The dark, expansive patches visible on the Moon’s face have been known for centuries as the lunar maria, a Latin term meaning “seas.” Early astronomers mistook these smooth plains for bodies of water due to their visual contrast with the brighter, rugged terrain surrounding them. These maria are vast sheets of solidified volcanic rock, not liquid oceans, that cover about 16% of the lunar surface, predominately on the side facing Earth. The brighter regions are the heavily cratered lunar highlands, which represent the Moon’s oldest crust. The formation of the maria required a complex, two-part geological process involving sudden, powerful impacts and a long period of internal volcanic activity.
The Impact Origin of Lunar Basins
The story of the maria begins with immense collisions that occurred billions of years ago, creating the deep depressions that would later be filled with lava. This period of intense bombardment, often described as the Late Heavy Bombardment, took place approximately 4.1 to 3.8 billion years ago. During this time, large asteroids and planetesimals were flung toward the inner planets, scarring the Moon’s surface with impact craters and basins.
The projectiles responsible for the maria basins were far larger than typical meteoroids, creating colossal circular depressions spanning hundreds or even thousands of kilometers. For example, the Imbrium Basin, which later became Mare Imbrium, is estimated to be over 1,200 kilometers across. These extraordinary impacts did more than just excavate material; the immense kinetic energy fractured the lunar crust deep beneath the basin floor.
These basin-forming events established the initial conditions for subsequent volcanism by fundamentally altering the Moon’s structure. The enormous weight of the ejected material, coupled with the deep fracturing of the crust, created a region of lower pressure. This pressure difference and the weakened crust provided a pathway for material from the Moon’s interior to rise toward the surface. The impacts also created areas of concentrated mass beneath the surface, known as “mascons,” thought to be caused by uplifted mantle material or the sheer weight of the solidified lava fill.
Massive Volcanic Floods
The immense impact basins did not immediately fill with lava; instead, a significant delay occurred between the initial impact and the subsequent volcanic activity. The main phase of mare volcanism began hundreds of millions of years after the largest impacts, mostly between 3.9 and 3.2 billion years ago, though some flows are estimated to be as young as 1.2 billion years old. This delay suggests that the impacts did not directly trigger the eruptions but simply prepared the lunar crust for later internal melting.
The material that flowed into these basins was a type of fine-grained igneous rock known as basalt, which originated from the partial melting of the Moon’s mantle. This molten rock, or magma, was extremely low in viscosity, allowing it to flow rapidly and spread out over vast distances. The low viscosity enabled the lava to quickly cover the irregular basin floors, creating the characteristically smooth plains we observe today.
This filling process was not a single, massive event but rather a series of episodic eruptions that occurred over a long span of time. Lava rose to the surface primarily through the deep crustal fissures created by the initial impacts. Each eruption laid down a new layer of basalt, with individual flows typically ranging from 10 to 20 meters in thickness. Over time, the repeated flooding resulted in an immense accumulation of solidified basalt, with the total depth of the lava fill in some major maria estimated to be between one and four kilometers.
The massive weight of these dense, thick layers of basalt caused the basin floors to sink further, leading to the formation of tectonic features like wrinkle ridges within the maria. These features are evidence of the compressional forces acting on the lunar crust as the enormous lava load subsided. The scale of this sustained volcanism, which produced vast flood basalt provinces, fundamentally reshaped the Moon’s near side topography.
Why Maria Look Different From the Highlands
The striking contrast between the dark maria and the bright highlands is a result of both their age and their distinct chemical composition. The highlands are composed primarily of an aluminum-rich rock called anorthosite, which crystallized and floated to the surface early in the Moon’s history. This light-colored rock reflects sunlight efficiently, making the highlands appear bright.
In contrast, the maria are made of dense basaltic rock rich in heavy elements, particularly iron and titanium. These elements are highly effective at absorbing light, which is why the maria appear dark. The high concentration of dark minerals determines their visual appearance.
The difference in crater density provides a measure of the relative ages of the two terrains. The highlands, having formed first, have been exposed to impacts for the longest duration, resulting in a dense saturation of craters. The maria, which formed later, have a significantly lower concentration of craters. This difference indicates that the volcanic resurfacing of the maria occurred after the most intense period of the Late Heavy Bombardment had subsided.
The distribution of the maria is also a notable difference, as the vast majority of these large, dark plains are found on the Moon’s near side. The lunar crust is significantly thicker on the far side, which likely inhibited the ascent of magma from the mantle to the surface to fill any impact basins that formed there. This asymmetry in crustal thickness is thought to be the primary reason for the uneven distribution of the maria.