Partial melting

Definition

Partial melting is the process where only a portion of a solid rock melts, leading to the formation of magma. This phenomenon occurs due to the variations in temperature and pressure within the Earth's mantle and crust, which allows certain minerals to melt while others remain solid. The resulting magma can then rise towards the surface, influencing volcanic activity and contributing to the composition of continental crust.

5 Must Know Facts For Your Next Test

1. Partial melting typically occurs at tectonic plate boundaries, especially in subduction zones where oceanic plates are forced under continental plates.
2. The minerals that melt during partial melting are often those with lower melting points, such as feldspar and quartz, which can significantly influence the resulting magma's composition.
3. The depth at which partial melting occurs can vary widely, typically happening between 30 to 150 kilometers deep in the Earth’s crust and upper mantle.
4. Partial melting plays a crucial role in forming granitic rocks, which are prevalent in continental crust, as they are derived from the melting of sedimentary and igneous rocks.
5. Isotopic analysis of materials formed from partial melting can provide insights into the sources and processes involved in the evolution of the mantle and crust over geological time.

Review Questions

• How does partial melting influence the composition of magma generated at subduction zones?
  • In subduction zones, partial melting occurs when an oceanic plate is forced down into the mantle beneath a continental plate. The pressure and temperature conditions allow certain minerals in the subducting slab and surrounding mantle rocks to melt. This results in magma that has a distinct composition influenced by both the original materials being melted and any fluids released from the subducting plate, leading to more evolved magmas such as andesite or rhyolite.
• Discuss how partial melting contributes to the formation of continental crust and its mineral diversity.
  • Partial melting contributes significantly to the formation of continental crust by allowing specific minerals to melt and form magma that rises towards the surface. As this magma cools, it crystallizes into various rock types, including granites and other felsic rocks rich in silica. The diversity of minerals found in continental crust results from the selective melting of different parent rocks, alongside processes like fractional crystallization, where different minerals solidify at different temperatures.
• Evaluate the role of isotopic tracers in understanding the processes of partial melting within the Earth's mantle.
  • Isotopic tracers are essential for evaluating processes of partial melting as they provide information about the sources and evolution of both mantle-derived magmas and continental crust. By analyzing isotopic ratios (like Sr, Nd, and Pb), scientists can infer the characteristics of the source materials that underwent partial melting. This understanding helps reconstruct past geological events and dynamics within the mantle, revealing how these processes shaped the current structure and composition of Earth's lithosphere.

Related terms

Fractional Crystallization: A process that occurs when different minerals crystallize from magma at different temperatures, leading to a separation of minerals and changes in magma composition.

Magma Differentiation: The process by which magma evolves to form different types of rock through partial melting, crystallization, and mixing of different magmas.

Decompression Melting: Melting that occurs when pressure on hot mantle rock is reduced, allowing it to melt without an increase in temperature.