Question

Passage:

In the mid-twentieth century, the origin of the Earth's magnetic field remained one of geophysics' most vexing anomalies. For decades, the prevailing assumption had been that the Earth possessed a permanent magnetization, a theory rendered untenable by the discovery that the Earth’s core consists primarily of molten iron. Being well above the Curie temperature, this molten iron cannot retain permanent magnetic properties. In response, two competing models emerged in the late 1940s to explain planetary magnetism: the fundamental physical law hypothesis proposed by Nobel laureate P.M.S. Blackett, and the geodynamo theory advanced by Walter Elsasser and Edward Bullard.

Blackett hypothesized that any massive rotating body inherently generates a magnetic field due to a previously unknown fundamental law of physics. He argued that the magnetic moment of a rotating body is directly proportional to its angular momentum. This theory was elegant and testable: if true, it would apply not just to the Earth, but to the Sun and all rotating celestial bodies. Furthermore, it dictated that the magnetic field strength of the Earth should decrease measurably with depth. To investigate this, Blackett developed an exquisitely sensitive magnetometer to conduct experiments deep within mine shafts.

Conversely, Elsasser and Bullard proposed that the Earth’s magnetic field is sustained by a self-exciting dynamo operating within the liquid outer core. According to this geodynamo model, the field is not a fundamental property of rotating mass, but a complex consequence of fluid dynamics and electromagnetism. Convection currents within the molten iron, driven by heat escaping from the inner core and influenced by the Coriolis effect of the Earth's rotation, create a spiraling flow of electrically conductive fluid. This movement through an initial, weak magnetic field induces electrical currents, which in turn generate their own magnetic field, reinforcing the original field in a continuous feedback loop.

The decisive resolution between these models came not from theoretical debate, but from empirical observation. Blackett's own meticulous subterranean experiments in the early 1950s failed to detect the decrease in magnetic field strength with depth that his theory strictly required. Instead, the field strength increased, directly contradicting his fundamental law hypothesis. Furthermore, paleomagnetic studies in the 1960s revealed that the Earth's magnetic field had undergone numerous spontaneous reversals over geological time. The geodynamo theory could accommodate these chaotic reversals as natural consequences of turbulent fluid dynamics within the core. A fundamental physical law directly linking rotation to magnetism, however, could not account for magnetic polarity reversals without positing simultaneous, inexplicable reversals in the direction of the Earth’s physical rotation—an astronomical impossibility. Consequently, Blackett abandoned his hypothesis, and the geodynamo model achieved paradigm status in geophysics.

Based on the passage, which of the following statements regarding the mid-twentieth century theories of the Earth's magnetic field are accurate? Select all that apply.