1936

The Inner Core

Inge Lehmann (1888–1993)

The structure of the interior of the Earth has been revealed since the beginning of the twentieth century via the way earthquake seismic waves travel through the planet. Starting around then, seismologists—geologists who study earthquakes and the Earth’s interior—have been placing sensitive earthquake monitoring systems around the world, building up a network that can detect not only strong local earthquakes, but also the weak waves from distant temblors that have occurred across the globe. By monitoring how long it takes waves to travel through the Earth, and by triangulating signals from the same earthquake observed at many widely spaced stations, seismologists have been able to learn that the Earth is divided into onionskin-like layers of core, mantle, and crust.

The initial model of the Earth’s interior was relatively simplistic, and a number of seismologists were puzzled by conflicting signals in the ever-more-sensitive network of seismometers being deployed. A key discovery was made in 1936 by the Danish seismologist Inge Lehmann, who analyzed earthquake seismic data and hypothesized that the Earth’s core was actually divided into two zones: a liquid (molten) outer core comprising about 60 percent of the core’s volume, surrounding a solid inner core all the way to the center. Previously, partly because of the strength of the Earth’s magnetic field, the core was thought to be a single molten object. Lehmann’s calculations and analyses were heavily scrutinized but widely accepted by geologists within only a few years. She was widely honored in the scientific and geological community during her career, mostly for her seismological studies, but also for overcoming the significant challenges of what was still a mostly male-dominated field.

Today’s global seismometer network continues to expand in coverage and sensitivity, and even small earthquakes send waves through the Earth that are monitored and studied to refine our model of the Earth’s internal structure. The composition of the inner and outer core is now known to be mostly iron (with some trace heavy metals), and the mantle is now known to be divided into an inner and outer zone as well. The depths of the crust, inner and outer mantle, and inner and outer core boundaries are now known to high accuracy.

Main image: Illustrative cross-section of earthquake seismic waves traveling through the Earth’s interior, including the different ways that the waves are refracted (bent) at the boundaries between the mantle, outer core, and inner core. Inset: A 1932 photo of Danish geophysicist Inge Lehmann.