Volcanic hotspots, areas of volcanism well away from plate boundaries, remain a controversial feature of geology. Recently, scientists have discovered a new possible explanation for them using laboratory attempts to reproduce their formation conditions.
Where tectonic plates meet, there are opportunities for magma from the Earth's core to rise to the surface. However, the existence of volcanic provinces like Hawaii is more of a puzzle. The theory that magma rises in broad updrafts channeled through cracks in the plate was reitterated recently, challenging the dominant model of narrow mantle plumes.
Today, volcanic provinces make up only a small percentage of Earth's land mass, but regions such as the Deccan Traps demonstrate they can cover an area at least the size of France kilometers deep in basalt. Such events have been blamed for mass extinctions.
Either way, the magma is thought to be rising from just above the core. With 3000 kilometers of rock on top of it, almost the width of the United States, the pressures are incomprehensible. An experiment in Nature Communications explores what happens to two sorts of iron-enriched silicate glasses when exposed to pressures of 85 billion pascals, achieved by squeezing the glass between two diamonds. The figure approaches a million times atmospheric pressure.
Until around 50 GPa, the authors report an increase in radiation absorption coefficients, which they attributed to “changes in electronic structure.” Consequently, “The radiative thermal conductivity of dense silicate melts may decrease with pressure and so may be significantly smaller than previously expected under core–mantle boundary conditions,” they note.
Dubbing the materials that exist under such conditions “dark magmas”, the authors suggest that this reduced capacity to conduct heat could explain how and why magma rises in certain locations.
Co-author Dr. Alex Goncharov of the Carnegie Institution for Science notes that these results are in contrast to those found for perovskite, another constituent of the lower mantle, but similar to magnesiowüstite crystals. Lower thermal conductivity would cause magma to get hotter than the surrounding materials. “This means it could trap heat from the underlying core and could lead to large-scale thermal upwellings called superplumes,” says Goncharov.
The laboratory could not replicate conditions at such depths closely, and Goncharov acknowledges, “To gain a greater understanding of what is happening in this region, scientists need to come up with novel techniques for in situ measurements of thermal transport properties under simultaneous conditions of high pressure and high temperatures. Our goal is to look at the real melted silicate at very high temperatures, which in principle could reveal different properties than the glasses we looked at.”