How Do We Discover Impacts?

How Do We Discover Impacts?

It’s difficult to identify impact sites, particularly old ones, due to the natural erosion and ever-changing features of our planet’s surface. Nevertheless, there are some ways we can determine the original size of impact, size of the culprit meteor or asteroid, and the age of the site to help us learn more of Earth’s history and realize the frequency and significance of impacts of our past.

Out of the below tactics of identifying or discovering impact sites, the presence of just one or even most of these traits still may not be enough to identify a site as an impact site. Extensive research is necessary before any firm conclusions can be drawn on a potential impact site!

Macroscopic Evidence (can be seen via satellite)

Morphometry: Simply put, morphometry refers to traits regarding craters that cannot be seen with the naked eye but rather require aerial photography, satellite imagery, etc. Many craters cannot be determined craters from ground level due to erosion or other geological changes over the years, so aerial or space views are necessary.

Shoemaker Impact Structure, Western Australia

 Megascopic Evidence (can be seen with the naked eye)

Shatter ConesShatter cones consist of rock that has suffered shock pressures around 2-20 GPa (for perspective, our atmospheric pressure is 0.000101325 GPa.) These cones leave vivid and obvious shock lines in the rock which can be seen in cross-sections.

Shatter cones in the Santa Fe impact structure near Santa Fe, NM

Microscopic Evidence (requires a microscope)

Planar Deformation Features: These are similar to shatter cones except microscopic as they’re impact lines found in granules, particularly of quartz. They can also appear as cracks or fractures in the granules.

Thin section photomicropgraph of shocked quartz grain with two sets of decorated planar deformation features (PDFs) surrounded by cryptocrystalline melt matrix in impact melt rock from the Suvasvesi South impact structure, Finland

High Pressure Mineral Polymorphs: These are the presence of materials created due to the high pressure associated with impact. Most often found are high-pressure mineral polymorphs created from quartz, namely coesite and stishovite.

Impact Melt Rocks: Total rock melts are most easily caused by shock pressures in excess of 60 GPa. These rocks are melted, then essentially solidified, some even translucent or glass-like, depending on the extent of the impact.

Pseudotachylyte: This is a fine-grained quartz-bearing fault rock which appears glass-like, particularly similar to balsatic glass, in its veins. Pseudotachylyte also tends to have clusters of crystal and a radial appearance within the rock. These are surprisingly uncommon, though they also appear at land slide sites and fault lines.

Pseudotachylite from the Rochechouart impact structure


Resources

  1. http://www.passc.net/EarthImpactDatabase/Criteria.html
  2. http://www.impact-structures.com/impact-rocks-impactites/impact-melt-page-impact-melt-rocks-impact-glasses-and-congeners/
  3. http://www.ajsonline.org/content/262/8/1008.abstract
  4. http://www.impact-structures.com/impact-rocks-impactites/the-shatter-cone-page/
  5. https://journals.uair.arizona.edu/index.php/maps/article/view/14616
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