Shatter Cone

Shatter Cone is a distinctive macroscopic, conical-to-curviplanar fracture structure decorated with radiating, divergently branching ridges and grooves ("horsetail striations") that develops in target bedrock subjected to high-strain-rate shock-wave compression during a hypervelocity meteorite impact. Ranging in axial length from less than 2 millimeters in fine-grained micritic limestones to more than 10 meters in thick-bedded quartzites and granitoids (such as at the Slate Islands and Sudbury structures in Ontario, Canada), shatter cones are the only macroscopic, field-scale shock-metamorphic feature widely accepted by planetary geologists as an unambiguous diagnostic indicator of an astrobleme or meteorite impact crater.

First described in 1905 by Wilhelm Branco and Eberhard Fraas as enigmatic "Strahlenkalke" (rayed limestones) in the Steinheim Basin of southern Germany, shatter cones were proposed as impact indicators by Robert S. Dietz in 1947 and definitively confirmed in the late 1950s and 1960s through comparisons with underground high-explosive and nuclear detonation craters, hypervelocity light-gas-gun impact experiments, and co-occurring microscopic planar deformation features (PDFs) in quartz.

Apical Angles, Horsetail Striations, and Parasitic Cones

True impact shatter cones display a suite of geometric and morphological features that distinguish them from tectonic slickensides, sedimental cone-in-cone diagenetic concretions, wind-abraded ventifacts, and industrial blasting fractures:

1. Divergent Horsetail Striations: The conical fracture surface is covered by sharp, convex-upward ridges and narrow V-shaped grooves that radiate away from a point-like or curvilinear apex in a dendritic, "horsetail" bundle. Unlike fault slickenlines—which consist of uniform parallel linear scratches—shatter-cone striations splay outward down the cone flanks at angles correlated with the apical angle.
2. Hierarchical Parasitic Stacking: Smaller secondary and tertiary ("parasitic") cones nucleate shingled one upon another along the flanks of a primary master cone, all sharing subparallel axes and pointing their apices in the same general direction.
3. Characteristic Apical Angles (~75°–115°): Complete cones (often exposed around central uplifts where rock beds were overturned or rotated) exhibit apical angles averaging 85° to 95° (spanning roughly 70° to 120° depending on lithology and elastic anisotropy).
4. Lithological Grain-Size Control: Fine-grained, isotropic, mechanically homogeneous rocks—specifically micritic limestone, dolomite, chert, and dense orthoquartzite—preserve crisp, millimeter-scale striations, whereas coarse-grained crystalline granite, gabbro, and gneiss form cruder, larger, step-fractured cones up to several meters tall.

Pressure Window (2–30 GPa) and Shock Attenuation Zone

Within the concentric shock-zonation envelope of an expanding impact hemispherical wave, shatter cones form in the low-to-moderate shock-metamorphic regime between 2 GPa and 30 GPa (most commonly 2–10 GPa in sedimentary targets and up to 25–30 GPa in crystalline basement rocks). Near the impact point where peak shock pressures exceed 35–60 GPa, whole-rock shock melting, diaplectic glass formation (maskelynite), and intense brecciation obliterate brittle fracture cones. Conversely, beyond the crater rim where the compressive wave decays below the Hugoniot Elastic Limit (HEL, ~1.5–2 GPa) into an ordinary seismic P-wave, stress amplitudes are insufficient to nucleate conical branching fractures.

Consequently, in simple impact craters (diameters < 2–4 km on Earth), shatter cones occur primarily in autochthonous floor rocks beneath the breccia lens and as allochthonous clasts within fallout breccia. In complex impact craters (diameters > 4 km), rebound of the shock-compressed crater floor brings the 2–25 GPa target horizon directly to the surface inside the central uplift or collar ring (as seen at the Steinheim Basin, Ries, Vredefort, Charlevoix, Wells Creek, and Kentland structures).

