At divergent boundaries, where tectonic plates pull apart (primarily mid-ocean ridge systems or continental rift zones), molten rock rises from below to fill the gap. The plates’ separation acts as a geological trigger, allowing hot mantle rock beneath to flow upward passively. Magma generation here is unique because the rock melts due to a dramatic reduction in overlying pressure, not an increase in temperature. This mechanism, known as decompression melting, creates most of the planet’s new oceanic crust.
Understanding the Conditions for Rock Melting
The mantle, the layer beneath the crust, is composed mostly of solid rock called peridotite, despite its extremely high temperature. The temperature increases steadily with depth, a rate known as the geothermal gradient. Although temperatures can exceed 1,200°C at 100 kilometers, the immense pressure from overlying rock prevents the peridotite from liquifying.
Scientists use the solidus to define the temperature threshold at which a rock begins to melt at a given pressure. The solidus temperature increases significantly with pressure, which keeps the mantle solid under normal conditions. In stable sections, the geothermal gradient stays well below the solidus line, meaning the rock’s actual temperature is insufficient for melting.
Decompression Melting: The Primary Mechanism
The fundamental process for magma generation at divergent boundaries begins with the physical separation of the two tectonic plates. As the plates move away, they create a zone of low pressure, causing the underlying, hot asthenosphere—the ductile, upper part of the mantle—to ascend. This rising mantle material initiates the melting process.
As the solid peridotite is drawn upward, the massive weight of the rock column above it decreases rapidly. The rock rises so quickly that it does not have time to cool down significantly. This upward movement causes the rock’s pressure-temperature path to cross the solidus curve. Because the melting point is lowered due to the reduced pressure, the already-hot rock finds itself above its new, lower melting temperature, and partial melting begins.
The melting is a partial melting of only 1 to 10 percent of the source rock. The resulting liquid, or magma, is mafic in composition, meaning it is rich in magnesium and iron but low in silica. This mafic melt is known as Mid-Ocean Ridge Basalt (MORB), the most common volcanic rock on Earth. This continuous process sustains the volcanic activity along the entire mid-ocean ridge system.
The Journey and Fate of the Magma
Once the liquid magma is generated, its lower density makes it buoyant, driving it upward through the mantle. The liquid separates from the remaining solid rock—a process called melt migration—and begins its ascent toward the surface. This buoyant magma collects in a temporary storage area known as a magma chamber, typically located a few kilometers beneath the seafloor along the ridge axis.
From this shallow crustal reservoir, the magma follows one of two primary paths to become new oceanic lithosphere. It can be injected into vertical fractures and cracks, where it cools slowly to form intrusive igneous rock layers called gabbro and dikes. Alternatively, the magma can erupt directly onto the seafloor through fissures.
When the hot, mafic lava contacts the cold seawater, it cools almost instantaneously, forming characteristic rounded structures called pillow basalts. The combination of the intrusive gabbro and dikes, and the extrusive pillow basalts, continuously constructs the new oceanic crust. This constant emplacement of rock is the physical manifestation of seafloor spreading.