Plate tectonics: Magma plumes and their relationship to plate movement.

The idea

Magma plumes or hotspots are modelled as localised upwellings of anomalously hot mantle material that operate independently of most plate margins; some models trace deep plumes towards the core-mantle boundary. As mantle or plume material rises, falling pressure causes decompression melting and generates basaltic magma. Where that magma reaches the surface it builds volcanoes, commonly shield volcanoes. As a tectonic plate moves over a relatively stationary hotspot, older volcanoes are carried away and lose their magma supply, producing a chain whose ages generally increase with distance from the active centre, as in the Hawaiian-Emperor chain. The chain's position and ages provide evidence for past plate direction and rate, while bends can record changes in plate motion.

What earns the marks

• Define magma plumes as a theoretical model of anomalously hot mantle upwelling, with some deep-plume models extending towards the core-mantle boundary.
• Explain that decompression melting of rising mantle or plume material generates basaltic magma that can feed hotspot volcanoes.
• Describe how the continuous movement of a tectonic plate over a stationary plume creates a chronological chain of volcanic islands.
• Use the age and spatial distribution of island chains as evidence to calculate the direction and rate of historical plate movement.

Common mistakes and how to put them right

• Confusing magma plumes with constructive plate margins.

  Put it right by emphasising that plumes are intra-plate features occurring away from tectonic boundaries.

• Stating that the magma plume itself moves to create island chains.

  Put it right by explaining that the plume remains relatively stationary while the tectonic plate moves above it.

• Assuming all volcanoes in a hotspot chain are active.

  Put it right by noting that only the volcano currently directly above the plume is active, while the rest are extinct.

Exam tips

• Use the Hawaiian Islands as a primary case study, explicitly naming the active volcano and older extinct islands to illustrate plate movement.
• Be prepared to calculate or interpret the rate of plate movement using the age of volcanic islands and their distance from the active hotspot.
• Clearly distinguish between the stationary nature of the mantle plume and the dynamic movement of the overlying lithospheric plate.