Earth Sciences 240A Lecture 7

The Tunguska Event

Siberia, 30 June 1908

Enormous shock waves: Air compression, Seismic, Sound

Exploration of site: Delayed until 1927; Radial pattern of broken trees; 1000 km2

Object?

Small comet or 30m carbonaceous chondrite; No fragments found

Chesapeake Bay Crater

Tektites throughout east US soils

Evidence

1983: drilling 120 km E of Atlantic City

35mya horizon: tektites, shocked quartz

1986: drilling SE Virginia

35mya horizon: incredible rubble

1993: Texaco/Exxon seismic survey

Defined huge impact structure under Bay

90 km crater; 1.3 km depth; Est. 4 km diameter asteroid

Misc. Impacts

James River, Alberta (top of Cambrian)

Multiple impacts

Shoemaker-Levy 9 and Jupiter

Clearwater, Quebec

Chad, Africa

Manicouagan (Que), Rochechouart (Fr), St. Martin (Man), Red Wing (USA) – all 214 mya (all one?)

Tagish Lake

18 Jan. 2000; BC near Yukon

Object:

200 tonnes; 15.8 km/s; density 1.5 g cm-3

Spectacular fireball entry

Recovery by UWO,CU, (NASA)

Trajectory

Carbonaceous chondrite (new category:CI-2)

Petrography

Chemistry

Conclusion: “Most primitive ever recovered”

Implications

Highest content of carbon

Life components from asteroids/comets?

Primitive ocean composition

High salt content (ref: Knauth, 1998)

High contents of Mg, Ca, Na plus Cl, SO4 released on leaching

Next

Lecture 8: PART C: Mass Extinctions and the Impact Connection