Here's why:

* Volcanic Origins: The Maria are vast, dark plains of volcanic rock formed by ancient volcanic eruptions.

* Basaltic Composition: The lava that flowed from these volcanoes was primarily composed of basalt, a dark, fine-grained, igneous rock rich in iron and magnesium.

* Differences from Earth's Basalt: Lunar basalt differs slightly from Earth's basalt due to the Moon's different chemical composition and geological history. It tends to be poorer in silica and richer in iron and titanium.

So, while we might not call the Maria "seas" in the traditional sense, the rocks that form their surfaces are indeed volcanic, and mainly composed of basalt.