Although all 3 types of rock are present, the earth’s crust is 90% igneous rock so it makes up the large majority of the crust. Sedimentary and metamorphic rock only makes up a mere 5% each of the earth’s crust.
The reason for this lopsided distribution of rock is because of the process by which these types of rocks are formed. Sedimentary rock formation occurs on the earth’s surface while igneous and metamorphic rocks are formed deeper within the earth’s crust.
What is the earth’s crust?
The earth’s crust is its thin, hard outer layer. Even though the crust may be anywhere from 3.1–43.5 mi deep at any given point, it makes up only about 1% of the earth’s total volume.
The earth’s mantle lies between the crust and the core and is over 1800 miles thick in comparison. The mantle is composed of mostly rock like the crust but makes up 84% of the earth’s volume.
How is igneous rock formed?
The short answer for how igneous rocks are formed through the cooling and then solidification of liquid magma, or lava. As the hot magma from the earth’s core rises to the surface, it cools and undergoes and chemical change causing it to harden and crystalize.
There are thought to be over 700 different types of igneous rock on earth. Most of these are formed below the earth’s crust.