Geological activity refers to processes that change a celestial body’s surface or interior structure, typically driven by internal heat. The Moon’s heavily cratered surface suggests such changes ceased long ago. However, despite lacking Earth-like plate tectonics, the Moon is not entirely dormant. It experiences limited, measurable geological activity driven by residual internal heat, gravitational forces, and external influences. This means the Moon is minimally active, with changes occurring much slower than on Earth.
The Current State of Lunar Activity
The Moon’s small size allowed its internal heat to dissipate much faster than Earth’s, preventing the sustained mantle convection required for plate tectonics. Current geological processes are driven primarily by the dissipation of residual heat and continuous gravitational stresses from Earth. The Moon’s activity is best described as geologically dormant, characterized by slow, localized changes that still manage to alter the lunar crust.
Internal Activity: Moonquakes and Thermal Contraction
Evidence for the Moon’s internal stresses comes directly from the four types of moonquakes detected by Apollo-era seismometers. Deep moonquakes, occurring hundreds of kilometers below the surface, are primarily caused by the rhythmic tidal forces exerted by Earth’s gravity as the Moon orbits. Shallow moonquakes, which originate just tens of kilometers beneath the surface, are the most energetic and are directly linked to tectonic activity. These shallow events, sometimes reaching magnitude 5.0, are caused by the Moon’s global thermal contraction. As the interior cools and shrinks, the crust wrinkles and breaks, forming steep, cliff-like features known as thrust faults or scarps. Imagery from the Lunar Reconnaissance Orbiter (LRO) has confirmed that these scarps are geologically young and actively forming, indicating the Moon has shrunk by approximately 50 meters in diameter over the last several hundred million years. A third type, thermal moonquakes, are tiny, recurring events caused by the extreme temperature difference between the two-week-long lunar day and night, which stresses rocks near the surface.
Past Activity: The Volcanic Era
The Moon’s ancient past was dominated by volcanic activity that far surpassed its current minimal state. This intense period began over 4 billion years ago, peaking between 3.8 and 3.0 billion years ago. Driven by significantly higher internal temperatures, this outpouring of lava created the vast, dark plains visible on the near side known as the maria, Latin for “seas.” These basaltic lava flows filled in the lowest-lying impact basins, creating the smooth, dark surfaces that contrast sharply with the brighter, heavily cratered highlands. The geological engine responsible for forming the maria largely ceased around one billion years ago as the Moon lost most of its internal heat. However, recent evidence from the LRO suggests that small, localized eruptions may have occurred as recently as 50 million years ago, indicating that pockets of magma persisted long after the main volcanic era ended.
External Activity: The Role of Impacts and Regolith Movement
While internal heat drives moonquakes, external forces provide another constant source of geological change on the Moon’s surface. The lack of a protective atmosphere means the surface is continuously bombarded by micrometeorites and larger impactors. This constant bombardment continuously churns the loose, powdery surface layer known as the regolith. This churning process, called “gardening,” is surprisingly rapid, with the top two centimeters of regolith being completely overturned approximately every 81,000 years. Another external process is “space weathering,” where the solar wind, cosmic rays, and micrometeorite impacts alter the chemical and optical properties of the surface materials. This results in the darkening and reddening of the surface over time, in part by creating tiny particles of metallic iron embedded in the regolith grains. Gravity also drives mass wasting, causing regolith to slowly slump and move down slopes, contributing to the gradual erosion and smoothing of ancient features.