My Blogs (olelog) are mainly based on my daily reading of earth science news.
Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.
Bosumtwi Lake in Ghana
The two best known impact craters in the world are probably
The three largest impact craters on earth are:
but there are many others, and here I shall mention but a few.
This crater is located in Ghana and is one of the few (less than 20) known impact structures in Africa. It is the youngest known (age of 1.07 million years) LARGE impact crater known on earth. It is about 10.5 kn in diameter and the structure is almost completely filled by lake Bosumtwi (see picture above).
This crater is a 35 million year old shallow marine, complex impact structure with a diameter of ca. 85 km. It was discovered in the early 1990's and it is probably the biggest impact structure in the US. The Chesapeake bay Impact Event is believed to be the source of North Americam microtectites.
The Ries crater is with a diameter of 25 km the largest and best reserved meteorite crater on earth. It was formed about 15 million years ago by the impact of an asteroid presumably measuring 1 km in diameter. I wrote about a visit to this crater at http://www.ougseurope.org/agm/2005/ries.asp.
In Norway only 3 round structures have so far been certainly identifed as impact craters. The numbers for Sweden and Finland are greater (6 in Sweden and 10 in Finland).In Norway they ask themself why. Have they really had less impacts? - or didn't they search well enough?
The University of Oslo has asked all young Norwegian school students to help in the search. This project is described in Norwegian! at http://www.geo.uio.no/groper/ and it takes and extremely long time to load.
Impact Crater found in Sahara
Olelog of Sunday, 5 March 2006
The crater is about 31 kilometres wide, more than twice as big as the next-largest Saharan crater known.
It may have been formed by a meteorite impact tens of millions of years ago with a diameter of roughly 1.2 km.
The Kebira Crater lies in the Western Desert on the northern tip of the Gilf Kebir region in southwestern Egypt, at the border with Libya. "Kebira" means "large" in Arabic.
Kebira's shape is reminiscent of the many double-ringed craters on the Moon
Lonar crater is about 170m deep and has an average diameter of 1830m. The elevated
rim consists of 25m of bedrock and 5m of ejecta over it. This ejecta blanket
is spread over about 1350m away from the crater rim and slopes away by 2-6 degrees.
Its age is estimated to be 52,000 ± 6,000 years. (Pleistocene)
The Lonar cratrer is the largest crater in basaltic rock (the Deccan traps). It is partially filled by a salt water lake.
Location 19.975° N 76.51° E.
The Lonar Crater is created by the hypervelocity impact of either a comet or meteorite. The impact origin of this crater is clearly demonstrated by the presence of plagioclase that has been either converted into maskelynite or contains planar deformation features (PDFs). Only shock metamorphism caused by a hypervelocity impact could either have transformed plagioclase into maskelynite or created PDFs in it. The impact origin of Lonar Crater is further substantiated by the presence of shatter cones; impact deformation of basalt layers comprising its rim; shocked breccia inside the crater; and a nonvolcanic ejecta blanket surrounding the crater.
The Manicouagan crater was formed by the impact of a 5 km diameter asteroid which excavated a crater originally about 100 km wide although erosion and deposition of sediments have since reduced the visible diameter to about 72 km. It is the fifth largest impact crater known on earth. Geologists used to think the Manicouagan impact caused the Triassic-Jurassic extinction 200 million years ago. Recent U-Pb zircon dating of the impact melt proved that the crater has an age of 214 ± 1 million years. As this is 12 ± 2 million years before the end of the Triassic, the crater cannot be the cause of the Triassic-Jurassic extinction event.
Recently, the completion of the Manicouagan Dam filled the annular moat to its
present depth creating a circular reservoir for hydroelectric power. This circular
lake accentuates the contour of the eroded impact-brecciated ring area of the
Manicouagan Impact Crater Monday, 11 February 2008
Kebara Crater, Sahara - see above
Chesapeake Bay, Virginia, USA
Tuesday, 17 January 2006, 18:18:07
Sunday, 4 December 2005, 09:41:39
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