Contact Metamorphism

Definition

Contact metamorphism is a type of metamorphism that occurs when rocks are heated and altered due to their proximity to hot magma or lava. This process typically leads to localized changes in the mineral composition and texture of the surrounding rocks, resulting in the formation of metamorphic rocks that reflect the conditions near the intrusive igneous structures that caused the metamorphism. The relationship between contact metamorphism and these structures highlights how heat transfer from magma can dramatically influence nearby rock formations.

5 Must Know Facts For Your Next Test

1. Contact metamorphism usually produces non-foliated metamorphic rocks, such as marble and quartzite, because it is primarily driven by heat rather than pressure.
2. The degree of metamorphism in contact zones can vary widely, often resulting in a gradient from high-grade metamorphic rocks closest to the intrusion to little or no change further away.
3. Metamorphic facies related to contact metamorphism are defined by specific mineral assemblages that form under certain temperature and pressure conditions, reflecting the thermal effects of the nearby magma.
4. The size and temperature of the igneous intrusion significantly influence the extent and characteristics of contact metamorphism, with larger intrusions generating more extensive aureoles of altered rock.
5. Common examples of contact metamorphic environments include areas around granite plutons, where surrounding sedimentary or volcanic rocks experience heat from the intruding magma.

Review Questions

• How does contact metamorphism differ from regional metamorphism in terms of processes and outcomes?
  • Contact metamorphism differs from regional metamorphism mainly in its driving factors; while contact metamorphism is primarily driven by high temperatures from nearby magma or lava, regional metamorphism involves both high temperature and significant pressure over large areas. The outcomes also vary: contact metamorphism tends to produce non-foliated rocks due to a lack of directed pressure, while regional metamorphism often results in foliated rocks with layered textures. These differences highlight how varying conditions affect rock transformation processes.
• Discuss the significance of a metamorphic aureole and how it relates to contact metamorphism.
  • A metamorphic aureole is crucial for understanding contact metamorphism because it represents the zone of altered rock surrounding an igneous intrusion. Within this aureole, temperature gradients create varying degrees of metamorphic change, leading to distinct mineral assemblages. The characteristics of these aureoles provide insight into the thermal history and intensity of the magmatic activity, demonstrating how close proximity to hot magma can significantly alter existing rock types.
• Evaluate how contact metamorphism can impact the classification of nearby sedimentary rocks when an igneous intrusion occurs.
  • When an igneous intrusion leads to contact metamorphism, it can drastically alter the classification of nearby sedimentary rocks due to changes in mineral composition and texture. For example, limestone may transform into marble through recrystallization under heat, changing its classification from a sedimentary rock to a metamorphic one. This process not only affects individual rock types but also provides valuable information about geological history and thermal events in an area, highlighting the dynamic interactions between igneous activity and sedimentary formations.

Related terms

Thermal Metamorphism: A synonym for contact metamorphism that emphasizes the role of heat in altering rocks, primarily due to nearby molten material.

Metamorphic Aureole: The zone of altered rock surrounding an igneous intrusion, characterized by different degrees of metamorphic change depending on the distance from the heat source.

Foliation: A texture in metamorphic rocks where minerals are aligned or layered due to pressure, which is typically more associated with regional metamorphism rather than contact metamorphism.