Geochemistry, Mineralogy, Volcanology Editors' Vox

The Earth’s Elastic Crust

A recent paper in Reviews of Geophysics discussed how the mineral composition and microfabric of the continental crust influences its seismic properties.

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The structural properties of rock on a micro scale, such as crystallographic orientation, grain shape and mineral composition, can have an impact on the macro scale, for example, the ways in which rock deforms under stress, at elevated pressure and temperature. A review article recently published in Reviews of Geophysics discussed the seismic and elastic properties of the Earth’s continental crust, providing a summary of the single crystal elastic properties of crustal minerals, and in what tectonic situation these minerals are expected to be important. The editors asked the authors to explain some of the concepts and methods relating to this research field, and their broader application.

Can you describe to a non-scientist how something solid (i.e. rock) can be “elastic”?

The principle of elasticity in solid materials can be illustrated using a spring, for which the theory was originally developed by 17th century English physicist, Robert Hooke. The principle states that when a force is used to extend (pull) or compress (push together) the spring, it is proportional to the amount it is extended or compressed. In practice, this means that a stiff spring will require a large force to extend it a certain distance, whereas a soft spring requires less force to extend it the same distance. Although it is difficult to perceive, rocks behave in a similar manner during the passing of a seismic wave. The atomic structure of minerals is distorted very slightly, with very small extension and compression, and mineral structure returns to original configuration after the passage of the seismic wave.

Why are the seismic and elastic properties of the Earth’s continental crust of particular significance?

Most of the Earth’s crust, both under the continents and the oceans, is inaccessible for direct observation and sampling so alternative methods that provide information about the crust are needed. Seismic (and elastic) properties are useful to obtain indirect information about several aspects of the crust, including its structure, composition and temperature gradient. The speed of seismic waves is directly related to mineral elasticity and density. Because minerals and rocks have particular elastic properties and density, seismic waves may be used as an indicator for mineral composition. In addition, seismic wave speed is often directionally dependent, which is known as seismic anisotropy. This parameter may reveal valuable information on how the crust is deforming.

What methods are used to observe and measure elasticity?

On the scale of the crust, seismic waves can be used to investigate its elastic behavior. On the scale of rock samples and minerals there are two general methods to measure elasticity that can be divided into two categories based on ultrasonic waves or Brillioun scattering. In the case of ultrasonic waves, it is possible to measure their time-of-flight through a mineral and rock, providing a quantitative measurement of ultrasonic velocity. Like seismic body waves, ultrasonic waves can propagate with both compressional and shear particle motion. In the case of Brillouin scattering, it is interaction of incident light with thermally generated acoustic vibrations that allows the measurement of elastic wave speeds. It is an optical noncontact technique suitable for measuring the elasticity of very small crystals (0.1 mm in diameter).

How have these methods changed over time and enabled measurements to become more accurate?

Ultrasonic instrumentation was developed in the early 1940s by scientists such as the American acoustical physicist, Floyd Firestone, which led to breakthroughs in many scientific areas. This technology paved the way for measurements of ultrasonic wave speeds in rocks and minerals in the following decades. Although ultrasonic methods remain popular to measure acoustic properties, stimulated light techniques based on Brillouin scattering using laser light sources have become a powerful addition to the techniques used to measure elasticity. The latter set of techniques offer high accuracy and precision in determining elasticity of single crystals, with the additional advantage that pressure and temperature conditions can effectively be simulated using high pressure assemblies with diamond anvil cells and laser heating.

An important addition in the last 15 years has been major progress in computational mineral physics driven by the development of quantum mechanical atomic scale modeling codes and the increasing computational capacity of supercomputers. These relatively new methods are capable of modeling complex mineral structures with hundreds of atoms at pressures and temperatures beyond the current experimental possibilities. Such techniques have been used to predict the elastic properties of complex minerals of the Earth’s crust and at extreme conditions of the center of the Earth and other planetary bodies.

How has research in this field contributed to better understanding of the behavior of the Earth’s crust?

Seismological methods have been tremendously important in revealing physical and chemical information on the crust (and the entire interior of Earth). The main constraint on interpreting this seismic data comes from laboratory measurements and predictions based on rock composition and their texture. Without these data there would simply be no way to use seismological data to constrain what the crust is composed of and how it is structured.

What are some of the unresolved questions where additional research is needed?

One of the main challenges that has existed since the beginning in this research field is how to scale results collected from rock and mineral samples, which exist on sub-millimeter to cm-scale, to the scale of seismic waves with wavelengths of 10s of meters to several kilometers. In addition, there is still a large gap in our knowledge on the high pressure and high temperature elastic and seismic properties of the crustal mineral inventory. Such data are crucial in extending the interpretative power of seismic data.

Another important challenge is the separation of so called intrinsic versus apparent sources of seismic anisotropy in the crust (and deeper parts of the Earth). The intrinsic source arises because of the crystal properties, whereas the apparent sources include factors such as layering, and crack and fracture networks. We envisage that tackling this problem will require joint efforts by different geoscientific communities, including seismology, geodynamics, rock physics, and structural geology.

—Bjarne S. G. Almqvist, Department of Earth Sciences, Uppsala University, Sweden; email: [email protected]; and David Mainprice, Géosciences Montpellier, Université de Montpellier, France