significance to other materials. However, volcanic flows are advantageous examples of colloidal suspensions. Magma is a colloid consisting of various rock particulates in many shapes and crystal sizes, with the concentration depending on the geological features of the underground mantle. (1, 2) It is no wonder that geologists have demonstrated that magma is viscoelastic (1, 2, 3), having mechanical properties of both a solid and a liquid. With the effective viscosity of magma depending on the flow rate, temperature, and structure, magma can be analyzed as any other viscoelastic suspension, paste, or gel. Due to alignment of the mineral-rich particles within the suspending liquid, magma displays shear thinning behavior as it flows up from the ground and is extruded from a volcano. (1, 2) The degree of shear thinning impacts how much the lava will flow, (1, 2) which is important for understanding the geographic impact of previous eruptions for predicting the lava spread of future eruptions.
Testing Molten Rocks
As one may imagine, studying lava in a laboratory is not a conventional rheometry project. Instead of using shear rheometers, tensile-testers are equipped with custom high-temperature units to clamp a volcanic rock, heat it to a molten state, and then slowly apply tension as a strain rate (Figure 1). in addition to rocks excavated from past eruptions, experimentalists also use partially molten granite (1) and silicate suspensions (2) to mimic magma. A viscosity value can be deducted from stretching the molten sample, although the data is not directly comparable to other viscosity measurements made with other methods. This “elongational viscosity” helps geologists understand the strength and microstructural changes that occurred in the original magma during the initial flow from the earth, with magma viscosities having been recorded to span fifteen orders of magnitude depending on their composition. (4) From the tensile elongation measurements, magma either deforms like a liquid (ductile) or a solid (brittle) depending on the strain rate. (3) This flow vs. fracture behavior is reminiscent of concentrated colloidal suspensions. Due to the difficulties of directly evaluating magmas, mathematical models are frequently developed based on these experiments to estimate and predict viscosities based on mineral content and temperature.
We can Learn from Geologists: How Bubbles Impact Flow
Beyond melting volcanic rock, geological studies interpret the microstructure of hardened rocks with chemical analyses to glimpse the behavior of the lava flow at the point of solidification. Microscale cross sections of volcanic rocks display the presence of bubbles, which tend to elongate in the direction of flow and reduce shear stresses prior to the point of hardening. (4) Bubble dispersity, size, and direction inform geologists of the flow patterns of the magma, (4) including slip layers and shear banding (2, 5) (generic example shown in Figure 2). This analysis is like the analyses under in microscopy-coupled rheometry as well as particle-tracking and light scattering rheological techniques. A “map” of the microstructure of is generated for a given point in time during flow. The influence of bubbles, shear banding, and other flow instabilities are detectable, allowing for better interpretation of how the sample interacts with surfaces and flow hindrances.
Natural materials can serve as examples and inspiration for synthetic products. Volcanic flow properties demonstrate the ubiquity of rheology beyond the lab along with opportunity for applying knowledge from one science to another.
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