Volcanic rocks form when magma (molten rock) cools and solidifies. The composition and texture of volcanic rocks can vary greatly, depending on the composition of the magma and the eruption conditions. However, there are several main types of volcanic rock that geologists commonly refer to. These include basalt, andesite, rhyolite, obsidian, and pumice.
Basalt
Basalt is a very common volcanic rock that makes up most of the world's oceanic crust. It is a fine-grained, dark-colored rock that forms from the rapid cooling of low-viscosity magma erupted at or very near the surface. Basalt magma has high levels of iron, magnesium, and calcium, with low levels of silica. As the molten rock erupts onto the surface, these minerals rapidly crystallize to form the rock basalt. Basalt often has visible crystals of olivine, pyroxene, and plagioclase feldspar. When conditions allow the molten basalt to cool very slowly below the surface, it forms a coarser-grained rock called gabbro. Basalt and gabbro make up the ocean floor and parts of the continental crust.
Andesite
Andesite is an intermediate volcanic rock, meaning it has an intermediate composition between rhyolite and basalt. Andesite magma forms when some of the magnesium and iron in basaltic magma is removed. This increases the silica content of the residual magma. Andesite lava erupts at temperatures between 900-1000°C. It is usually light to dark gray in color and fine-grained. Andesite lava tends to be thick and flows slowly, so it piles up around the volcanic vent. This forms a dome volcano or lava dome. Andesite is found in oceanic crust and parts of the continental crust above subduction zones, where oceanic crust descends beneath continental crust.
Rhyolite
Rhyolite is a felsic volcanic rock made up primarily of quartz and feldspar minerals. It forms from silica-rich magma with high levels of gases like water vapor and carbon dioxide. Rhyolite magma is very viscous due to its high silica content. This causes it to erupt explosively, producing fragmented lava like volcanic ash, pumice, and obsidian. The high silica also means rhyolite lava flows are thick and slow-moving when they do erupt onto the surface. Rhyolite is typically light gray, brown, or pinkish in color. It is found mainly in continental crust above subduction zones. The magma differentiates from basaltic magma by leaving behind iron and magnesium minerals as it rises.
Obsidian
Obsidian is a natural glass formed by the rapid cooling of rhyolite lava without crystallization. It is rich in silica, typically 70% or more. Obsidian is black or very dark in color. It forms when rhyolite lava erupts with a high content of gases. The gases make the lava foam and fragment into small particles that cool so rapidly they cannot crystallize. This creates obsidian glass. Obsidian breaks with very sharp edges, which made it useful for ancient cutting tools and weapons. Areas with obsidian deposits include Yellowstone National Park in the western United States.
Pumice
Pumice is a porous volcanic rock formed during explosive eruptions from magmas that contain high levels of gases. The gases exsolve as the magma rises rapidly, foaming it up and making it viscous. When the magma hits the surface, the trapped gas bubbles freeze into the structure of the lava as it solidifies into pumice. The many holes and channels give pumice an extremely low density, allowing some pieces to float on water. Pumice ranges from white to light gray or light brown in color. It commonly forms from silica-rich rhyolite magmas but can also form from andesite magmas. Pumice deposits are found near volcanic vents and calderas worldwide. The lightweight rock is used for abrasives and skincare products.
So, the composition and eruption style of a volcano's magma determines what type of volcanic rock it will produce, from mafic basalts to felsic rhyolites. Examining the volcanic rocks in a region provides insights into the area's geologic history and formation. The characteristics and uses of these rocks also make them an important natural resource.
