About 90% of all volcanic rock on Earth (6)

I believe the answer is:

basalt

(Other definitions for basalt that I've seen before include "Dark rock of volcanic origin" , "Common volcanic rock" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Black volcanic rock" , "Rock containing olivine" .)

I've seen this clue in The New York Times.

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