1. Impact Craters on Earth Searching for the End of the World

2. Craters – a new study From the status 50 years ago of "minor curiosity", impact cratering has now been elevated to be one of the three most important geologic processes. Impact craters are common on all four inner Solar System planets - Mercury, Venus, Earth, and Mars. About 200 craters are known on Earth (compared with millions on the Moon that survive because tectonic and fluvial erosion processes have not destroyed them).

3. Study Questions 1)what happens to make a crater? 2) how are impact craters found and recognized? 3) WHY STUDY CRATERS?

4. Craters are COOL! For years, impact cratering was esoteric. Now, impacts are hot topics, especially since they have been identified as the likely immediate cause of dinosaur extinction. Where in the 1960s the number of scientists working on impact craters were probably less than 20, now there are hundreds. Scientists and the general public now realize that impacts by extra-terrestrial bodies (asteroids, comets, meteorites) are genuine dangers to mankind, capable of causing catastrophes greater than any other natural process known to affect the Earth's surface. Plus, they are almost certainly to occur (as they already have) as huge events sometime in the future of civilization. This subject of impacts is pertinent and interesting,

5. Impact seems responsible for events in the Earth's past that made significant changes in the evolutionary history of animals and plants.

6. The catastrophic damage done by impacts has become a popular topic for TV and Hollywood movies.

7. Most compelling is the knowledge that at least one major extraterrestrial object has caused widespread damage barely over a century ago! The message is: impacts still happen and may do so again…

8. Huh? What event? The Tunguska event of 1908 that took place over east-central Siberia. This was manifested by an explosion (equivalent to about 15 megatons of TNT) in the lower atmosphere of an incoming comet or meteorite. This occurred in a then uninhabited part of Siberia. Here is its location:

10. If the ferocity of the Tunguska event didn't get your attention, the graph below should. It shows the energy release caused by asteroidal impacts, expressed in terms of megatons of Hydrogen bomb equivalents, and the likelihood in terms of average number of years between cratering events of different magnitudes (frequency of recurrence):

11. Distribution of Impact Craters on Earth

12. Any particular pattern? Why or why not?

13. Meteor Crater, AZ, USA

14. Craters in Texas? YES!

15. So, why is the study of impact craters important? Because over the last 40 years scientists have come to realize that this mode of crater formation is one of the three or four most fundamental natural processes affecting the Earth's (and planets' in general) surface. The Moon, Mercury, Venus, and Mars, and the satellites of the Giant Planets, are all thoroughly peppered with impact craters. Earth has around 200 known impact craters but once had many more; active degradational geologic processes (stream erosion; tectonic deformation; burial by sediments, glaciation) that have erased most of the earlier ones. In the last 100 years, several other meteorite impacts leading to small craters have occurred on the land surface around the globe; even more frequently incoming bodies have fallen into the oceans, potentially causing tsunamis.

16. From studies of terrestrial impact craters in terms of size and age, from planetary surface studies, and from monitoring asteroid and comet distributions, a rather accurate estimate of cratering frequency (size versus occurrence in time) has led to plots like this:

18. No other natural event is as powerful, devastating, or potentially catastrophic as a major impact. Consider one capable of producing a 50 km (31 mi) wide crater, excavated to a depth of 5 km (3 mi): the energy expended is thousands of times greater than the simultaneous detonation at one point of all the nuclear explosive devices (euphemism for bombs) manufactured to date.

21. The Nature of Craters

22. So how does this happen? Ground zero…

23. 0.15 seconds after impact

24. 0.6 seconds after impact

25. 6.9 seconds after impact

26. 11.0 seconds after impact

27. 25.0 seconds after impact

28. 26.0 seconds after impact

29. 35 seconds after impact

30. 1.0 minute after impact

31. 30 minutes after impact

32. Cross-section of a buried Crater

33. OK – now let’s play God… http://impact.ese.ic.ac.uk/ImpactEffects/

34. So did Impacts REALLY cause mass extinctions? http://rst.gsfc.nasa.gov/Sect18/Sect18_4.ht ml

35. Crater Show and Tell Tunguska - http://www.youtube.com/watch?v=EiXpp-i442s&feature=player_embedded http://www.youtube.com/watch?v=EiXpp-i442s&feature=player_embedded Chicxulub http://www.youtube.com/watch?v=5qJPTjMnwNk&feature=related http://www.youtube.com/watch?v=0EyDd7rZWnE Wilkes Land http://researchnews.osu.edu/archive/erthboom.htm http://www.youtube.com/watch?v=mx-cD2NQ-64 http://www.youtube.com/watch?v=u_5MU8ZQN-E http://www.youtube.com/watch?v=tLXpIev71OM&feature=related http://www.youtube.com/watch?v=tl4ZUkL-3U8&feature=related http://www.youtube.com/watch?v=TfB8kU8mqGI&feature=related Meteor craters from around the world http://www.youtube.com/watch?v=7axLsX7zxmo

36. More Websites http://www.lpi.usra.edu/publications/slideset s/craters/ http://www.thelivingmoon.com/43ancients/02 files/Earth_Images_09.html http://webecoist.com/2010/01/12/heavy- hitters-earths-most-amazing-meteor- craters/