What other evidence supported the impact hypothesis?

We’ve noted that “several other lines of evidence” also support the hypothesis of an impact at the time of the dinosaur extinction, so you may wonder what these are. Overall, there are four basic lines of evidence found in the sedimentary layer marking the time at which the dinosaurs went extinct:

1. We’ve already noted that the layer has an iridium concentration more characteristic of an asteroid than of Earth. This has also proved to be true for several other elements, including osmium, gold, and platinum.
2. The layer contains small grains of “shocked” quartz, which means quartz crystals with a structure telling us that they formed under very high-pressure conditions. Such conditions are known to occur in impacts. For example, the same type of shocked quartz is found at Meteor Crater in Arizona (see Figure 5.2.3–18).
3. The layer also contains small “rock droplets” (sometimes called tektites) that are known to form when drops of molten rock cool and solidify in the air. This is expected from an impact, since the impact would have splashed molten rock into the air. Similar rock droplets are found at other impact sites, including Meteor Crater.
4. The layer contains a lot of “soot,” which is presumed to have come from vast forest fires ignited by impact debris. The fires would have burned worldwide because debris from the impact would have been blasted high enough to reach space and spread around the world. Then, when the debris plunged downward, atmospheric friction would have heated it until it became a rain of hot, glowing rock – hot enough to ignite fires wherever it landed.

Together, these four lines of evidence made a strong case for an impact even before the crater was found in 1991, and the crater’s discovery then sealed the case.