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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Scientists date suitable minerals in impact melt or nearby igneous rocks, then use fossils, sediment layers and impact debris to place that age in context. They generally cannot radiometrically date a fossil-bearing sedimentary bed itself. At Chicxulub, radiometric ages of impact material and its connection to ejecta at the Cretaceous/Paleogene (K/Pg) boundary support an impact around 66 million years ago.
What a radiometric date actually dates
Radioactive parent isotopes decay into daughter products at known rates. By measuring their proportions—often with a mass spectrometer—geochronologists estimate when a mineral formed or when its isotope system was reset. The result is an age for that mineral’s recorded event, not automatically for every rock or fossil nearby. The U.S. Geological Survey explains the available approaches and why geologists choose methods to suit the material and question in its guide to dating rocks and fossils.
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At an impact crater, molten target rock can cool and crystallize as impact melt. Suitable minerals in it may record that cooling, which is closely tied to the impact. But context matters: a mineral grain inherited from older rock can predate the impact, while later heating or alteration can reset or complicate its isotopic record. The USGS account of Chicxulub describes argon dating of melt rocks and evidence from core samples.
How scientists establish the age of fossil-bearing layers
Common sedimentary rocks such as sandstone, limestone and shale are not generally dated directly by standard radiometric methods. Instead, geologists establish the order of layers from their position and contents. Fossil succession helps identify which beds are relatively older or younger and correlate layers at different locations. Dated igneous rocks or other time markers can then bracket those fossil-bearing strata and help place them on a numerical timescale. The USGS explains how radiometric ages and fossil-based relative ages work together in its geologic-time overview.
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So a fossil can be useful for dating in the broad geological sense, but its presence does not itself provide a direct radiometric age for the sediment around it. The fossil helps identify and correlate the layer; other evidence supplies numerical ages.
How Chicxulub’s impact is linked to the K/Pg boundary
The case for linking Chicxulub to the boundary is built from multiple, mutually reinforcing clues rather than simply noting that a crater and an extinction are both ancient. Core samples contain shocked breccia clasts, and impact melt has chemistry and radiometric ages resembling material in boundary ejecta. The distribution of ejecta thickness and shocked-grain sizes with distance from the crater also helps connect the crater to deposits elsewhere.
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The older studies and records sometimes call this the Cretaceous/Tertiary (K/T) boundary; K/Pg is the current name. A USGS record of a 1992 study describes a structure about 200 km in diameter and reports core evidence, iridium-bearing melt and argon dating. David A. Kring’s 2016 Lunar and Planetary Institute review gives an approximate diameter of 180 km and describes the impact as contemporaneous with the mass extinction around 66 million years ago. These are publication-specific approximate diameter figures, not a single reconciled exact measurement. The 66-million-year figure is likewise an approximate context, not a high-precision age with a stated uncertainty here.
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What different dating methods can—and cannot—tell us
| Method | What it can establish | Important limit |
|---|---|---|
| Radiometric dating | A numerical age for a suitable mineral’s formation or isotopic resetting. | The mineral’s history and geological setting matter; an inherited grain may be older than the impact. |
| Fossil succession and stratigraphy | The relative order of sedimentary layers and correlations between them. | They do not independently give a direct radiometric age for a fossil or its surrounding sediment. |
| Paleomagnetism | Magnetic directions recorded in rocks can supply relative age information or, in a well-calibrated context, help constrain age. | The USGS recommends pairing paleomagnetic evidence with radiometric evidence. |
| Carbon dating | The age of suitable organic carbon material over comparatively recent timescales. | It does not directly date rock and is not suitable for a 66-million-year-old impact. |
Carbon dating is therefore not the tool for ancient crater rocks. Geologists instead select isotope systems and minerals suited to the event and material being studied. As the USGS puts it, “There are many ways to date rocks and geologic events. Not every technique will be applicable to every situation, so geologists have to be strategic with the geochronologic methods they apply.”
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Why several independent clues matter
A crater’s history can include older target rocks, impact melt, inherited minerals, post-impact sediments and fossils in surrounding layers. Those materials need not share one age. Geologists build a timeline by determining what each dated mineral records, establishing the relative order of sedimentary layers, and checking whether impact deposits and regional boundary evidence align. At Chicxulub, the agreement between radiometric ages, shocked material, melt chemistry and ejecta patterns supports the impact’s placement around the K/Pg boundary.
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