The term “igneous rock” is one of the three major classifications of rock, alongside sedimentary and metamorphic types. The name “igneous” is derived from the Latin word ignis, which translates directly to “fire.” This fiery name points to the intense heat involved in the formation process of these rocks.
The Source of the Fire: Melting and Magma
The fiery process begins deep within the Earth’s crust or mantle, where existing rock is subjected to immense heat and pressure. It takes temperatures between approximately 700°C and 1,300°C for rock to melt and transform from a solid state into a liquid. This molten material, located beneath the Earth’s surface, is called magma.
Magma is a complex, high-temperature fluid mixture that often contains suspended solid crystals and dissolved gases. The exact temperature needed for melting depends on the rock’s chemical composition and the surrounding pressure. Mafic, or dark-colored, rocks generally require higher temperatures to melt than lighter, silicic rocks.
When this superheated material is forced up toward the surface and erupts, it is known as lava. Whether the molten rock remains trapped below or flows across the surface, its origin is always this extremely hot, liquid state.
How Location Determines the Rock Type
The final character of an igneous rock is determined almost entirely by where the molten material solidifies. Geologists classify these rocks into two major groups based on their cooling environment. The first group, known as intrusive or plutonic igneous rocks, forms when magma cools very slowly beneath the surface, insulated by surrounding rock layers.
Because the cooling process can take thousands or even millions of years in this underground environment, the mineral crystals have a long time to grow. This slow crystallization forms rocks with a characteristically coarse-grained appearance. Intrusive rocks are often exposed at the surface only after significant uplift and erosion have worn away the overlying material.
The second major group is extrusive, or volcanic, igneous rocks, which form when lava cools rapidly on or very near the Earth’s surface. As lava is suddenly exposed to the relatively cool atmosphere or water, it solidifies quickly. The rapid cooling drastically limits the time available for crystals to develop and grow to any significant size.
Textures and Common Igneous Examples
The differing cooling rates lead directly to the visible texture of the final rock, which is a significant feature used for identification. Intrusive rocks, formed by slow cooling, display a phaneritic texture, meaning their individual crystals are large enough to be easily seen with the naked eye. Granite, a common light-colored rock used in construction, is a prime example of a phaneritic, intrusive igneous rock.
Conversely, extrusive rocks exhibit an aphanitic texture, where the crystals are so small they are not visible without magnification. Basalt, a dark and dense rock that forms the ocean floor, is a common aphanitic extrusive rock. If the lava cools almost instantaneously, such as when it hits water, the rock forms a glassy texture with no crystalline structure at all. Obsidian, a natural volcanic glass, is the most well-known example of this extremely rapid solidification.