Contact Metamorphism

Definition

Contact metamorphism occurs when rocks are altered by high temperatures due to nearby molten magma or lava. This process typically results in localized changes in the mineralogy and texture of the affected rocks, creating new metamorphic rocks in the surrounding area. The impact of contact metamorphism is crucial for understanding the broader rock cycle and how metamorphic facies evolve under varying temperature and pressure conditions.

5 Must Know Facts For Your Next Test

1. Contact metamorphism generally occurs at high temperatures but low pressures, making it different from regional metamorphism which involves higher pressures.
2. The zone of contact metamorphism can create distinct bands of altered rock surrounding an igneous intrusion, known as an aureole.
3. Common minerals formed during contact metamorphism include garnet, hornfels, and various feldspar types, depending on the protolith.
4. The extent of contact metamorphism is influenced by the size and temperature of the intrusion, as well as the nature of the surrounding rocks.
5. Contact metamorphism can help geologists identify past volcanic activity and understand the thermal history of a region.

Review Questions

• How does contact metamorphism differ from regional metamorphism in terms of pressure and temperature conditions?
  • Contact metamorphism is characterized by high temperatures but relatively low pressures, occurring near igneous intrusions where heat from magma alters nearby rocks. In contrast, regional metamorphism involves both high temperatures and high pressures over larger areas, usually associated with tectonic forces and mountain-building processes. This distinction is important because it affects the types of minerals that form and the overall texture of the resulting metamorphic rocks.
• Discuss how contact metamorphism can lead to the formation of distinct mineral assemblages and textures in affected rocks.
  • Contact metamorphism alters the mineral composition and texture of protoliths due to exposure to intense heat from nearby molten magma. As temperatures rise, specific minerals can become unstable and transform into new minerals that are stable under these conditions. For example, shale may change into hornfels, exhibiting a fine-grained texture with new mineral assemblages such as garnet or pyroxene. The variations in mineralogy are influenced by factors like the original rock type and the degree of thermal exposure.
• Evaluate the significance of studying contact metamorphism in understanding geological processes and the rock cycle.
  • Studying contact metamorphism is essential for understanding geological processes because it illustrates how heat from igneous activity can affect surrounding rocks. This phenomenon provides insights into the dynamics of the rock cycle, showcasing how different types of rocks can be transformed through processes like heat and pressure. Moreover, identifying contact metamorphic rocks helps geologists reconstruct past tectonic environments and volcanic activity, linking surface features with deep Earth processes. Ultimately, these studies enhance our comprehension of Earth's history and its ongoing geological evolution.

Related terms

Protolith: The original, unmetamorphosed rock from which a metamorphic rock forms through the processes of heat and pressure.

Metamorphic Rock: A type of rock that has been changed from its original form due to heat, pressure, or chemically active fluids, often resulting from processes like contact metamorphism.

Thermal Gradient: The rate of temperature change in a given direction, particularly relevant in contact metamorphism as it determines how far the effects of heat from a magma body will reach.