Shatter cone
Rare conical fractures formed by high-pressure shock.
Wikipedia / Wikimedia Commons
Shatter cones are uncommon rock formations found only in the bedrock under meteorite impact craters or underground nuclear blasts. Their presence shows that the rock experienced a shock pressure between 2 and 30 GPa (290,000–4,350,000 psi).
In shape, they are distinctly conical, with the cone’s apex as the top point. These cones repeat in a cone-on-cone pattern at both large and small scales within the same sample. Sometimes a larger cone has a spoon-shaped side. In fine-grained rocks like limestone, they create an easily spotted "horsetail" pattern of thin grooves, or striae. Coarser rocks tend to produce less clear shatter cones, which can be hard to tell apart from other geological features such as slickensides. Geologists have several ideas about how shatter cones form—either from compression as the shock wave passes through the rock, or from tension when the rock rebounds after the pressure drops. The result is a network of branching fractures, large and small, throughout the rock.
Shatter cones range in size from microscopic to several meters across. The world’s largest known example, over 10 meters long, is found at the Slate Islands in Terrace Bay, Ontario, Canada. The axes of the cones typically radiate outward from the impact point, with the cones pointing upward and toward the crater’s center. However, some rocks’ orientations have been altered by later geological processes at the site.
- field
- Geology
- known_for
- Evidence of high-pressure shock events
- formation_cause
- Meteorite impacts or underground nuclear explosions
- pressure_range
- 2–30 GPa (290,000–4,350,000 psi)
- largest_known_example
- More than 10 meters in length at Slate Islands, Terrace Bay, Ontario, Canada
Lore & Background
Shatter cones have a distinctively conical shape that radiates from the apex, repeating cone-on-cone in large and small scales in the same sample. Sometimes they have a spoon shape on the side of a larger cone. In finer-grained rocks such as limestone, they form an easily recognizable 'horsetail' pattern with thin grooves (striae). Coarser grained rocks tend to yield less well developed shatter cones, which may be difficult to distinguish from other geological formations such as slickensides.
Geologists have various theories of what causes shatter cones to form, including compression by the wave as it passes through the rock or tension as the rocks rebound after the pressure subsides. The result is large and small branching fractures throughout the rocks. Shatter cones can range in size from microscopic to several meters. The largest known shatter cone in the world (more than 10 metres in length) is located at the Slate Islands in Terrace Bay, Ontario, Canada.
The azimuths of the cones' axes typically radiate outwards from the point of impact, with the cones pointing upwards and toward the center of the impact crater, although the orientations of some of the rocks have been changed by post-cratering geological processes at the site.
Reader's Guide
Shatter cones are significant as diagnostic indicators of high-pressure shock events, specifically meteorite impacts or underground nuclear explosions. Their presence confirms that the host rock experienced pressures between 2 and 30 GPa, a range not typically achieved by ordinary geological processes. This makes them a key tool for identifying ancient impact craters, especially when other shock-metamorphic features are absent or ambiguous. The cones' orientation—radiating outward from the impact point and pointing upward and toward the crater center—can help geologists locate the source of the impact, though post-cratering processes may alter these orientations. Their size range, from microscopic to over ten meters, allows for study at multiple scales. The largest known example, at the Slate Islands in Ontario, Canada, provides a reference for field identification. Despite their utility, the exact formation mechanism remains debated, with competing theories involving compression or tension during the shock wave's passage and subsequent rebound. This uncertainty underscores the need for continued research into shock metamorphism.
Did You Know?
- Shatter cones are only known to form in bedrock beneath meteorite impact craters or underground nuclear explosions.
- They can range in size from microscopic to several meters, with the largest known example exceeding 10 meters in length at the Slate Islands, Ontario, Canada.
- In finer-grained rocks like limestone, shatter cones display a distinctive 'horsetail' pattern with thin grooves (striae).
- The azimuths of shatter cone axes typically radiate outward from the point of impact, pointing upward and toward the crater center.
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