Class 9 : The K-T impact(the Alvarez theory)

Geological anomalies at K-T boundary. The K-T crater. Timeline for devastation.

I : Geological anomalies at the K-T boundary

Detailed investigation finds the following in the K-T transition layers…

Iridium In the K-T boundary everywhere in world! Higher levels of iridium in North America. Large impact or volcanism (e.g. Deccan Plateau).

Shocked quartz Find very strongly shocked quartz. Such quartz only thought to be found in impact sites. Find more weakly shocked quartz in obvious volcanic

sites. Soot – and lots of it!

Feldspar spherules Found spread

throughout the KT boundary.

Formed as vaporized minerals started to recondense.

No known volcano can spread such spherules across the whole globe.

II : The K-T crater

Coastline

Cenotes

Seismic reflections

Alvarez theory given huge boost by discovery of the Chicxulub crater: Circular crater revealed through pattern of

cenotes (small sinkholes). Crater confirmed by detection of

gravitational anomalies. Also measured by seismic reflections.

Crater is ~150 km diameter. Exactly the kind of crater predicted by the

Alvarezes. Crater dated at 65 Myr.

Zircon

Zircon dating

Zircon is a mineral (containing mainly zirconium) that contains some uranium.

Uranium decays into lead. There are two isotopes of uranium (U-235 and

U-238) that decay radioactively into two isotopes of lead (Pb-207 and Pb-208).

This gives us two independent “clocks” (both based on radioactivity)…

Gives us enough information to determine the age of zircon crystal despite possibility of partial lead loss due to heating.

See page 117 of textbook.

III : Timeline for devastation

Initial impact of a 10km asteroid or comet Atmosphere has almost no effect on incoming

object… strikes surface at full speed. Explosive yield of approximately 100 million

megaton TNT equivalent. Impacting asteroid and approx 100 km3 of Earth rock

are vaporized and thrown 100 km into the atmosphere.

Intense fireball and blast wave flattens and ignites everything for 1000s of km across.

Molten debris rains down across planet, seeding massive wildfires – 90% of all plant life burns.

Huge earthquakes cause massive Tsunamis… devastate coastlines around the globe.

Next 10-1000 years… Atmospheric debris and smoke completely blocks the

Sun from reaching the surface. Temperature plummets (like “nuclear winter”). Photosynthesis stops and most plants die. Food chain collapses. Eventually, debris settles out of atmosphere and the

Sun starts to shine again… But then there’s dramatic global warming…

Carbon dioxide from fires causes a greenhouse effect… temperature rises and planet becomes too hot!

This lasts for an unknown length of time… Few species survive this ordeal…