GEOL 102 Historical Geology

Spring Semester 2011
The Mesozoic Era VI: The K/Pg Extinction

Probably the most intensively studied of all mass extinctions in the rock record

Victims include:

Of course, many survivors as well.

Although many untestable hypotheses suggested (hunting by aliens, supernova radiation, etc.), three contributing factors have strong independent physical evidence:

The Maastrichtian Regression:
Draining of epeiric seas would alter terrestrial climate by:

Increased Maastrichtian volcanism, especially the Deccan Traps:

The Chicxulub Impact
1980: Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. Wanted to determine length of time represented by the clay layer. Consulted dad (Nobel winning physicist Luis Alvarez) for possible solution. Suggestion:

The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).

When examined Gubbio clay, found a huge increase in iridium ( iridium spike) at base of clay: clearly not an "average" of infall.

Hypothesized: an asteroid impacted Earth at the K/Pg boundary

Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter, the likely consequence to a large-scale nuclear war first proposed shortly after (and suggested by) the Alvarez scenario


Biotic prediction fits most of the predictions; search for geological signature was on.

Shocked Quartz:

Melt Glass (Tektites):

Tsunami ("tidal wave") and ejecta deposits:


So, great evidence for an impact at K/Pg independent of extinction. Also, pattern consistent with proposed effects (although some versions of the superacid rain, global fires, and global super tsunamis do not have good evidence and are probably "overkill" scenarios).

Suggestions that all these systems were in effect:

Extinction of non-avian dinosaurs paves the way for the rise of mammals as the dominant group of terrestrial animals. Marine realm recovery represents survival of many groups, but less change in structure.

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Last modified: 14 January 2011