Crustal structure refers to the arrangement and composition of the Earth's outermost layer, known as the crust. This layer is relatively thin compared to the underlying mantle and is composed of various types of rocks, including igneous, sedimentary, and metamorphic rocks. The crust is divided into two main types: the continental crust, which forms the continents, and the oceanic crust, which underlies the oceans. The thickness of the crust varies, averaging about 30 kilometers under continents and about 5-10 kilometers under oceans. The crust is also characterized by geological features such as mountains, valleys, and faults, which result from tectonic processes. Understanding crustal structure is essential for studying earthquakes and volcanic activity.