Wave-Interference and Dynamic Fracture Front Models

Several complementary physical models explain how a passing shock wave carves striated cones:

1. Johnson–Talbot Wave-Scattering Model (1964): When a compressive shock front propagating at velocity U_s encounters a small mechanical heterogeneity (such as a pebble, fossil fragment, mineral grain, or pore cavity), the heterogeneity scatters a spherical elastic precursor or rarefaction wave moving at longitudinal sound speed c_L. Interference between the planar shock wave and the scattered spherical wave along the Mach-like locus cos(θ/2) = c_L/U_s concentrates tensile hoop stresses along a cone whose apex sits right at the heterogeneity and points toward the oncoming shock source.
2. Sagy–Reches–Fineberg Dynamic Fracture-Front Instability Model (2002, 2004): High-speed laboratory fracture experiments show that when a tensile crack front is driven near the Rayleigh wave speed (v_R) by the tensile tail (rarefaction wave) immediately behind a compressive shock pulse, encountering a small inhomogeneity generates solitary fracture-front waves. The nonlinear track of these counter-propagating front waves leaves V-shaped, branching horsetail ridges on a curved fracture surface with an opening angle 2α determined by the ratio of crack velocity v to front-wave speed c_f.
3. Baratoux–Melosh Heterogeneity Expansion Model (2003): Numerical hydrocode simulations demonstrate that differential compressibility between a small inclusion and its host rock during decompression after shock passage generates a conical hoop-tension zone radiating outward from the inclusion, reconciling why some nested cones show opposing (base-to-base) orientations at a single outcrop.

At eroded terrestrial astroblemes where the original crater rim and ejecta blanket have been stripped away by erosion, structural geologists measure the three-dimensional orientations of dozens of striation rays around each shatter cone at multiple outcrops, fit a mathematical cone to each dataset by least squares to determine the cone axis vector, and then rotate the host sedimentary beds back to their pre-impact horizontal orientation (correcting for central-uplift overturning). Plotting the restored cone apices on a map consistently converges inward and upward toward a single aerial focal point—pinpointing the exact ground zero and depth of burst of the ancient impacting projectile, often alongside cross-cutting veins of impact pseudotachylite.

How can a geologist tell a shatter cone apart from a fault slickenside in the field?

Fault slickensides form by planar frictional sliding of one rock block past another, producing parallel linear grooves, step risers, and oriented mineral fibers on a flat or gently curved fault plane. Shatter cones are three-dimensional conical surfaces covered with divergent, branching ("horsetail") striations that splay outward from a pointed apex, with smaller parasitic cones stacked like shingles along the flanks of larger cones.

Which way does the apex of a shatter cone point relative to the meteorite impact?

Before tectonic tilting or overturning of the rock layers during central-uplift rebound, the pointed apex of a primary shatter cone generally points upward and inward toward the point of impact (the source of the oncoming shock wave), while the open base flares away in the direction of shock-wave propagation.

Can volcanic explosions or tectonic earthquakes produce shatter cones?

No. Even the largest volcanic caldera eruptions and magnitude-9 tectonic earthquakes generate overpressures below ~0.1–0.5 GPa and strain rates several orders of magnitude below the Hugoniot Elastic Limit (~2 GPa) required to initiate dynamic shock-front branching. Aside from artificial underground nuclear or high-explosive detonations, shatter cones in nature are formed only by hypervelocity meteorite impacts.

1. Dietz, R. S. (1960). "Meteorite Impact Suggested by Shatter Cones in Rock." Science, 131(3416), 1781–1784. https://doi.org/10.1126/science.131.3416.1781
2. French, B. M. (1998). Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Lunar and Planetary Institute, LPI Contribution No. 954. https://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html
3. Sagy, A., Reches, Z., & Fineberg, J. (2002). "Dynamic Fracture by Large Extraterrestrial Impacts as the Origin of Shatter Cones." Nature, 418(6895), 310–313. https://doi.org/10.1038/nature00903
4. Baratoux, D., & Melosh, H. J. (2003). "Shatter Cones: AnUniquely Impact-Related Phenomenon? New Insights from Numerical Modeling." Earth and Planetary Science Letters, 216(1–2), 43–54. https://doi.org/10.1016/S0012-821X(03)00474-600474-6)
5. Osinski, G. R., & Ferrière, L. (2016). "Shatter Cones: (Mis)understood?" Science Advances, 2(8), e1600616. https://doi.org/10.1126/sciadv.1600616