Sea-floor spreading opens the axial rift valley. Rising asthenosphere melts(decompression melting), forming mafic magma. 減壓熔融Pooled magma solidifies into oceanic crustal rock.Pillow basalt枕狀玄武岩—magma quenched at the sea floor (rapid cooling; equal pressure)Dikes—preserved magma conduitsGabbro—deeper magmaOphiolite: 蛇綠岩系