The Moon’s surface displays prominent dark patches, known as lunar maria, which have captivated observers for centuries. These vast, flat areas are not bodies of water, despite their name. Understanding their origins provides insights into the Moon’s geological past. This article explains the processes that led to their formation, from immense impacts to widespread volcanic activity.
Defining the Lunar Maria
The lunar maria are characterized by their dark, smooth appearance, contrasting sharply with the brighter, heavily cratered lunar highlands. These dark regions cover approximately 16% of the Moon’s surface, mostly on the side visible from Earth.
Early astronomers mistakenly believed them to be actual seas or oceans, leading to the Latin term “maria,” meaning “seas.” While no liquid water exists on the Moon’s surface today, the name persists, reflecting a historical understanding.
Their immense scale makes them easily discernible even to the unaided eye, forming the familiar patterns often described as the “man in the moon.”
The Precursor: Impact Basin Formation
The initial stage in maria formation involved colossal impacts that created massive, circular depressions known as impact basins. This intense bombardment occurred primarily during the early history of the solar system, specifically during the “Late Heavy Bombardment,” roughly 4.1 to 3.8 billion years ago.
During this era, the Moon experienced frequent collisions with numerous asteroids and comets. These high-energy impacts excavated vast depressions in the Moon’s solidifying crust, some hundreds of kilometers in diameter. These large basins were a foundational event, setting the stage for subsequent geological processes.
The Filling: Volcanic Activity
Following the creation of these massive impact basins, molten rock, or magma, from the Moon’s interior began to rise towards the surface. This fluid, basaltic lava erupted through deep fissures and cracks in the lunar crust, often created or reactivated by the immense impact events. The lava flowed across the basin floors, gradually filling them and forming the flat, expansive plains observed today.
The maria’s dark color comes from the basaltic rock, rich in iron and magnesium, making it less reflective than the surrounding highland material. Most eruptions occurred between approximately 3.8 and 2.5 billion years ago, long after the initial basin-forming impacts. However, some evidence suggests smaller-scale volcanism might have continued as recently as 123 million years ago in certain regions.
Volcanism concentrated within these basins due to thinner crust and lower elevations, providing easier pathways for magma. Lunar volcanism differed from Earth’s in its effusive nature, producing widespread flood basalts rather than tall volcanic cones, partly due to the Moon’s lower gravity and lack of a significant atmosphere. This outpouring of lava, often in multiple flows, solidified to create the smooth, dark surfaces of the lunar maria.