Unconformity

Nonconformities separate sedimentary rock layers from metamorphic rock layers and from intrusive igneous rock (like granite). In a step-by-step process similar to the other two unconformities, sediment accumulates and becomes rock. Then plate collisions deform these layers and change them into metamorphic rocks. Associated with this mountain-building, molten rock often squeezes upward into the metamorphic rock fractured by the deformities and solidifies, forming igneous rock (usually granite). In phase three, the mountains erode to a peneplain. Then, finally, new layers collect over the flattened metamorphic and igneous rocks. As described earlier, the Tapeats Sandstone layer in the Grand Canyon forms an angular unconformity where it overlies the tilted Grand Canyon Series and a nonconformity where it rests on the Vishnu Schist.