In 1980 the Alvarez team proposed that an asteroid impact ended the Cretaceous. The idea was widely disliked. What eventually made it unarguable was not the iridium anomaly that started the discussion but a mineralogical detail: quartz grains at the boundary layer carried a deformation texture that nothing on Earth can produce except an impact.
What shock does to a crystal
A meteorite impact generates a shock wave with pressures in the tens of gigapascals arriving in microseconds. That is a completely different regime from the slow, high-pressure conditions of regional metamorphism, where rock has millions of years to adjust by recrystallising.
Under shock there is no time to adjust. Instead the quartz lattice fails along specific crystallographic planes, producing planar deformation features: sets of parallel, microscopically thin lamellae of amorphous glass running right through the grain, usually in multiple intersecting orientations.
Under a petrographic microscope they are unmistakable. Volcanic quartz does not have them. Tectonically deformed quartz does not have them. No industrial process makes them. Pressure that high, delivered that fast, happens in exactly one natural setting.
The high-pressure polymorphs
Push harder and silica changes structure entirely. Coesite forms above roughly 2 gigapascals and stishovite above roughly 8, both denser rearrangements of the same silicon dioxide, both polymorphs of ordinary quartz.
Coesite was synthesised in a laboratory in 1953 and found in nature at Meteor Crater in 1960, which is what confirmed Meteor Crater as an impact structure rather than a volcanic feature, an argument that had run for decades. Stishovite turned up at the same crater shortly after. Their presence in surface rock is close to proof of impact, because nothing else at the surface reaches those pressures.
Why it mattered at the K-Pg boundary
The thin clay layer marking the Cretaceous-Paleogene boundary is found worldwide. It contains anomalous iridium, which is rare in crust and common in asteroids, and it contains shocked quartz with multiple sets of planar deformation features.
Volcanism, the main competing explanation, cannot produce shocked quartz. The mineralogy did what the geochemistry alone could not: it eliminated the alternative. When the Chicxulub crater was identified off the Yucatán in 1991, at the right age and the right size, the case closed.
Impact glass, the other product
Enough energy melts rock outright. The melt cools too fast to crystallise and freezes as glass, structurally disordered in the same way as obsidian, but with a completely different origin.
Libyan desert glass is the most striking example anywhere: nearly pure silica, pale yellow-green, scattered across the Great Sand Sea on the Egypt-Libya border, formed roughly 29 million years ago. Its purity and its formation mechanism are still debated, with an airburst and a ground impact both on the table. We carry it as a three-piece lot and as a larger single piece, and the fuller story is in the stone that fell from the sky.
Glass fractures along smooth curves rather than flat planes, which is the behaviour described in conchoidal fracture and the reason desert glass was worked into tools in the Neolithic. A carved scarab in Tutankhamun’s pectoral was cut from it, which means the material sat in a pharaoh’s tomb for three thousand years before anyone knew where it came from.
Not the same as a meteorite
Impact glass is melted Earth. A meteorite is the visitor itself. Both come from the same event and they are different objects, a distinction gone through properly in telling a space rock from a hot rock.
There is something pleasing about the fact that the most consequential geological argument of the twentieth century was settled by looking at scratches inside sand grains.
