Extrusive rock

Extrusive rocks are a specific category of igneous rocks formed on the Earth's surface. They result from the process of extrusion, where hot magma moves from inside the planet to the outside. Once magma reaches the surface, it is called lava. This material can flow out smoothly or explode violently into the atmosphere. In the case of explosions, the material falls back to the ground as pyroclastics or tuff. This process is the opposite of intrusive rock formation, where magma cools deep below the surface.

The primary factor determining the appearance of these rocks is the cooling rate. When magma is exposed to open air or seawater, it loses heat very rapidly. This rapid cooling leaves very little time for mineral crystals to grow. Most extrusive rocks have an aphanitic texture, meaning the crystals are too small to be seen by the human eye. However, some crystals may begin forming slowly in a magma reservoir before the eruption occurs. These larger, visible crystals are called phenocrysts. When a rock contains both these large phenocrysts and a fine-grained groundmass, it has a porphyritic texture.

Sometimes, the cooling process is so fast that a portion of the rock matrix does not crystallize at all. Instead, it becomes a natural glass, such as obsidian. Other extrusive rocks contain many small, bubble-shaped cavities called vesicles. These vesicles form when the magma contains abundant volatile components, which are released as free gas. As the lava cools, these gas bubbles become trapped inside the solidifying rock. Rocks like scoria and pumice are defined by this vesicular texture. While they look similar, pumice is a light-weight glass with a silicic composition that allows it to float.

Different volcanic structures produce specific types of extrusive rocks. Shield volcanoes are large, slow-forming structures that erupt fluid basaltic magma. This magma cools to form basalt, a rock composed of minerals like feldspars and pyroxenes. Fissure volcanoes also produce basalt by pouring low-viscosity magma from long vents. In contrast, composite volcanoes, also known as stratovolcanoes, often erupt andesitic magma. This magma is made of melted mantle rocks and contains many gases. These eruptions typically result in the formation of the extrusive rock known as andesite.

Other volcanic features create distinct rock types through different mechanical processes. Cinder or scoria cones expel lava that has a very high gas content. This process creates scoria, which is a mafic, vesicular rock. Lava domes form when high-viscosity lava piles up into a mound shape. These domes can solidify into obsidian or sometimes dacite, as seen at Mount St. Helens. Large volcanic depressions called calderas can form after a volcano collapses. A resurgent caldera, such as the Yellowstone Caldera, may refill with rhyolitic magma to produce rhyolite.

The variety of extrusive rocks is closely linked to the chemical makeup of the magma. For example, submarine volcanoes erupting on the ocean floor produce pumice. The specific weight contents of silica and alkali elements determine where a rock falls on a classification scale. This relationship between chemistry and texture helps scientists understand the history of volcanic activity. By studying whether a rock is aphanitic, porphyritic, or vesicular, researchers can determine how fast the lava cooled and how much gas was present during its formation.