Geologica 3 - Gutenberg's soft core
The German Jewish seismologist who found the Earth's mantle-core boundary
Hello to everybody. First of all, I’m sorry I was not able to mantain a steady flow of publications. Life is busy and I haven’t got much spare time to write about Geology. I can’t promise I will follow up with another article sooner than I did with this one. I’ll keep trying, I’m doing what I can. Thank you for following me…
In this article I will summarize some of the life and achievements of another great personality who strongly contributed to Geology and Plate Tectonics: Beno Gutenberg, the discoverer of the core-mantle boundary and of the upper mantle’s low velocity zone.
Beno Gutenberg was born in Darmstadt, Germany, on the 4th of June 1889, to a Jewish family. His intellectual journey began at the University of Göttingen, where he earned his doctorate in physics in 1911. During World War I, he served in the German Army as a meteorologist. His mentor, Emil Wiechert, had ignited his passion for geophysics, leading him to explore the enigmatic world of seismology.
In 1913, together with Ludwig Geiger, Gutenberg gained his most outstaning achievement. His fascination with the Earth's interior had led him to study seismic waves. There are two main types of seismic waves that are generated during fault slip at an epicenter: compressional waves, or P waves, and shear waves, or S waves, which are slower but larger in amplitude and are not transmitted in fluds. Geophysicists knew that refracted P waves traveling to the surface were recorded only until a certain distance from the epicenter. Between 103° and 143° angular distances from earthquake epicenters there were no arrivals. This is the so called “shadow zone”. How can this be explained?
Refracted waves are continuosly deviated away from the vertical as they propagate further deep in the Earth, across denser and denser material (Snell’s law defines how a wave is deviated at the boundary with a medium of a different density - think of a straw in a glass of water that looks broken at the air-water boundary). Eventually, seismic waves resuface as they are constinuously refracted into a curved path. In simple words, when an earthquake occurs, refracted P waves are not deteced by seismometers placed where they are expected to resurface. Gutenberg suggested that a strong, sudden density increase within the Earth could deviate the seismic rays toward the center, making them surface again further away than expected, thus explaining the shadow zone.
Gutenberg calculated this discontinuity to be at a 2900 km of depth, more than half the Earth’s radius (6378-6356 m). This discontinuity took his name and is considered to be the outer boundary of the Earth’s core, which contains most of the planet’s mass. S waves are not transmitted in the core, or they are strongly slowed down. This is typical of fluid materials. Shear waves can’t be generated in fluids, let alone transmitted: therefore, the Earth’s core must be composed of a melted material denser than the overlying mantle.
Great discoveries notwithstanding, in the 1920s Gutenberg still had to rely on working at his father’s factory, while continuing his research in his spare time. And in 1926, he identified another crucial feature of the Earth's interior: the low-velocity zone (LVZ). This layer, located between around 80 and 300 kilometers deep (50 to 100 beneath the oceans), within the upper portion of mantle, slows down seismic waves, identifying a possible interval of partially melted material also referred to as Astenosphere (in contrast to the overlying rigid Lithosphere). Basically these are materials in a state similar to that of a toothpaste that reacts rigidly to rapid impulses such as seismic waves, and fluidly to slow impulses such as tectonic movements.
The portion of mantle (mantle lid) above the LVZ is therefore rigid and, together with the crust, it forms the Lithosphere. The Lithosphere-Asthenosphere Boundary (LAB) is defined as a rheological boundary (RBL) between the higher-viscosity lithosphere above and the lower-viscosity (and warmer) Asthenosphere below. But this boundary can also be defined by temperature (Thermal Boundary Layer = TBL) that separates areas where heat transport occurs by conduction (the Lithosphere) versus areas where the heat transport happens by convection. Convection cells may only develop in the less viscous Asthenosphere. Also composition can vary across the LAB, as the ultramafic lithospheric mantle (the lid) is depleted of its volatile constituents.
In 1928 Gutenberg tried succeeding to his mentor Wiechert but, possibly due to his Jewish background, he failed; the university of Potsdam also refused his application. Eventually he was forced out of Germany if he wanted to have any chance in continuing his scientific career. Eventually, he accepted a position as Professor of Geophysics at the California Institute of Technology (Caltech) and became the founding director of the Seismological Laboratory there when it was transferred to Caltech from Carnegie.
Under Gutenberg, Caltech’s Seismological Laboratory became the number one in the world. He was mentor to Charles Francis Richter and together they developed the Richter scale for measuring earthquake magnitudes. They published seminal papers on seismic waves and earthquake energy, which laid the foundation for modern seismology. They developed the relationship between seismic magnitude and energy:
and the Gutenberg–Richter law, which provides probability distribution of earthquakes for a given energy.
In the US, he and his wife, Hertha, played a crucial role in aiding individuals seeking refuge from Nazi persecution during the 1930s, demonstrating their commitment to humanity in the middle of political turmoil. They sponsored former colleagues, students, and friends, including Professor Helmut Landsberg and Professor Victor Conrad (who will discover a discontinuity within the Earth’s crust).
Hertha Gutenberg's involvement extended beyond financial support; she played a crucial role in facilitating connections and assistance through organizations like the Catholic group in New York, which helped Professor Victor Conrad secure his relocation to the United States.
In 1952, Gutenberg received the Prix Charles Lagrange from the Académie royale des Sciences, des Lettres et des Beaux-Arts de Belgique. Gutenberg remained director of the Seismological Laboratory until 1957. He was succeeded by Frank Press. He died of cancer in California at age 70.
(Some passages are taken from Wikipedia)