For decades, the Moon has been commonly described as a cold, geologically inactive body, often summarized by the question, “Is the Moon dead?” Scientifically, a “dead” body has ceased large-scale internal processes like volcanism and plate tectonics, which are driven by internal heat. While the Moon fits this description compared to the dynamic Earth, it is far from inert.
Major geological processes stopped long ago, but minor activity persists. To fully answer whether the Moon is truly geologically dead, we must examine its internal heat engine, physical structure, and the small, measurable events that still reshape its surface today. The Moon’s small size and formation history have dictated a unique, relatively quiet geological fate.
Is the Moon Geologically Dead?
A planetary body is considered geologically dead when its internal heat has largely dissipated, halting the processes that reshape its surface, such as plate tectonics and large-scale volcanism. The Moon fits this description in a broad sense, having ended its major period of volcanic activity billions of years ago. This activity formed the dark, flat plains known as the lunar maria. Unlike Earth, the Moon lacks global plate tectonics, which is the movement of large sections of crust driven by heat convection in the mantle.
The Moon underwent differentiation early in its history, where denser materials sank to form a core while lighter materials formed the mantle and crust. Due to its relatively small size, the Moon rapidly lost the heat generated from its formation and from the decay of radioactive elements, cooling much faster than Earth. This meant that the internal energy required to sustain a magnetic field or drive significant crustal movement faded long ago.
Despite the cessation of major activity, the Moon is not entirely static, and new evidence challenges the idea of complete dormancy. Recent studies have identified small, young ridges on the lunar surface, some of which appear to have formed within the last 200 million years. These formations suggest that the Moon’s crust is still experiencing subtle shifts, possibly due to its gradual contraction as it continues to cool and shrink. This slow, ongoing tectonic deformation indicates a low level of geological activity, distinguishing the Moon from the true geological dormancy of an asteroid.
Understanding the Moon’s Internal Structure
The Moon’s internal structure provides the primary reason for its low level of geological activity. It is a differentiated body, meaning it consists of a crust, mantle, and a core, similar to Earth, but with a much smaller scale and thermal budget. The crust is estimated to be approximately 50 kilometers thick on average, composed largely of anorthosite, a lighter rock type.
Beneath the crust lies a thick, rocky mantle, which is richer in iron than Earth’s mantle. While the mantle was once partially molten, driving the ancient volcanism that created the maria, it is now primarily solid. Models suggest that the deepest part of the mantle, near the core boundary, may still contain a soft layer, possibly partially molten, which is kept warm by tidal forces exerted by Earth’s gravity.
The Moon’s core is relatively small, making up only about 20% of its radius, compared to Earth’s core (55% of its radius). Seismic data indicates that the core structure is layered, featuring a solid, iron-rich inner core surrounded by a liquid outer core. This small core size and the rapid cooling were insufficient to sustain the long-term internal heat necessary to drive a powerful, enduring magnetic field or the robust convection required for plate tectonics.
Modern Lunar Activity: Quakes, Impacts, and Ice
While the Moon lacks Earth-like plate movements or active volcanoes, it exhibits four distinct types of measurable seismic activity, known as moonquakes.
Deep Moonquakes
The most common are deep moonquakes, which occur hundreds of kilometers below the surface. They are directly linked to the tidal stresses caused by Earth’s gravitational pull. These tidal forces continuously tug and deform the Moon’s interior, leading to subtle seismic releases with a monthly periodicity.
Shallow Moonquakes
More significant, though less frequent, are shallow moonquakes, which originate just 20 to 30 kilometers below the surface. These events are more powerful, sometimes registering up to 5.5 on the Richter scale, and can last for over ten minutes. This long duration is due to the Moon’s dry, rigid structure that transmits vibrations efficiently. Scientists believe these shallow quakes are caused by the Moon’s ongoing, slight contraction as its interior cools, stressing the brittle outer crust and forming thrust faults.
Impact and Thermal Moonquakes
The lunar surface is also constantly shaped by external forces, primarily continuous micrometeorite impacts that strike the surface due to the lack of a protective atmosphere. These impacts cause vibrations that register as a third type of moonquake and contribute to the slow process of space weathering, which modifies the surface dust, or regolith. A final type, thermal moonquakes, occur as the lunar surface expands and contracts when the two-week-long lunar night transitions into the scorching lunar day.
Another form of modern activity involves the presence of water ice, particularly in permanently shadowed regions (PSRs) within deep craters near the lunar poles. These areas never receive direct sunlight, maintaining extremely cold temperatures that allow water ice to remain stable. Data from orbiting probes has confirmed that this water ice is mixed into the lunar regolith, and its presence is a resource for future human exploration.