The Earth's outer layer is broken into moving, interacting plates whose motion at the surface generates most earthquakes, creates volcanoes and builds mountains. In this image, the orange layer represents the deformable, warm asthenosphere in which there is active mantle flow. The green layer is the lithospheric plate, which forms at the mid ocean ridge, then cools down and thickness as it moves away from the ridge. The cooling of the plate overprints a compositional boundary that forms at the ridge by dehydration melting and is preserved as the plate ages. The more easily deformable, hydrated rocks align with mantle flow. The directions of past and present-day mantle flow can be detected by seismic waves, and changes in the alignment of the rocks inside and at the bottom of the plate can be used to identify layering.