I’m basing this on what I remember from high school physical science.
Are there really only three types of rocks? Sedimentary, metamorphic and igneous? I’d have to think that igneous rock is rare. There isn’t volcanic activity in most of the Earth. What type of rock is the most common?
The entire interior of the planet is brimming with volcanic activity. Igneous rock is the default starting point for the other two kinds. All rocks started out as igneous rocks. Sedimentary rocks are made up of the eroded grains of igneous rocks that were carried by water and deposited on sea- or riverbeds. Metamorphic rocks are sedimentary rocks that have been subjected to great heat and pressure and changed in that way. For instance, shale is sedimentary, but if a shale bed is subjected to heat and pressure, it turns into slate. Limestone is sedimentary but when subjected to heat and pressure turns into marble.
There are only those 3 types, AFAIK, but they are broad categories, each containing many familiar kinds of rock. Igneous are cooled from molten mantle material, sedimentary are composed of particles eroded from other rocks and pressed into a solid, and metamorphic are formed by applying heat and pressue short of causing a change of state. As for relative abundances, my WAG is that igneous wins by a landslide. While very little of the Earth’s surface is given to volcanism, igneous rock can also form below the surface in plutons, batholiths, dykes and the like. Plus, the entire sea floor is composed mostly of basalt, originating in the mid-oceanic rift zones.
Igneous rock can be metamorphosed as well.
However, there used to be igneous activity in almost all parts of the earth. All granite is classified as igneous, even though none of it was ever erupted in a volcano. As a result, some old igneous rock has been around for a long time. Precambrian granite is igneous rock from more than 2 billion years ago.
Some sedimentary rocks are composed of carcasses of dead sea animals 
Forgot about that! :smack: