Lecture Notes: How Rocks Melt to Produce Magma

Introduction

• Explanation of rock melting in various plate tectonic settings.
• Companion video available on partial melting.
• Earth's interior is extremely hot, with temperatures increasing with depth, known as the geothermal gradient.

Key Concepts

Temperature and Pressure

• Geothermal gradient: rate at which Earth's temperature increases with depth.
• Earth's core temperature: estimated greater than 6000°C.
• Pressure also increases with depth, preventing rocks from melting except under specific conditions.

Melting Conditions

• Focus on the upper few hundred kilometers of Earth where most melting occurs.
• Graph Explanation:
  • Represents melting conditions for peridotite (typical mantle composition).
  • Solidus Line: Temperature to start melting peridotite at different depths.
  • Liquidus Line: Temperature needed to completely melt peridotite.
  • Partial Melting: Conditions between solidus and liquidus lines.
  • Geothermal Gradient Curve: Typical temperature of rocks at given depths.
  • Mantle rocks require higher temperatures to melt with depth due to pressure.

Melting Processes at Plate Boundaries

Divergent Plate Boundaries

• Occur at oceanic ridges where plates move apart.
• Decompression Melting:
  • As lithosphere thins, asthenosphere rises.
  • Rising hot rocks lower in pressure, prompting partial melting.
  • Example: Decompression melting below hotspots like Hawaii.

Convergent Plate Boundaries

• Occur at subduction zones with descending plates.
• Flux Melting:
  • Water-rich minerals in descending plate release water into mantle above.
  • Water addition causes mantle rocks to melt, forming magma.
  • Example: Volcanic arcs and island arcs.

Heat Transfer Melting

• Occurs when rising mafic magma heats the base of the continental crust.
• Causes partial melting of the crust to form felsic magma.
• Associated with continental rifts.

Summary of Magma Production Methods

1. Decompression Melting:
   • Dominant at oceanic ridges and hot spots.
   • Also contributes to magma production in continental rifts.
2. Flux Melting:
   • Main source for magma in island arcs and volcanic arcs (convergent boundaries).
3. Heat Transfer Melting:
   • Associated with continental rifts.

Learning Objective

• Understand the processes and conditions under which magma is produced by rock melting in the mantle and crust.