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Fusion Crust |
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| Fusion Crust Formation and Atmospheric Entry |
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| Figure 1. Allende (Pueblito de Allende, Chihuahua, Mexico; fell February 8, 1969). CV3 carbonaceous chondrite, complete 660 g stone with fusion crust. Note exposed chondrules (upper left), impact-related broken surface (lower right), and contraction cracks in the glassy crust. Scale bar=8 mm. |
| Fusion Crust The final major event in a meteoroid’s multi-billion-year journey is atmospheric passage, which generates the diagnostic fusion crust—a thin, thermally altered exterior layer that distinguishes freshly fallen meteorites. During hypervelocity entry, compression of air ahead of the meteoroid produces intense frictional heating and ablation. Surface materials melt (and in some cases vaporize), forming a thin glassy to microcrystalline rind typically 1–2 mm thick (variable with entry angle, velocity, size, and composition). Ablation removes substantial mass, sculpting characteristic features such as regmaglypts (thumbprint-like depressions). Once the meteoroid decelerates below ~3–5 km/s, ablation ceases; it then enters “dark flight,” cooling rapidly before reaching the surface. Witnessed falls are frequently reported as “frosting over” due to rapid condensation of atmospheric moisture on their cold interiors. The primary cooling of iron meteorites effectively ceases between approximately 400°C and 350°C, at which point significant long-range diffusion in the Fe-Ni system becomes negligible. Subsequent thermal or mechanical events, including impacts and atmospheric entry, produce secondary structures. The same applies to stony meteorites. Fusion crust is a classic secondary structure resulting from this brief but extreme thermal event. On freshly fallen stones it is typically black and glassy; on irons it is thinner, often magnetic, and may appear metallic or oxidized. Terrestrial weathering gradually alters the crust from black to brownish tones over centuries to millennia, eventually causing flaking or loss through contraction cracking, hydration, and biological activity. Selected Examples Figure 1. Allende (Pueblito de Allende, Chihuahua, Mexico; fell February 8, 1969). CV3 carbonaceous chondrite, complete 660 g stone with fusion crust. Note exposed chondrules (upper left), impact-related broken surface (lower right), and contraction cracks in the glassy crust. Scale bar=8 mm. Figure 2. Dimmitt (Castro County, Texas; found 1942). H3.7 unequilibrated ordinary chondrite regolith breccia, complete 1.78 kg stone. Weathered fusion crust oxidized to brown. Scale bar=14 mm. Figure 3. Peekskill (Westchester County, New York; fell October 9, 1992). H6 brecciated ordinary chondrite. Section showing ~2 mm thick fusion crust. Scale bar=2 mm. Figure 4. Bruderheim (Alberta, Canada; fell March 4, 1960). L6 ordinary chondrite, complete 416 g fusion-crusted individual with regmaglypts. Scale bar=12 mm. Figure 5. Bruderheim fusion-crusted fragment. Crust thickness 1–1.2 mm, charred appearance. Scale bar=2 mm. Figure 6. Chiang Khan (Loei, Thailand; fell November 17, 1981). H6 ordinary chondrite, sectioned end piece with 1.5–2 mm fusion crust. Scale bar=3 mm. Figure 7. Wold Cottage (Yorkshire, Wold Newton, UK; fell December 13, 1795). L6 ordinary chondrite. Patches of black fusion crust on a specimen curated for over 200 years. Contraction cracks have allowed moisture and contaminants to penetrate, leading to localized flaking. Scale bar=1.5 mm. Figure 8. Texline (Dallam County, Texas; found 1937). H5 ordinary chondrite with partial, weathered fusion crust remnants. Terrestrial age unknown. Scale bar=10 mm. Figure 9. NWA 869 (Northwest Africa; found ~2000). L3-6 ordinary chondrite. Weathered relic fusion crust with minute contraction cracks on the upper surface; highly weathered broken faces. Terrestrial age likely several thousand years. Scale bar=20 mm. |
| Note: Buchwald's "Handbook of Iron Meteorites" 1975, Chapter, "The Physics of the Fall," expertly discusses this atmospheric passage and is highly recommended reading. Rubin, "Meteorite Mineralogy" (2021), subchapter 12.1, 12.2, presents a comprehensive discussion of atmospheric passage and mineral alteration from terrestrial weathering. |
| The following images highlight the fusion crust on stone and iron meteorites. They are mostly self-explanatory but will have a descriptive sentence or two. |
Figure 2. Scale bar 14 mm. |
| Dimmitt, |
| Castro County, Texas. |
| Found, 1942 |
| Stone, H3.7, chondrite, unequilibrated, regolith breccia |
| Complete specimen, 1.78 kg. The fusion crust of this stone meteorite has weathered to an oxidized brown color. |
Figure 3. Scale bar 2 mm. |
| Peekskill |
| Westchester County, New York |
| Fell, October 09, 1992 |
| Stone, H6 chondrite, brecciated |
| Peekskill, sectioned, showing 2mm thick fusion crust. |
Figure 1. Scale bar 12 mm. |
| Bruderheim |
| Alberta, Canada |
| Fell, March 04, 1960 |
| Stone, L6 chondrite |
| Complete fusion-crusted specimen, 416 grams. Note the regmaglypted depressions. |
| Bruderheim - continued next image. |
Figure 1. Scale bar 2 mm. |
| Bruderheim |
| Fusion-crusted fragment. The crust is 1-1.2mm in thickness presenting as a charred exterior. |
Figure 1. Scale bar 3 mm. |
| Chiang Khan |
| Chiang Khan, Loei, Thailand |
| Fell, November 17, 1981 |
| Stone, H6 chondrite |
| Sectioned end piece, note the 1.5 mm - 2 mm thick fusion crust. |
Figure 1. Scale bar 1.5 mm. |
| Wold Cottage |
| Yorkshire, Wold Newton, UK |
| Fell, December 13, 1795 |
| Stone, L6 chondrite |
| The Wold Cottage meteorite presents with paches of black fusion crust. This meteorite fell in 1795 and was curated to the standards of the day for over 200 years exposed to atmospheric water, miscellanous fungi and bacteria from handling, temperature changes, etc. Over time, contraction cracks in the crust can wick all of this into the interior causing some of the fusion crust to flake or lift off the surface. |
Figure 1. Scale bar 10 mm. |
| Texline |
| Dallam County, Texas |
| Found 1937 |
| Stone, H5 chondrite |
| A weathered chondrite with partial or patches of fusion crust. The terrestrial age is unknown. |
Figure 1. Scale bar 20 mm. |
| NWA 869 |
| Northwest Africa |
| Found 2000 |
| Stone, L3-6 chondrite |
| The terrestrial age of this meteorite is unknown but estimated to be several thousand years or more. This specimen was likely buried in sand and exposed over and over. The top surface has a weathered relic fusion crust with minute contraction cracking still visible. The rest of the specimen is also highly weathered along the broken side surfaces. |
| References • Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. (Especially Chapter on “The Physics of the Fall.”) • Rubin, A. E., & Ma, C. (2021). Meteorite Mineralogy. Cambridge University Press. (Subchapters 12.1 and 12.2 on atmospheric passage and terrestrial weathering.) These examples illustrate the variability of fusion crust morphology, thickness, and preservation state across chondrite groups and terrestrial ages, highlighting its utility as a key indicator of recent atmospheric entry. Detailed petrographic and geochemical study of fusion crusts further reveals information on entry dynamics, ablation rates, and early-stage terrestrial alteration |
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