Geologists develop an understanding of the geologic history of an outcrop by considering rock type, mineral composition, and the type of environment in which rock units formed.

Some key ideas to keep in mind include the following:

• Sedimentary rocks are often well layered, and those formed on land are often oxidized (e.g., red in color) due to weathering in an oxygen-rich environment.
• Igneous rocks crystallize from magma at fast rates on the surface and slower rates in the subsurface, producing tiny and large mineral crystals, respectively. The compositional spectrum of igneous rocks is fundamentally determined by the silica content. Basalts and gabbros are low silica and dark in color, compared to rhyolite and granite, which are high in silica and lighter in color.

Look at the types of characteristics listed below, and then assign them to their respective rock unit. Some characteristics will fall within one rock unit or the other. All characteristics will fall within one of the two rock unit categories.