Shatter cones are distinctive geological features that serve as compelling evidence of hypervelocity impact events, such as those caused by meteorites. They are cone-shaped, striated rock structures formed under extremely high pressures, typically exceeding 2 GPa (gigapascals), in the moments following an impact.
2 GPa is approximately 20,000 times the pressure at sea level. 2 GPa is about 40 to 200 times higher than the pressures in heavy-duty industrial hydraulic systems. 2 GPa is almost 20 times the pressure found at the bottom of the Mariana Trench. 2 GPa corresponds to the pressure at a depth of 43.49598 miles in the Earth’s crust, where conditions approach those of the upper mantle. 2 GPa is roughly equivalent to the pressure required to deform or crush extremely strong materials like diamond or high-strength metals in controlled experiments. 2 GPa would be utterly fatal and far beyond anything survivable by any living organism.
These features are considered diagnostic of impact structures, as their formation mechanisms are unique to the shock waves generated during such catastrophic events. Shatter cones are generally conical in shape, with their apex pointing toward the source of the impact.
Their surfaces are characterized by striations1 that radiate outward from the apex, giving them a “horsetail” or fan-like appearance. These striations are a direct result of the shock waves fracturing the rock in a distinct pattern. Shatter cones range in size from microscopic to several meters in length,
depending on the scale of the impact and the properties of the affected rock. The materials in which shatter cones form are typically fine-grained, homogeneous rocks such as limestone, dolomite, or quartzite, but they can also occur in heterogeneous rocks like granites.
The degree of cone preservation can vary, influenced by post-impact processes like erosion or metamorphism. In some cases, the apexes may be blunted or worn away, but the striated surfaces remain identifiable.
Shatter cones have been discovered in numerous confirmed impact craters worldwide. Some notable locations include:
- Sudbury Basin, Ontario, Canada: This large and ancient impact structure contains well-preserved shatter cones in a variety of rock types.
- Vredefort Crater, South Africa: As one of the oldest and largest known impact structures, the Vredefort Crater features extensive shatter cones, particularly in its central uplift region.
- Ries Crater, Germany: Shatter cones here are well-documented and serve as classic examples used for scientific study.
- Chicxulub Crater, Mexico: Associated with the mass extinction event 66 million years ago, this site has yielded shatter cones in deeply buried rocks.
- Wells Creek Crater in Tennessee, USA: An impact crater located near Cumberland City, Tennessee. It is 7.5 miles in diameter and the age is estimated to be 200±100 Ma (million years old), placing it in the Jurassic or a neighboring period.
- Slate Islands in Lake Superior, Canada: The island group, consisting of 15 islands in total, was created by a meteorite impact which formed a crater about 20 miles wide.
- Serpent Mound in Ohio, USA: An eroded meteorite impact crateran effigy mound2, located on a plateau in Brush Creek Valley within the crater. European-American explorers had already noted the unusual terrain in the 19th century, and it is often speculated that the odd geological features inspired Native Americans to construct the mound at that location.
The formation of shatter cones is attributed to the propagation of shock waves through rocks during an impact event. As these waves travel through the target rock, they interact with inherent flaws or heterogeneities in the material. This interaction generates complex stress fields that fracture the rock in a conical pattern.
The exact physics behind the striation patterns is still an area of ongoing research, but they are thought to represent zones of localized shear stress caused by the passage of the shock wave. Shatter cones are invaluable to geologists for confirming the extraterrestrial origin of impact structures. Unlike other indicators, such as high-pressure minerals or melt rocks, shatter cones are macroscopic and often easier to recognize in the field.
They provide insights into the dynamics of the impact process, including the orientation and magnitude of shock waves. Additionally, studying shatter cones can reveal information about the post-impact history of the site, such as erosion and tectonic activity.
Footnotes
- Striations are linear grooves or ridges that form on a surface due to abrasive or shear forces. They appear as parallel lines or marks and can be found in various geological, biological, and mechanical contexts. In geology, striations often result from glacial activity, where rocks embedded in ice scratch against bedrock, or from shock processes like meteorite impacts, creating patterns on shatter cones. In biology, muscle fibers exhibit striations under a microscope due to the arrangement of actin and myosin filaments. In engineering and material sciences, striations may occur during machining or from fatigue cracking. These features provide clues about the forces and conditions that shaped the surface. ↩︎
- An effigy mound is a raised pile of earth built in the shape of a stylized animal, symbol, religious figure, human, or other figure. The Effigy Moundbuilder culture is primarily associated with the years 550–1200 CE during the Late Woodland Period, although radiocarbon dating has placed the origin of certain mounds as far back as 320 BCE. ↩︎
Further Reading
Sources
- Wikipedia “Serpent Mound crater” https://en.wikipedia.org/wiki/Serpent_Mound_crater
- Wikipedia “Slate Islands (Ontario)” https://en.wikipedia.org/wiki/Slate_Islands_(Ontario)
- Wikipedia “Wells Creek crater” https://en.wikipedia.org/wiki/Wells_Creek_crater
- Wikipedia “List of impact structures in North America” https://en.wikipedia.org/wiki/List_of_impact_structures_in_North_America
- Wikipedia “Shatter cone” https://en.wikipedia.org/wiki/Shatter_cone
- Hillbilly U “Shatter Cones – Shatter Cones are made by shock waves. Shock waves are in dynamic change while they travel. A shatter cone is just one of many types of resonate body effects from a meteor impact.” https://www.hillbillyu.com/shatter-cones-impact-crater
- Science Direct “Shatter Cones” https://www.sciencedirect.com/topics/earth-and-planetary-sciences/shatter-cones
- Wiley Online Library “The current state of knowledge about shatter cones: Introduction to the special issue” https://onlinelibrary.wiley.com/doi/full/10.1111/maps.12678
