What are the maria composed of? Is this material found elsewhere in the solar system?

Basaltic Lava. It is very similar in composition to the basaltic lava on Earth.

Explanation:

Maria (Plural for Mare) are the huge dark plains on the Lunar surface. They were name as such as the ancient astronomer thought them to be sea (Latin meaning of mare is sea). As the astronomer's studied and observed Lunar surface further we got to know that these are not sea of water but huge plains filled with lava. These are easily visible to naked eye as dark spots on the Lunar surface.

These were formed due to ancient volcanic eruptions when the Moon was young. The study of these with Earth based observatories and Lunar missions have revealed a lot about the composition of these Lunar Maria. Major composition of them is Basaltic Lava very similar to that of Earth. They are iron rich which makes them less reflective in comparison to the highlands.

Elemental composition of the Lunar Maria is as follows:

1. Silica (SiO₂) - 45.4%

2. Alumina (Al₂O₃) - 14.9%

3. Iron Oxide (FeO) - 14.1%

4. Lime (CaO) - 11.8%

5. Magnesia (MgO) - 9.2%

6. Titanium Oxide (TiO₂) - 3.9%

7. Sodium Oxide (Na₂O) - 0.6%

The percentage of these elements is very similar to that on Earth.

The high percentage of Aluminum can be attributed to the replacement of Iron by anorthosites (which are mostly composed of plagioclase feldspar). Anorthosites replaced iron and magnesium in the continental areas.