All rocks are made up of minerals. The most common Iowa minerals are gypsum, halite, calcite, galena, pyrite, sphalerite, quartz, limonite, and marcasite. The unique combination of these minerals and how they were formed determines if the rock is igneous, sedimentary, or metamorphic.
Iowa’s oldest rocks are igneous rocks, found deep underground. It is estimated these rocks are 2.9 billion years old.