What's the diff between a gabbro & a basalt (chemically & texturally), a rhyolite & a granite, a granite & a gabbro? Why are basalts & gabbros generally darker in appearance than rhyolites & granites?

Question

Earth and planetary science: Geology focused

Diff: Difference

Answer 1

Gabbro and basalt are both mafic igneous rocks, however a basalt is extrusive. Gabbro on the other hand, is extrusive with both rocks having crystals of plagioclase and augite, with the visible difference being their size.

Rhyolites and granites come from the same magmas, however the difference is that rhyolites are extrusive, while granites are intrusive. You can see this by the identifiable crystals in the granite, whereas rhyolites do not have visible crystals.

Gabbro is an intrusive mafic rock while granite is an intrusive felsic rock. Granite is also made up of >69% SiO2.

Basalts and gabbros (both mafic rocks) are darker because their chemical makeup is different from that of rhyolites and granites (both felsic rocks). Mafic rocks have a higher concentration of Magnesium and Iron in them, giving them a darker appearance than felsic rocks which are rich in feldspar and quartz.