Class 9 : The K-T impact
(the Alvarez theory)
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Geological anomalies at K-T boundary.
The K-T crater.
Timeline for devastation.
I : Geological anomalies at the
K-T boundary
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Detailed investigation finds the following in the
K-T transition layers…
Iridium
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Shocked quartz
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In the K-T boundary everywhere in world!
Higher levels of iridium in North America.
Large impact or volcanism (e.g. Deccan Plateau).
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!
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Feldspar spherules
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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
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Alvarez theory given huge boost by
discovery of the Chicxulub crater:
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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
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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
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Initial impact of a 10km asteroid or comet
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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.
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Next 10-1000 years…
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But then there’s dramatic global warming…
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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…
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…