Meteorite Impact Craters on Earth, or

When Good Asteroids Go Bad

Chair: Nancy K. Cox, RN

Having a great interest in meteorites and asteroids, when I began looking into this topic, I thought it would mainly be just a laundry-list of the known impact structures on Earth. But the subject became more and more interesting. As any NCHALADA topic, there is a lot of history involved. Just as with meteorites (regarding which it was thought “impossible that stones could fall from the sky”), it was thought just as unlikely that any meteorites large enough to leave impacts could have hit the Earth. And, even before that possibility was considered, first there had to be identified probable impact sites.

The earliest and best known candidate (being “fresh” – only ~50,000 years old) is the famous Meteor Crater in Arizona (also known as Barringer Crater, and Canyon Diablo). People began to take an interest in it in the 1890s. Daniel M. Barringer, in the early 1900s (by 1905 he had visited it 5 times) believed it was caused by a meteorite impact (although he incorrectly thought the main mass would be found buried under the crater pit). It was hotly debated by G. K. Gilbert and others that the cause was volcanic or a steam explosion (for this and a few other sites in the US). It took years to gather evidence and decide what factors would constitute evidence for meteorite impact structures.. This would apply also to other sites around the world. In the 1920s, the renowned meteorite hunter Harvey Harlow Nininger also joined the impact debate (and Meteor Crater) – Norm can tell us some tales about Nininger.

One of the next identified impact structures was the New Quebec (formerly Chubb) crater in Canada, a perfectly round crater lake photographed from the air in 1943.

From 2 impact sites in 1950, by the late 1950s there were 12 identified impact sites. In 2005 there are 162.

In the 1950s and ’60s, when debate raged over the origin of the craters of the Moon (volcanic or meteorite impact), this dovetailed nicely into the investigation into the planet Earth impact debate, especially when the side came down for meteoritic craters on the Moon. Investigators such as the renowned, late, great Gene Shoemaker studied Meteor Crater in Arizona, and made comparison with nuclear bomb crater test sites in the Southwest, to further the conclusion that there are indeed impact structures on the Earth, although due to erosion by wind and water, Earth’s craters get obliterated over time, unlike the Moon’s.

Eventually, criteria were established as evidence necessary to determine if a geologic formation is an impact site [ed: “crateria”?]. These all have to do with evidence of shock, and include the finding of:

1. Shatter cones

2. Impact breccias

3. PDFs (more later) (Every subject has its acronyms, and meteorite impacts are no exception.)

Our discussion will also include other minerals that are found, such as coesite (a previously unknown high-pressure phase of silica) as well as tektites (earlier thought to be pieces of the Moon, now shown to be impact-melted bits of the Earth). Also discussed will be the fact that these impacts are explosion phenomena (when a fast-moving object of a certain mass hits another object, an explosion occurs, accounting for the fact that no one large meteorite piece is found under a crater, but scattered fragments around it). This took years to figure out, too.

We will also review and look at some of the now-famous impact sites around the world, including:

USA: Meteor Crater Arizona; Manson, Iowa; Beaverhead, Montana; Odessa, Texas

Canada: New Quebec (Chubb Crater); Manicouagan, Quebec.

South America: Campo del Cielo, Argentina

Eurasia: Ries Germany; Sikhote Alin, Siberia

Africa: Vredefort, South Africa

Australia: Henbury; Gosse’s Bluff; Wolfe Creek

I’ll bring a number of show-and-tell materials:

Posters: detailing the impact structures on Earth, their relationship to meteorites and asteroids, lists of impact sites by location, and a timeline for formation, from recent to millions of years old.

Actual mineral samples of impact materials: shatter cone, coesite, Ries breccia, K/T boundary material, and meteorites.

References

Meteorite Craters by Kathleen Mark. University of Arizona Press, 1987, 1995. A good survey, with numerous photos, of the history of the study of impact sites.

Meteorite Craters and Impact Structures of the Earth by Paul Hodge. Cambridge University Press, 1994. Very good, well organized (by continent) of the 150 then-known impact sites. Many photos and diagrams.

Australia’s Meteorite Craters by Alex Bevan and Ken McNamara. Western Australian Museum, 1993. Nice photo guide and maps to Australia’s impact sites.

Tektites by Ken McNamara and Alex Bevan. Western Australian Museum, 3rd edition, 2001. Nice summary and photos of the history of the study of tektites.

Comets, Asteroids and Meteorites. Time Life, 1990. Luscious illustrations.

Impact Cratering – A Geologic Process by J. Melosh. Oxford University Press, 1998. A standard work on the subject.

A Comet Strikes the Earth by Harvey Harlow Nininger. American Meteorite Laboratory, Denver, 1942, 1996. Covers Nininger’s studies of Barringer Crater.

Sky & Telescope had a good article a few years ago about the Camp del Cielo craters.

-- Added for the web: a couple of resources to start searching --

The Open Directory Project. A list of web resources, more complete than this one, with links to related subjects. Also, an outdated copy of the same directory (but running on much faster servers) at The Google Directory.

Earth Impact Database contains an overview of impact craters, a database sorted by name, size, or location, and images (photographic and other) of various craters.