Inge Lehmann was a Danish seismologist who, in 1936, overturned established scientific understanding of Earth’s structure. Her work proved that the planet possesses a solid inner core—a discovery overlooked for decades, yet now foundational to modern geology. Lehmann’s success occurred despite working in a male-dominated field and largely in isolation, making her story a testament to individual brilliance and perseverance.
The State of Seismology in the 1930s
Prior to Lehmann’s work, the Earth’s interior was thought to be predominantly molten. Seismology, the study of earthquake waves, was a nascent science, still largely theoretical. Researchers used seismic data—the way earthquake waves travel through Earth—to infer the planet’s composition. But the model lacked a critical piece: a deep, dense core.
Lehmann’s Breakthrough Discovery
Lehmann’s pivotal insight came while analyzing seismic readings from a 1929 New Zealand earthquake. She noticed that certain primary (P) waves, which can pass through both solids and liquids, behaved in a way that didn’t align with the prevailing molten-core theory. Specifically, some P waves increased in velocity at a certain depth, rather than slowing down as expected in liquid.
Lehmann theorized that this acceleration was due to a distinct change in density—a solid inner core within Earth’s molten outer core. Her paper, “P’,” published in 1936, meticulously detailed her calculations, graphs, and observations. The theory was initially met with skepticism, yet later confirmed by other scientists.
Seismic Waves Explained
To understand Lehmann’s discovery, it helps to know the basics of seismic waves:
- P waves (Primary): Longitudinal waves that travel fastest through any medium.
- S waves (Secondary): Transverse waves that travel only through solids.
- Surface waves: Move along Earth’s surface, causing the most damage during earthquakes.
Lehmann’s observation that P waves accelerated at depth indicated that they were passing through a solid material—the inner core. Secondary waves cannot pass through liquids, which meant that the inner core was not molten.
A Lasting Legacy
Inge Lehmann’s discovery revolutionized our understanding of Earth’s internal structure. The solid inner core, now known to be primarily iron and nickel, plays a critical role in generating Earth’s magnetic field. Lehmann’s work remains foundational to geophysics, and her name is memorialized in the “Lehmann discontinuity,” the boundary between Earth’s inner and outer core. Despite her groundbreaking achievement, she remained a relatively obscure figure for decades. Only now is her contribution to science receiving the recognition it deserves.
Lehmann’s story demonstrates that scientific progress often comes from unexpected places—from those working outside mainstream institutions and challenging conventional wisdom. Her legacy is a reminder that persistence, meticulous observation, and a willingness to question established theories are essential for advancing our understanding of the world.
