Scientists usually obtain information about Earth's crust in a few different ways. One way is to collect samples from the Earth's crust by drilling for rock cores, which give them a cross-section of various layers of rock. Scientists also collect rocks, some of which give information about the history of the crust's formation and movement. Another way to study the crust is by measuring seismic waves that occur throughout the crust. These waves travel at different speeds depending on what kind of material they pass through, so scientists can characterize the materials in the crust in this way. Another method of studying the Earth's crust is through various imaging techniques, such as sonar (for the ocean's crusts), or GPS (to show how the crust moves/changes with shifts in the tectonic plates).