What are the lunar maria composed of and how did they form?
Lunar maria are actually pools of once-molten rock. They were formed while the Moon was still solidifying, when meteors impacting the Moon broke through its thin crust.
The maria actually were seas at one time.
When the Moon first formed, it was hot enough for the surface to be covered by a great ocean of molten rock. As this solidified, on occasion large meteors would hit the Moon and crack the thin, still-forming crust. Then molten rock came to the surface once more, becoming the maria we see today.
Read more anout the Moon and how its features came to be here: