Magma forms in the crust and upper mantle when solid rock partially melts. The formation of magma depends on several factors, including heat, pressure, and water content.

Origin of Magma

• At a depth of 100 km, the temperature of the mantle rock ranges from 1400C to 1600 C.

• At these temperatures, the solid rock of the lower crust and upper mantle is near, but not quite at, its melting point.

​

Heat

• The additional heat needed to produce magma comes from three sources:

  • First, friction generates heat as huge slabs of lithosphere slide past each other in subduction zones.

  • Second, the mantle itself heats these subducting slabs.

  • Third, hot mantle rock can rise and intrude into the cooler lithosphere, heating it.

Heat

Pressure

Pressure increases with depth inside Earth!

• Increasing pressure raises the melting point of rock deep beneath the surface.

• Decreasing pressure, in contrast, lowers rock's melting point.

• Decompression melting- occurs when rock rises and melts due to reduced pressure.

Decompression Melting

• This process typically takes place as hot yet solid rock rises because it is less dense than the surrounding rock.

• As the rock rises, pressure on the rock decreases.

• Increasing water content of rock also lowers the rock's melting temperature.

• Because of this, "wet" rock deep beneath the surface melts at a much lower temperature than does "dry" rock of the same composition and under the same pressure.

​

Water Content

• Fortunately, magma only reaches the surface in certain areas.

• Most volcanoes form along divergent and convergent plate boundaries.

• Some volcanoes form far from plate boundaries above "hot spots" in the crust.​

Volcanoes and Plate Boundaries

Divergent Boundary Volcanism

• At divergent boundaries, volcanic activity occurs where the plates pull apart.

• Mantle rock rises to fill the gap between plates.

• As the rock rises, decompression melting occurs.

• This forms magma, which erupts through a spreading center.

Divergent Boundary Volcanism

• Although most spreading centers are located along mid-ocean ridges, some are not.

• The Great Rift Valley in Eastern Africa is an area where continental crust is being pulled apart along a divergent boundary.

• Mount Kilimanjaro in Tanzania is one of many volcanoes that have formed near the rift valley.

Mount Kilimanjaro