Shock Metamorphism
Introduction
Shock metamorphism is a process that occurs when high-speed impacts from meteorites or cosmic bodies cause significant changes in the physical and chemical properties of rocks. This process is characterized by the formation of high-pressure mineral phases and microscopic shock lamellae, which are thin layers of material that have been significantly deformed due to the impact.
Formation and Characteristics
Shock metamorphism occurs when a meteorite or other cosmic body impacts the Earth's surface at high velocities, typically exceeding 10 km/s. The energy from the impact is transferred to the rocks at the impact site, causing a rapid increase in pressure and temperature. This sudden increase in pressure and temperature causes the rocks to deform and undergo significant changes in their physical and chemical properties.
The most common characteristics of shock metamorphism include the formation of high-pressure mineral phases and the development of microscopic shock lamellae. High-pressure mineral phases, such as stishovite and coesite, are formed as a result of the extreme pressures generated during the impact. These mineral phases are typically not found in rocks that have not been subjected to shock metamorphism.
Shock lamellae are thin layers of material that have been significantly deformed due to the impact. These lamellae are typically parallel to each other and can be observed under a microscope in thin sections of the rock. The presence of shock lamellae is one of the most definitive indicators of shock metamorphism.
Processes Involved
The processes involved in shock metamorphism are complex and involve several stages. The initial impact causes a shock wave to propagate through the rock, causing a rapid increase in pressure and temperature. This shock wave can cause the rock to melt or vaporize, depending on the intensity of the impact.
Following the initial shock wave, there is a release wave, which causes a rapid decrease in pressure and temperature. This release wave can cause the high-pressure mineral phases formed during the shock wave to revert back to their original forms. However, some of these high-pressure mineral phases can be preserved if the release wave is not strong enough to completely reverse the changes caused by the shock wave.
The final stage of shock metamorphism involves the cooling and solidification of the rock. This stage can result in the formation of new mineral phases and the development of shock lamellae.
Evidence of Shock Metamorphism
Evidence of shock metamorphism can be found in several types of rocks, including meteorites, impactites, and rocks from the Earth's crust. The most definitive evidence of shock metamorphism is the presence of high-pressure mineral phases and shock lamellae.
High-pressure mineral phases, such as stishovite and coesite, are typically not found in rocks that have not been subjected to shock metamorphism. These mineral phases can be identified using various analytical techniques, such as X-ray diffraction analysis and electron microprobe analysis.
Shock lamellae can be observed under a microscope in thin sections of the rock. These lamellae are thin layers of material that have been significantly deformed due to the impact. The presence of shock lamellae is one of the most definitive indicators of shock metamorphism.
Impact on Earth's History
Shock metamorphism has played a significant role in Earth's history. The impacts that cause shock metamorphism have been responsible for several major events in Earth's history, including mass extinctions and the formation of large impact craters.
One of the most well-known examples of shock metamorphism is the Chicxulub impact event, which is believed to have caused the extinction of the dinosaurs. The impact caused significant shock metamorphism, resulting in the formation of high-pressure mineral phases and shock lamellae.