What makes the exposed rock of the Earth's mantle, that is on the ocean floor, dark green?

Answer 1

I take it you mean volcanic rock--nowhere is the mantle exposed on the surface of the earth, even in the deep ocean basins.

Dark green material may contain minerals such as Olivene (we geologists remember the phrase-"Olive Green is Oliv-ene". If the rock is sedimentary some of the green may come from clay or clay-like minerals such as chlorite or glauconite.

Answer 2

I guess that you mean basalt when you speak of "the exposed rock of the Earth's mantle". Basalt is a dark colored rock, usually black or brown, and covers the ocean floors. Green colors could come from olivine or pyroxene, or from other minerals such as serpentine. I doubt if it is due to algal growth, unless it was a shallow ocean floor.

Answer 3

It becomes dark green because of the algee that is growing on it. Since the rock in is such moist conditions, algee will gather upon the rock and colonize