Which difference between gabbro bedrock and granite bedrock causes seismic waves to travel faster in gabbro than in granite?

The primary difference between gabbro and granite that affects the speed of seismic waves is their mineral composition and density. Gabbro typically has a higher density and a more uniform grain structure compared to granite, which contains lighter minerals like quartz and feldspar. This increased density in gabbro allows seismic waves to travel faster through the rock compared to the less dense and more variable structure of granite. Additionally, the presence of mafic minerals in gabbro contributes to its overall rigidity, further enhancing wave propagation speed.