The Igneous Rocks of Volcanoes: From Basalt to Rhyolite
Every volcano is defined by the rock it produces. The composition of its magma, especially how much silica it contains, controls whether a volcano erupts gently or explosively, builds a broad shield or a steep cone, and produces fluid lava flows or clouds of ash. Understanding the main volcanic rocks, from dark basalt to pale rhyolite, is the key to understanding the diversity of volcanoes around the world.
Volcanic versus plutonic rocks
Igneous rocks form from cooling magma, and they fall into two broad groups. Volcanic, or extrusive, rocks form when magma erupts at the surface and cools quickly, giving them fine crystals or a glassy texture. Plutonic, or intrusive, rocks form when magma cools slowly deep underground, growing large crystals. The same magma can produce either kind depending on where it solidifies; this article focuses on the volcanic rocks erupted at the surface.
Basalt: the most common volcanic rock
Basalt is the most abundant volcanic rock on Earth, dark and low in silica. It forms from fluid, runny lava that erupts at high temperatures, building shield volcanoes and vast lava plains. The ocean floor is largely paved with basalt, and the shields of Hawaii and Iceland are built from it. Its fluidity is why basaltic volcanoes tend to erupt gently, producing flows rather than violent explosions.
Andesite: the rock of the Ring of Fire
Andesite, named after the Andes, is intermediate in silica content and characteristic of the volcanoes of subduction zones around the Pacific Ring of Fire. More viscous than basalt, andesitic magma traps more gas, making its volcanoes more prone to explosive eruptions. The classic steep stratovolcanoes, from the Andes to Japan, are largely built of andesite and related rocks.
Dacite: thick and explosive
Dacite is richer in silica than andesite, making it more viscous still. It is associated with explosive eruptions and the growth of lava domes, where thick magma piles up rather than flowing. Many dangerous dome-building volcanoes, such as those that produce deadly pyroclastic flows, erupt dacite, reflecting the link between high silica content and explosive behaviour.
Rhyolite: the most explosive
Rhyolite is the most silica-rich common volcanic rock, pale in colour and extraordinarily viscous. Rhyolitic magma is so thick that it can barely flow, trapping enormous amounts of gas. This makes rhyolitic systems capable of the most violent eruptions on Earth, including the caldera-forming super-eruptions of volcanoes like Yellowstone. Rhyolite is the rock of the planet's most explosive volcanism.
Pumice, obsidian, and glass
The same silica-rich magmas can also produce distinctive textures. Pumice is a frothy, gas-filled volcanic rock so light it can float on water, formed when gas-charged lava is blasted apart. Obsidian is volcanic glass, formed when silica-rich lava cools so quickly that crystals cannot grow. Both are products of explosive, silica-rich volcanism and are easily recognised in the field.
Reading a volcano's rocks
For geologists, the rocks a volcano produces are a key to understanding its behaviour and history. The silica content and texture reveal how explosive the volcano is likely to be, while the layering of different rock types records its eruptive past. By studying these rocks, scientists can assess the hazards a volcano poses and reconstruct the story of its eruptions.
Explore on the map
From the basaltic shields of Hawaii to the andesitic stratovolcanoes of the Andes and the rhyolitic calderas of supervolcanoes, the world's volcanoes reflect the rocks they produce. Explore them on the interactive map — filter by type to compare the gentle and the explosive volcanoes shaped by their different magmas.