The Moon, Earth’s natural satellite, lacks tectonic plates similar to those on our planet. While Earth’s geology involves constant surface reshaping, the Moon’s activity stems from different processes. Understanding these differences requires examining their distinct internal structures and thermal histories.
Earth’s Tectonic Plates Explained
Earth’s outer shell, known as the lithosphere, is divided into several large pieces called tectonic plates. These plates consist of the planet’s crust and uppermost mantle, moving continuously at rates up to 10 centimeters per year. This movement is primarily driven by convection currents within Earth’s mantle, where heat from the interior causes solid rock to slowly flow in a circular pattern.
As hot material rises and cooler material sinks, it creates forces that drag and push the overlying plates. The interactions along plate boundaries result in significant geological phenomena. Where plates pull apart, new crust forms at mid-ocean ridges, while their collision can lead to mountain building, volcanic activity, and the formation of ocean trenches where one plate slides beneath another in a process called subduction. Earthquakes frequently occur along these plate boundaries, releasing energy built up from the plates’ movements.
The Moon’s Internal Structure and Activity
The Moon’s internal structure, like Earth’s, is differentiated into a crust, mantle, and core, but with significant distinctions that explain its lack of Earth-like plate tectonics. Its smaller size, approximately one-quarter that of Earth, caused it to cool much faster after its formation about 4.5 billion years ago. This rapid cooling resulted in a largely solid interior and the absence of the vigorous, large-scale mantle convection necessary to drive global plate movement.
The Moon’s crust averages about 50 kilometers thick, though it is thinner on the side facing Earth and thicker on the far side. Beneath the crust, the mantle is composed predominantly of minerals like olivine and pyroxene, and it is more iron-rich than Earth’s mantle. The lunar core is proportionally smaller than those of other rocky bodies, with a solid iron-rich inner core and a fluid outer core, but it lacks the dynamic convection that generates Earth’s strong magnetic field and contributes to plate motion.
Despite lacking tectonic plates, the Moon experiences geological activity, detected by seismometers left by Apollo astronauts. “Deep moonquakes” originate far beneath the surface, around 1000 kilometers deep, and are primarily caused by tidal forces exerted by Earth’s gravity as the Moon follows its elliptical orbit. These quakes show a monthly periodicity linked to the Moon’s orbital position.
Shallow moonquakes occur at depths up to 125 miles and are more intense, sometimes lasting for minutes or hours. These are thought to be caused by the Moon’s slow cooling and contraction, which causes its brittle surface to break and form thrust faults. Over the past billion years, this shrinkage has reduced the Moon’s diameter by approximately 90 to 100 meters.
Surface features like lobate scarps are direct evidence of this contraction, appearing as cliff-like structures formed by compressional forces where one section of the crust is thrust over another. These scarps are considered geologically young. Additionally, wrinkle ridges, found within the dark, smooth plains known as maria, are also features created by compressive tectonic forces. The maria are vast basaltic plains formed by ancient volcanic eruptions that filled large impact basins. These activities differ from Earth’s plate tectonics, reflecting a body whose surface changes are driven by internal cooling and external tidal stresses, not a globally dynamic mantle.