Week 10.1

Question 1

Case Study I – Barringer Crater

Answer 1

• Barringer (aka Meteor Crater) is probably the most typical simple crater on Earth, well-preserved in the Arizona desert at 49,000 yrs old
• The crater is ~1.0km across, making an almost-perfect circle
• Depth: 185m
• Rim: +45m
• Fracture Depth: 265m
• Impactor Size (at impact): 30m, 300kt
• Impactor Speed: 15,000 m/s (or 54,000km/h)
• Surprisingly, it took a while to convince people this is a meteor crater from an extraterrestrial origin
• Uniformitarians didn’t believe in single-catastrophes shaping geology, so for many years this was passed as an explosive hole full of gas
• Daniel M. Barringer was one of the first to call this a meteor crater, and purchased the mining rights search of the buried, interstellar rock – unfortunate for Dan, but most chunks were found outside the crater!
• Analyses of the Canyon Diablo Meteorite show a composition distinct of Iron Meteorites
  7.9% Ni, 0.3% Co, ~91% Fe

Question 2

Case Study II – Chesapeake Bay Crater

Answer 2

• Crater wasn’t discovered until 1993, despite Chesapeake Bay being consistently settled since 1607
• Through the 1900s, many tektites of unknown origin were discovered around the bay
• In the 1980s, interests in oil drilling found impact ejecta and shocked quartz
• In 1993, Texaco and Exxon discovered a huge rim-and-crater structure at the bottom of the bay
• Use of the geological record has shown this impact took place 35 million years ago
• Width: 40,000m (~85,000m after rim collapsed)
• Depth: 500m
• Rim: n/a
• Fracture Depth: 11,000m
• Impactor Size (at impact): ~3,300m, ?
• Impactor Speed: 20,000 m/s (or 72,000km/h)
• The event sent a column of dust and rock up to 50km into the sky
• Believed to be part of a heavy Meteor Shower, containing a few items that made it to the surface
• We can only speculate how many more large meteors fell into the oceans, rather than creating terrestrial craters

Question 3

Case Study III – The Tunguska Event

Answer 3

• In some cases, an impactor passing through the atmosphere can burn up, or even explode, before it hits the ground
• Occurring in 1908, the Tunguska Event is the most recent severe meteor-related catastrophe
• Unfortunately, the Russian Revolution and onset of World War I stalled investigations
• The only surface-expression of the event left was a 1000km2 area of flattened trees pointing away from the centre
• Where scientists expected a crater, they instead found an 8km-across area of upright trees, although heavily scorched
• Depth: 0m
• Fracture Depth: n/a
• Impactor Size: ~50m
• Impactor Speed: 15,000 m/s (or 54,000km/h)

Question 4

How Meteoroids Break

Answer 4

• Many meteors explode in the Earth’s atmosphere every year, much smaller than those at Tunguska, Chelyabinsk, or Tagish Lake
• Gravity often plays a large role in pulling apart meteors before they strike a planet, such as the string of pearls noted behind
• Shoemaker-Levy before it slammed into Jupiter in 1994
• Some meteoroids may break but not explode before hitting the ground; this often leads to twin craters
• The craters in Clearwater, QC, are the result of one large meteoroid breaking in two before hitting the ground; the larger chunk fell to form a complex crater (left), where the smaller formed a simple crater (right)