And Then There Were None: Major Extinction Events in Earth’s History - Geology - page 328

Kreidezeit; the T is for Tertiary, which is the period between the Cretaceous and the Qua-
ternary (65 to 2.8 million years ago). Modern geologists who no longer recognize the
Tertiary period have begun to refer to this transition as the K-Pg boundary (Cretaceous-
Paleogene) instead.
Many events occurred during this time that may have, together, resulted in the extinc-
tion of so many animals. The supercontinent of Pangaea was breaking up, and the Dec-
can Traps of India (see Figure 22-1) were erupting. As I explain earlier in the chapter, tec-
tonic plate movements, as well as massive volcanic activity, can change global atmo-
spheric, climate, and ocean conditions, resulting in animal extinctions.
At the K/T (or K-Pg) boundary, however, there is also clear evidence of an impact event.
In the Gulf of Mexico, just off the north side of the Yucatan Peninsula, scientists have
identified a massive crater. The Chicxulub Crater illustrated in Figure 22-3 is the result of
a rocky body at least 10 kilometers (6 miles) wide hitting the earth around 65 million
years ago.
Figure 22-3: The
location of the Ch-
icxulub crater in
the Gulf of Mexico.
According to the impact theory (which I describe earlier in this chapter), such an im-
pact could have created long-lasting darkening of the atmosphere, interfering with
plants first, and then rippling up the food chain to devastate the largest creatures: dino-
saurs.
The strongest line of evidence for this explanation is the amount of iridium found in lay-
ers of sediment dating to this time. Iridium is an element that is rare in earth's crust but
much more common in meteors. Its worldwide presence in layers of clay and sand that
must've been at the surface of the earth during the K-Pg boundary indicates that some-
how, large amounts of iridium were introduced to earth's atmosphere. Scientists accept
that a large meteor impact is an obvious explanation.