* The crust is heterogeneous: The Earth's crust is not uniform. It varies greatly in composition and thickness across the globe, with different types of rocks dominating different regions.
* Exposure and accessibility: We primarily know about the composition of the crust through geological studies and rock samples collected from the surface. This means our understanding is limited by what we can access.
* Continents vs. ocean basins: Continental crust is significantly thicker and is dominated by igneous and metamorphic rocks (granite, gneiss, etc.), while oceanic crust is thinner and primarily composed of basalt (an igneous rock).
However, we can make some general statements:
* Continental Crust: The majority of continental crust is composed of igneous and metamorphic rocks. Estimates suggest that over 90% of continental crust could be made up of these rock types.
* Oceanic Crust: Oceanic crust is largely composed of basalt, an igneous rock. This makes up the majority of its composition.
Overall: While we can't pinpoint an exact percentage, it's safe to say that igneous and metamorphic rocks make up a substantial portion of the Earth's crust, particularly in the continental regions.
