What Is The Rarest Volcanic Rocks?

Volcanic rocks form when magma (molten rock) cools and solidifies. The type of volcanic rock that forms depends on the chemical composition of the magma and how quickly it cools. While basalt and pumice are relatively common volcanic rocks, others are quite rare. Here are some of the rarest volcanic rocks found on Earth:

Kimberlite

Kimberlite is an ultrapotassic volcanic rock known for containing diamonds. It forms deep underground in the upper mantle where there are high pressures and temperatures. Kimberlite magma is rich in volatiles like water and carbon dioxide which lower its melting point, allowing it to melt and rise rapidly from great depths. This speed means kimberlite volcanoes are small and the magma chills quickly when it reaches the surface, creating a fine-grained volcanic rock.

Kimberlite pipes, the vents that channel kimberlite to the surface, are the primary source of mined diamonds today. Notable kimberlite mines include those at Kimberley, South Africa which lent the rock its name, and mines in Yakutia, Russia. Kimberlite deposits are rare, found scattered in ancient stable cratons across the continents. The difficulty in finding diamond-bearing kimberlite pipes makes this volcanic rock extremely valuable.

Carbonatite

Carbonatite is an extremely rare type of igneous rock made almost entirely of carbonate minerals like calcite, dolomite, and ankerite. Most lava is silicate-rich, so carbonatite is highly unusual. Carbonatite magma forms from partial melting of carbon dioxide-rich mantle rock. The magma is hot but highly viscous, allowing distinctive carbonatite lava domes to form.

Major carbonatite outcrops have been found near Lake Natron in Tanzania, the Kola Peninsula of Russia, and Oka, Quebec. These igneous carbonate rocks often contain rare earth metals, copper, and niobium-tantalum ore deposits, adding to their economic value. The composition and formation of carbonatites are still not fully understood.

Rhyolite

Rhyolite is a felsic volcanic rock high in silica and aluminum. It has a very high viscosity that can allow gas bubbles to be trapped as the magma solidifies. This gives rhyolite a porous texture full of holes from gas vesicles. Pumice and obsidian are forms of glassy rhyolite.

While not the rarest rock, rhyolitic lava and tuffs are relatively uncommon compared to basalts and andesites which dominate most volcanic regions. Few volcanoes actually erupt rhyolite though it may be more common in certain places like the central Andes Mountains. Yellowstone Caldera holds one of the largest rhyolitic magma chambers in the world. Violent rhyolitic eruptions at Yellowstone were responsible for giant ash flows that blanketed much of the western United States.

Komatiite

Komatiite is a type of ultramafic volcanic rock almost devoid of silicon. It has a high amount of magnesium with over 18% MgO by weight. Komatiites formed predominantly in the hot Archaean Eon when Earth’s mantle temperatures were higher. The komatiite lavas could reach astounding temperatures of 1600°C (2900°F).

Little komatiite has survived from the Archaean as it weathers easily. Outcrops have been found near volcanoes and in greenstone belts in Canada, South Africa, and western Australia. The turbulent lava flow textures and spinifex crystal shapes of komatiites are relicts of their ancient, super-hot eruption environments. No modern komatiite flows exist, making these ultramafic volcanics exceedingly rare today.