True. Most of the Earth's crust is made up of a mixture of metamorphic and igneous rocks. Metamorphic rocks are formed when existing rocks are changed by heat, pressure, or chemical reactions. Igneous rocks are formed when magma or lava cools and solidifies. Sedimentary rocks, which are formed from sediments that have been deposited and compacted over time, make up a relatively small portion of the Earth's crust.