The Moon does not have tectonic plates like Earth, meaning it lacks the dynamic, planet-wide system of shifting crustal segments that drives terrestrial geology. Plate tectonics is tied directly to a planet’s internal heat engine, which the Moon is not capable of sustaining. While the Moon is not geologically dead, its active processes are driven by different forces, leading to unique surface features and seismic events.
Defining Plate Tectonics and the Earth Model
Plate tectonics is the theory explaining how Earth’s outer layer, the lithosphere, is divided into large, rigid plates that move relative to one another. This movement is powered by the convection of material in the underlying mantle. The lithosphere includes the crust and the uppermost, solid part of the mantle, riding atop the weaker, ductile layer known as the asthenosphere.
Mantle convection involves the slow, circular movement of hot rock carrying heat from the planet’s interior to the surface. Hot, less dense material rises, while cooler, denser material sinks, creating a continuous current within the mantle. This conveyor belt motion is the fundamental driver that pushes, pulls, and slides the tectonic plates across Earth’s surface at speeds of a few centimeters per year.
The interaction of these plates at their boundaries defines Earth’s most active geological features, such as deep-sea trenches and volcanic arcs. Plates are consumed at subduction zones, where one plate sinks beneath another, and new crust is created at spreading centers, like mid-ocean ridges. This cycle of creation and destruction is the hallmark of Earth’s dynamic, plate-driven geology.
The Moon’s Internal Structure and Thermal State
The reason the Moon lacks plate tectonics lies in its small size and cold internal structure. Although it is a differentiated body with a crust, mantle, and core, its proportions and thermal state differ vastly from Earth’s. Its smaller size caused it to lose most of its internal heat early in its history, preventing the development of a vigorous, long-lived convection system.
The lunar lithosphere, the rigid outer shell, is exceptionally thick, estimated to be hundreds of kilometers deep, which essentially freezes the surface in place. Beneath this thick layer, the Moon’s mantle is mostly solid, lacking the hot, partially molten, and easily deformable asthenosphere found beneath Earth’s plates. The absence of this dynamic layer means there is no mechanism to break the outer shell into moving plates.
The Moon does possess a core, which includes a solid inner core and a fluid outer core, but this central region is small, extending only about 20% of the Moon’s radius. This small core and the overall cold mantle cannot generate the heat required to sustain the large-scale mantle convection necessary to mobilize tectonic plates. The Moon’s geology is therefore dominated by a single, rigid lithosphere, not a fractured network of moving plates.
Lunar Seismic Activity and Surface Features
Despite the absence of plate tectonics, the Moon is not seismically inert; it experiences events called moonquakes, which reveal its current activity. These events are classified into four main types:
- Deep moonquakes
- Shallow moonquakes
- Thermal moonquakes
- Meteoroid impact quakes
The deepest quakes, occurring hundreds of kilometers below the surface, are primarily caused by the tidal forces exerted by Earth’s gravity as the Moon moves in its orbit.
The most powerful moonquakes, however, are the shallow ones, which originate at depths of about 20 to 30 kilometers and can reach magnitudes up to 5.5 on the Richter scale. These shallow events are directly associated with the Moon’s slow, global contraction as its still-warm interior continues to cool. As the Moon shrinks, its brittle outer crust must accommodate the reduced volume, generating stress and causing the surface to fracture.
This contraction creates distinct surface features known as lobate scarps, which are small, cliff-like landforms formed by thrust faults. A thrust fault occurs when one section of the crust is pushed up and over an adjacent section. High-resolution images have revealed thousands of these young features, some estimated to be less than 50 million years old, suggesting the Moon is still actively shrinking today.
The timing of many shallow moonquakes is also influenced by Earth’s tidal stress, which acts as a trigger, causing a fault to slip when the compressional stress is at its peak.