Which Are Nonpermeable Rocks?

Intrusive Igneous Rocks

• Intrusive rocks are igneous rocks that solidify below the earth's surface. Seven miles beneath the earth's surface rock is semi-plastic where the temperature is 1,500 degrees Fahrenheit and the pressure 50,000 lb. per square inch. When rocks solidify under such conditions they do so without and pores or fissures and are completely nonpermeable to water. Perhaps the best example of a nonpermeable intrusive igneous rock is granite, which forms the inner core of most large mountain ranges.

Extrusive Igneous Rocks

• Extrusive rocks are igneous rocks that have cooled and hardened above the earth's surface. These rocks are formed as a result of liquid lava coming to the earth's surface in erupting volcanoes. When the lava solidifies on the earth's surface it usually forms a nonpermeable rock such as obsidian or rhyolite, although an exception to this is pumice. Basalt is an example of lava that has solidified under the ocean.

Metamorphic Rocks

• Metamorphic rocks are rocks that have undergone a change due to heat, pressure or fluid permeation on other rocks. Igneous, sedimentary or previous metamorphic rocks can all undergo further metamorphism. Not all metamorphic rocks are nonpermeable; their permeability depends on their parent rocks and the conditions of metamorphism. Examples of nonpermeable metamorphic rocks are marble, quartzite, slate, gneiss and anthracite coal.

Sedimentary Rocks

• By far the majority of sedimentary rocks are permeable to water, including such common rocks as chalk, limestone and sandstone. However, there are a few exceptions, notably amber or succinite. Limestone is a rock in which the permeability varies from one place to another. Generally speaking, limestone is permeable, although there are large limestone deposits that are essentially nonpermeable due to the conditions of their formation.