Definition of Igneous Rock
An igneous rock is one of the three main rock types, formed from the cooling and solidification of molten rock. This molten rock originates deep within the Earth as magma or erupts onto the surface as lava. The name "igneous" comes from the Latin word "ignis," meaning fire, reflecting their fiery origins.
Intrusive vs. Extrusive Formation
Igneous rocks are categorized based on where they solidify. Intrusive (Plutonic) rocks form when magma cools slowly beneath the Earth's surface, allowing large, visible crystals to grow (e.g., granite). Extrusive (Volcanic) rocks form when lava erupts and cools rapidly on the surface, resulting in very fine-grained or glassy textures (e.g., basalt, obsidian).
Common Examples and Characteristics
Granite, a common intrusive igneous rock, is known for its visible mineral crystals (quartz, feldspar, mica) and is often used in construction. Basalt, a common extrusive igneous rock, is fine-grained, dark, and forms oceanic crust. Other examples include gabbro (intrusive), rhyolite (extrusive), and pumice (a porous extrusive rock formed from frothy lava).
Importance in Earth Science
Igneous rocks provide crucial insights into the Earth's internal processes, including volcanism, plate tectonics, and the composition of the mantle and crust. They are also sources of valuable mineral resources and form significant geological features like mountain ranges, volcanic islands, and vast continental shields, shaping our planet's surface.