The Earth’s most dramatic surface events are driven by molten rock. Molten rock found beneath the surface, deep within the Earth’s crust or mantle, is known as magma—a hot mixture of liquid melt, suspended crystals, and dissolved gases. Lava is the term used to describe this same material once it breaches the Earth’s surface through a volcanic vent or fissure. The process begins with the partial melting of solid rock deep underground.
The Earth’s Interior: Where Magma Originates
Magma originates primarily in the upper mantle, specifically within the asthenosphere, and sometimes in the lower crust. The asthenosphere lies beneath the rigid lithosphere and consists of rock that is solid but behaves plastically. This region is where the conditions for melting are most often met.
Despite the extremely high temperatures deep within the planet, the vast majority of the Earth’s interior remains solid. This is due to the intense pressure exerted by overlying layers, which raises the rock’s melting point. Magma formation requires a shift in the balance between temperature and pressure in specific, geologically active zones, rather than just high temperature alone.
The Three Ways Solid Rock Melts
The creation of magma requires a mechanism to disrupt the solid state of rock. There are three primary melting processes, each associated with distinct tectonic settings, which explains why volcanoes occur only in certain places on Earth.
Decompression Melting
Decompression melting occurs when the pressure on hot rock is reduced while the temperature remains constant. As hot mantle rock rises beneath a thinning crust, the pressure drop lowers the rock’s melting point below its ambient temperature, causing partial liquefaction. This is the dominant mechanism at divergent plate boundaries, such as mid-ocean ridges, and beneath mantle plumes (hot spots). The magma produced by decompression melting is typically basaltic and low in silica.
Flux Melting (Volatile Addition)
Flux melting is triggered by introducing volatile substances, primarily water and carbon dioxide, into hot rock. Volatiles act like a fluxing agent, lowering the rock’s melting temperature by breaking chemical bonds. This process drives volcanism at convergent plate boundaries, particularly in subduction zones. As an oceanic plate sinks, water contained within its minerals is released into the overlying mantle wedge. This fluid lowers the mantle rock’s melting point, resulting in magma formation.
Heat Transfer Melting
Heat transfer melting is the least common of the three processes. It occurs when magma rising from the mantle stalls in the crust, forming a magma chamber. The superheated magma transfers thermal energy to the cooler surrounding crustal rock, causing it to melt. This process is common beneath continental hot spots or where large volumes of mantle-derived magma are injected into the crust. The resulting magma is often high in silica and highly viscous, reflecting the continental crust’s silica-rich composition.
Magma’s Journey to the Surface and Eruption
Once formed, magma is less dense than the surrounding solid rock, driving its ascent toward the surface via buoyancy. The molten material rises through cracks and fissures, accumulating in large reservoirs called magma chambers, typically located a few kilometers beneath the surface. Magma is approximately 5% to 10% less dense than the solid rock from which it melted, sustaining this upward movement.
As magma rests in chambers, its composition can undergo significant changes through magma differentiation. Crystals with high melting points may settle out, or the magma may assimilate portions of the surrounding crustal rock. These changes can alter the magma from a low-silica, fluid basalt to a high-silica, viscous rhyolite, influencing the style of the ensuing eruption.
The final stage of the journey is dictated by the behavior of dissolved gases, or volatiles, such as water vapor and carbon dioxide. These volatiles are held in solution within the magma under high pressure, similar to carbonation in a sealed soda bottle. As the magma rises closer to the surface, the confining pressure drops, causing the dissolved gases to exsolve and form bubbles.
The rapid formation and expansion of gas bubbles drives most volcanic eruptions. If the magma is viscous, the bubbles cannot escape easily, leading to immense pressure buildup and an explosive eruption. If the magma is fluid, the gases escape more readily, resulting in effusive eruptions where molten rock flows calmly onto the surface, becoming lava.