The outer layer of the Earth is called the crust. It is the thinnest layer of the Earth, varying in thickness from about five kilometers beneath the oceans to up to 70 kilometers beneath mountain ranges. The crust consists of solid rocks and minerals, and it is divided into several tectonic plates that float on the underlying semi-fluid mantle. These plates are responsible for geological activities such as earthquakes, volcanic eruptions, and the formation of mountains. The Earth's crust is crucial for sustaining life, providing natural resources like minerals, fossil fuels, and soil necessary for agriculture and human habitation.