Other important sedimentary rock types include shale and coal. Shale layers can serve as important aquitards. Intrusive rock is formed when the molten material solidifies at considerable depth below the earth's surface. A common example is granite. Although not important as aquifers, these materials are candidates for the host rock for high-level radioactive waste.
Extrusive rock is formed when the solidification process occurs at or near the ground surface.
An example of this type of rock is basalt. In some locations, extrusive rocks form significant aquifers. The Columbia River Plateau covering eastern Washington and Oregon, and Idaho, averages about 500 m in thickness and is one of the largest basalt deposits in the world. Basalt aquifers are critically important water sources for the Hawaiian Islands. They generally form poor aquifers.