Types of Igneous Rocks

Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill.

Granite is an intrusive igneous rock that has many uses in building construction and architectural design.

Limestone is not igneous; it is a sedimentary rock. Limestone is a common sedimentary rock of biochemical origin. It is largely composed of calcite. Sometimes it is almost pure calcite but more often it is mixed with many other minerals and sand.

Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO 3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite.

Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It usually forms from the metamorphism of sandstone.

Sandstone is a clastic sedimentary rock composed of sand-sized grains of mineral, rock or organic material.

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. The specimen shown above is about two inches (five centimeters) across.