How were the 'maria' and 'highlands' on the lunar surface formed? Are they truly oceans?

Hello, that's an excellent question! Many people wonder the same thing when they first look at the Moon through a telescope. Those dark patches on the Moon really do look like oceans on Earth!

Let me explain this in plain language and walk you through this chapter of the Moon's "geological history."

First, the most direct answer: There's not a single drop of water in the lunar maria. They are not real oceans.

The name "lunar sea" (月海, yuè hǎi) is purely a historical artifact. A few hundred years ago, when Galileo first observed the Moon through a telescope, he saw large, flat, dark areas contrasting sharply with the bright, rugged regions around them. People at the time, drawing on their experience with Earth, naturally assumed these dark areas were oceans. So, they named them using the Italian word "Mare" (meaning "sea"). This name has stuck ever since.

In reality, these so-called "seas" are vast, flat plains of volcanic lava.

So, how exactly did the "highlands" and "lunar maria" form?

We can think of their formation as happening in two stages: the Moon's "adolescence" and its "adulthood."

Stage One: Formation of the "Highlands" – The Moon's Ancient Skin

The "Highlands" (Lunar Highlands/Terrae) are the bright, heavily cratered, higher-elevation regions of the Moon. They represent the Moon's oldest crust, essentially its "original skin."

The Magma Ocean Era: About 4.5 billion years ago, shortly after the Moon formed, it was a hot, molten ball of rock, which scientists call a "Magma Ocean."
"Cream" Rising: In this vast molten pot, lighter minerals (mainly plagioclase) slowly "floated" to the surface, much like cream rises in milk.
Cooling into a Crust: These lighter minerals cooled and solidified at the surface, forming the Moon's initial crust. This crust is what we see today as the "Lunar Highlands." Because it's primarily composed of light-colored anorthosite (a rock rich in plagioclase), it appears very bright.
Weathering the Ages: Because the highlands are the Moon's oldest part, they have endured countless asteroid and comet impacts over billions of years. Their surface is heavily pockmarked, giving them a rugged, complex appearance.

Highlands Summary: They are the Moon's original, ancient, light-colored crust. Having existed the longest, they bear the most impact scars.

Stage Two: Formation of the "Lunar Maria" – The Moon's "Giant Scars" and "Lava Patches"

The "Lunar Maria" are the dark, flat, lower-elevation regions. Their formation occurred much later than the highlands and was far more dramatic.

Giant Impacts: Around 3.9 billion years ago, after the highlands formed, the solar system was a chaotic place during a period called the "Late Heavy Bombardment." Enormous objects, tens or even hundreds of kilometers in diameter, slammed violently into the Moon.
Basin Creation: These catastrophic impacts blasted huge, deep basins into the ancient "old skin" of the highlands, like smashing craters into concrete. The impact that formed the Imbrium Basin (Sea of Rains), for example, must have been unimaginably powerful.
Magma Upwelling: The lava didn't flow out immediately after the impacts. Instead, hundreds of millions of years later, heat generated by the decay of radioactive elements deep within the Moon melted the rock in the lunar mantle. This molten magma then slowly welled up ("gurgled up") through the weaknesses and fractures in the crust created by the earlier giant impacts.
Filling the Basins: This hot magma (primarily basaltic, similar in composition to lava from Hawaiian volcanoes on Earth) was very fluid. It flooded out like water, flowing across and filling those giant impact basins.
Cooling into "Seas": The lava eventually cooled and solidified, forming vast, flat, dark plains. Because basalt is dark in color, these regions appear very dark. These are the "lunar maria."

Maria Summary: They formed when dark lava from the Moon's interior welled up, flooded, and solidified within the giant basins blasted into the ancient highlands by late-stage impacts. Being much younger than the highlands, they have experienced far fewer impacts, so their surfaces are sparsely cratered and appear very smooth.

A Simple Analogy to Help You Understand

Think of the Moon's formation like making a slightly flawed pizza:

Highlands: You roll out a light-colored dough base. This is the Moon's original highlands. Its surface might be a bit uneven.
Giant Impacts: You use a spoon to forcefully scoop out several large depressions in the dough base. These are the impact basins.
Lunar Maria: Then, you pour dark tomato sauce (or black olive paste) into these depressions. The sauce flows out and fills the holes. This sauce, once cooled and set, represents the lunar maria.
Later Small Impacts: Finally, you sprinkle a few sesame seeds over the whole pizza. These represent the later, scattered impact craters. You'll notice far fewer "sesame seeds" on the dark sauce areas compared to the light dough areas because the sauce was added later.

I hope this explanation gives you a clearer picture of the Moon's "appearance"! Next time you look at the Moon, you'll be able to distinguish between the ancient "skin" and the younger "patches."