Lecture 7: Did An Asteroid Kill off the Dinosaurs?

See Chapter 5, Section 5 in the textbook

Impact Theory (movie)

What is it?

• asteroid, giant meteor, or comet struck the Earth at end of Cretaceous
• drastic, devastating global climate change
The K-T boundary refers to the dividing line between the Cretaceous and Tertiary geological periods -- the end of the age of dinosaurs. Paleontologists knew there was a mass extinction at this time, but not why. A thin clay layer marks this transition all over the world (unusual -- most rock layers are found only in small patches).


Luis and Walter Alvarez, discoverers of impact theory, at K-T boundary layer

Why an impact?

• elements found at K-T boundary that are rare on Earth, common in meteors (found worldwide)

  • Clay at the K-T boundary shows an enhancement of iridium, an element rare in Earth rock

    

  • The "iridium anomaly" is very narrow in geologic terms -- it must have been generated by a sudden event

    

  • The iridium is present all over the world, but is thicker in some places, providing a clue where the impact may hae occurred
  • In many places the KT boundary is the ONLY layer with elevated iridium levels

• Droplets of previously molten rock and shocked quartz

  • "Tektites" of once-molten rock seen at impact sites for smaller craters; seen worldwide at KT boundary
  • Indicate rock was melted, thrown into atmosphere, then re-cooled
• 2-inch deep deposit of tektites found at KT boundary in New Jersey -- very far from impact site!

• the "smoking gun" - a 200-km crater in Mexico (read a very interesting story about its discovery here.), or a good general Chicxulub site here

  • Crater discovered as gravitational anomaly by geologists looking for oil

    

  • Finally got widespread acceptance for the impact theory
• Allows estimation of impactor size -- about 10 km diameter

Impact candidates

• comets

• ``dirty snowballs'' -- mixtures of dust and ices that evaporate near Sun, producing visible tails of thousands of miles

 



• come from Oort Cloud, which surrounds Sun at distance of about 40,000 times Earth-to-Sun distance
• Halley recognized that comets periodic
• Shoemaker-Levy recently crashed into Jupiter (movie) -- others might have crashed into Earth!

Comet Impact Sites on Jupiter (black smudges)



asteroids

• made of stone or iron
• most lie between orbits of Mars and Jupiter
• some asteroids are even double (binary asteroids).

  

  Click here to see a movie of a double asteroid (90 Antiope) that Dr. Close helped find at the 10m KECK telescope. Click here to see images of more binary asteroids that we have found (they are quite rare only 4 known with orbits).

• many are in orbits that cross that of Earth --> collision possible!

 



• Meteor Crater near Flagstaff is one of the most famous (and the first-recognized) impact crater on Earth, from a meteorite impact about 50,000 years ago. (See more images of terrestrial impact craters here.

• in March 1969, previously undetected asteroid passed 690,000 km from Earth (less than twice distance to Moon)
• most Earth-crossing asteroids cannot have been in present orbits for long, else would have collided or have been ejected
• some unknown process must channel them into our neighborhood
• Scientists are actively looking for "Earth-crossing asteroids" and "Near-Earth Objects", and determining the chances that any of them will impact the Earth. NASA has a good site about this work.
   

   
   

To learn more about meteorites in Arizona, click here.

What are the results of a giant impact?

Terrible Destruction

• Kinetic Energy = (1/2) MASS x VELOCITY²
• comets and asteroids move at high speeds (15-70 km/sec, or 34,000-157,000 mph)
• impact produces tremendous shock wave

 

 



• impact that produces 50-km crater releases 10²⁸ ergs, or
• 10⁴ - 10⁵ times energy of Mount St. Helens eruption
• 10⁴ times energy of 1906 San Francisco earthquake and first thermonuclear (hydrogen) bomb
• about same energy as Earth's entire geological activity for one year
• 10-km asteroid envisioned by Alvarezes would have released 10³¹ ergs, or
• 100 million megatons of TNT
• more than 7 billion times energy of Hiroshima bomb (one bomb for every person on Earth and 10 for every square km of Earth's surface)
• more than explosion of all 60,000 nuclear weapons
• the total energy the Earth recieves from the sun in about two weeks

Short-term environmental effects (courtesy of D. Kring, University of Arizona)

• impact characteristics (simulation)
• impact in shallow sea (100-300 ft deep) near Yucatan peninsula, in Gulf of Mexico

 



• burrowed about 15 km into ground
• vaporized water, local crust, dented local mantle, melted crust elsewhere
• low energy ejecta blanketed surrounding terrain (``immediate vicinity'' = all of North America)
• high energy ejecta
• rose up from middle of impact cone into atmosphere, expanding in all directions in stratosphere
• material thrown out to half the distance to Moon, 10-15% escapes Earth forever
• remainder blankets entire globe
• after materials settle out of atmosphere, BIG impact crater
• about 180 km across (Tucson - Phoenix distance)
• km-high rims
• initially no water (vaporized), but then Gulf water rushes in --> km-high waterfalls!
• 4 inches of debris should have fallen on Tucson (2000 km from impact), but destroyed by erosion?
• local, regional effects occur immediately or over a few days
• tsunamis crash into Gulf coast (100-300 m high), waves cross Atlantic (25-50 m high in western Africa and Europe), other waves bounce back and forth in Gulf basin for days

 



• shockwave (1000 km/s winds)
• fireball
• magnitude 10 earthquake
• wildfires
• volcanic activity

 

Long-term environmental effects

• global effects from high energy ejecta last longer and cause mass extinction
• raining, burning dust heats atmosphere at impact site and antipode (near Madagascar), perhaps heating local surface to 500 degrees F and igniting wildfires over large part of globe
• on timescale of months, ejecta chokes atmosphere (``dust-loading'')
• blocks sunlight and photosynthesis --> damages/kills food-chain
• impossible to see anything on Earth
• Krakatoa-like consequences on much larger scale

 
• on timescale of years, ozone loss, acid rain, cooling, heavy metal poisoning takes place
• ``nuclear winter''
• limestone CaCO₃ --> CaO + CO₂ (greenhouse gas!) and anhydrite CaSO₄ --> CaO + SO₃ (sulfuric acid rain!)
• CO₂ enriched by factor of < 0.2 (more than 100,000 times contribution of fossil fuel burning today)



Greenhouse Effect: O'Keefe and Aherns (1989) estimated that impact of 12 km diameter comet or 14 km asteroid would have raised Earth's average temperature by over 10 F. This massive heating effect would have lasted for hundreds to thousands of years, probably making interior of continents virtually inhospitable. Eventually CO2 would be absorbed by oceans, plants, and weathering process. (courtesy Prof. Paul Eric Olsen, Columbia University)

• S by factor of 5.5-7x10⁵
• H₂0 by factor of 130-140
• nitrogen chemistry changes as atmosphere heated (N --> NO, nitric oxides that rain out as nitric acid!)
• chloride introduced by asteroid, vaporized, and blown into stratosphere destroys ozone layer --> intense UV radiation

 

 
 
 

• on timescale of decades, greenhouse warming occurs

To read some more about the dinos, check out the Hand-Outs and Reference Materials page.

To read some more about mass extinctions, the asteroid impact theory, and alternative theories, click here, here or here.

To read more about current issues and controversies in the field of dino death, click here.

If you want to see how nasty the fighting over this topic can get, click here.

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