How Shock Metamorphism Shaped and Altered the Alamo Rocks
Impact Metamorphism Processes
Shock metamorphism, also called impact metamorphism, occurs when high heat and pressures generated during an impact deform the underlying rock layers. Shock metamorphism can lead to planar deformation features, high-pressure polymorphs, and shatter cones, all of which provide evidence supporting a bolide impact.
Planar deformation features, or PDFs, are narrow planes of glassy material arranged in parallel sets. The large shock compressions needed to form PDFs are only associated with bolide impacts and nuclear explosions. They can easily be recognized under microscope, usually forming within quartz or feldspar minerals. PDFs found within quartz minerals are termed shocked quartz. Shocked quartz is a popular line of evidence supporting bolide impacts on Earth, including the Alamo Impact Event.
The pressures produced during an impact can lead to the formation of high-pressure polymorphs of several minerals within the target rock. A polymorph is a mineral that takes on different forms or crystal structures, while still retaining the original chemical composition. High impact pressures can alter the crystal structure of several minerals and generate new forms or high-pressure polymorphs. Examples of high-pressure polymorphs include diamond (converted from graphite minerals), stishovite (converted from quartz minerals under low shock pressures), and coesite (converted from quartz minerals under high shock pressures). In the early 1960s, the identification of stishovite and coesite minerals provided one of the earliest forms of evidence supporting bolide impact origins of numerous structures, most notably the Ries Crater in Germany.
Low impact pressures can also generate distinctive conical fracturing patterns in target rocks known as shatter cones. Shatter cones are striated, curved fractures that typically form partial to complete cone shapes, much like an ice cream cone. They are often found in place below the crater floor and rarely observed within individual rock fragments of breccia units. Shatter cones can occur as composite groups or as individuals, ranging in length from millimeters to meters. They are now generally accepted as unique indicators of shock pressures and bolide impacts.