magma

Magma comes from Greek magma (a thick unguent, a kneaded mass, the thick residue left after pressing), from the verb massein or mássein (to knead, to squeeze). The same root gives English mass and massage — both involving compression and working of material. The ancient Greek word was used for thick pharmaceutical pastes, for the residue left after pressing grapes or olives, and for any kneaded or compressed dense substance. Latin inherited the word in the same pharmacological sense. The geological application of magma — molten rock within the Earth's crust or mantle — does not appear until the early nineteenth century, when geologists began systematically describing volcanic phenomena and needed a term for the molten material before it erupted. The pharmaceutical residue became the planetary fluid.

Magma is not uniform. It varies in composition, temperature, gas content, and viscosity in ways that determine how volcanoes behave. Mafic magmas (basaltic, relatively low silica) are hot (1100–1200°C), low in viscosity, and low in dissolved gas — they flow readily and produce effusive eruptions, like the lava flows of Hawaii. Felsic magmas (rhyolitic, high silica) are cooler (700–900°C), highly viscous, and rich in dissolved gases that cannot escape through the thick melt — they produce explosive eruptions of extraordinary violence, fragmenting the magma into ash and pumice. The difference between a Hawaiian lava flow that you can approach and film and a Pinatubo or Krakatoa eruption that changes global climate is largely a difference in magma composition. The same basic process — melting of rock — produces these radically different behaviors based on chemistry.

Magma is generated in several distinct geological settings. At mid-ocean ridges, decompression melting occurs as mantle rock rises toward the surface along spreading centers and the drop in pressure lowers the melting point, generating basaltic magma. At subduction zones, oceanic crust descends into the mantle, releasing water that lowers the melting point of the surrounding rock and generates the magmas that feed the explosive volcanoes of the Pacific Ring of Fire. At hotspots, anomalously hot mantle plumes melt the overlying plate from below, producing the chains of volcanic islands like Hawaii. Each setting has a characteristic magma composition and eruptive style. Volcanology is in part the science of reading these compositions to understand which of these processes is operating and what kind of eruption to expect.

Magma chambers — the reservoirs of molten rock that feed volcanoes — were long imagined as simple pools of liquid sitting beneath volcanic systems. Modern seismology and geodesy have complicated this picture considerably. The magma plumbing systems beneath large volcanoes like Yellowstone, Long Valley, and the Campi Flegrei in Italy turn out to be complex networks of partially molten rock — mush zones in which perhaps only 5–50 percent of the material is actually liquid at any given time, with the rest consisting of crystals suspended in the melt. This has implications for eruption prediction: the transition from a crystal mush to a fully liquid eruptible magma requires a triggering event — the injection of fresh hot magma from below, or a sudden decompression — and understanding these triggers is one of the central problems of modern volcanology.