What happens during "Contact Metamorphism" that often leads to the formation of new materials?
ARocks are broken down by tectonic shearing only.
BRocks come into contact with hot intruding magma/lava.
CRocks are pushed to the upper layers of the atmosphere.
DRocks undergo change due to freezing temperatures.
Answer:
B. Rocks come into contact with hot intruding magma/lava.
Read Explanation:
Contact Metamorphism Explained
Definition and Process
- Contact metamorphism, also known as thermal metamorphism, occurs when pre-existing rocks (country rocks) are altered by the heat from nearby intrusive igneous bodies (magma) or extrusive igneous bodies (lava flows).
- The high temperatures associated with magma or lava bake the surrounding rocks, causing changes in their mineralogy and texture.
- This process is primarily driven by heat, with pressure playing a less significant role compared to regional metamorphism.
Formation of New Materials
- The intense heat can cause the minerals within the original rock to recrystallize, grow larger, or react to form entirely new minerals that are stable at higher temperatures.
- Examples of new minerals that can form include garnet, cordierite, andalusite, and various types of micas.
- The original rock's composition influences the specific new minerals formed. For instance, a shale subjected to contact metamorphism might transform into hornfels, a fine-grained rock rich in new minerals.
Key Characteristics and Zones
- Contact metamorphism typically creates a metamorphic aureole or contact halo around the igneous intrusion.
- The intensity of metamorphism generally decreases with distance from the heat source. This results in distinct zones of altered rock, with the most altered rocks closest to the magma.
- Rocks formed through contact metamorphism are often characterized by their fine grain size and lack of foliation (layered structure), though some exceptions exist.
Formation of Valuable Deposits
- Contact metamorphism is often associated with the formation of important economic mineral deposits.
- Hydrothermal fluids released from the cooling magma can transport dissolved minerals and deposit them in fractures and cavities in the surrounding rocks, forming veins of gold, silver, copper, lead, and zinc.
- The process can also lead to the formation of skarns, which are rocks rich in calcium-silicate minerals and often contain valuable metallic ores.
Distinction from Other Metamorphism
- Unlike regional metamorphism, which affects large areas and involves significant pressure changes, contact metamorphism is localized and primarily heat-driven.