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.
The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct.
Reduction of shallow continental shelves due to the formation of the super-continent Pangea.
Basaltic lava eruptions in Siberia. (See http://my.opera.com/nielsol/blog/show.dml/77787)
Volcanic eruptions in Siberia 251 million years ago may have started a cascade of events leading to high hydrogen sulfide levels in the oceans and atmosphere.
A leading theory for the P-T extinction is, that a warming of the earth and a slowdown in ocean circulation made it harder to replace the oxygen sucked out of the water by marine organisms. According to the theory, microbes would have saturated the water with hydrogen sulfide, a highly toxic chemical. (See http://my.opera.com/nielsol/blog/2006/10/21/permian-triassic-mass-extinction)
Is the impact crater in East Antarctica? (See http://my.opera.com/nielsol/blog/show.dml/282181)
The release of frozen methane hydrate from the ocean beds to cause a greenhouse effect. The release of massive clouds of methane from icy hydrates buried under shallow ocean floors is a suspect.
500 million years ago. 50% of all animal families went extinct.
The two most accepted current hypotheses for the Cambrian extinction are:
Other proposed causes:
65 million years ago at the Cretacious/Tertiary boundary.
This extinction is particularly famous for the extinction of dinosaurs.
Scientists have proposed many kinds of theories to account for it. They range from asteroid or comet impacts, volcanism, sea level changes, supernova explosions, and on and on.
Volcanism (Deccan traps)
In Late Cretaceous time, India was an isolated land mass drifting northward toward its collision with Asia. While India was east of Madagascar, and just south of the equator, it drifted over the head of a mantle plume. Mantle plumes are columns of bouyant molten rock material which rise through earth's mantle. They burn through the lithosphere plates to erupt as "hotspot" volcanos. Mantle plume volcanos release great volumes of basaltic lavas onto earth's surface. This type volcanism is also known as flood basalt volcanism because the relatively liquid lavas flood out over vast geographical areas.
Chicxulub Crater, an ancient impact crater buried underneath the Yucatán Peninsula, with its center located approximately underneath the town of Chicxulub, Yucatán, Mexico.
Chicxulub Crater in the Gulf of Mexico
Image by Natural Resources Canada
There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era
Radiation from a relatively nearby supernova explosion.
a) Flesh eaters ate all the plant eaters and starved.
b) Dinosaurs evolved such clumsy bodies that they could not breed or perhaps even move.
c) Small mammals ate all the eggs.
d) The new flowering plants poisoned the dinosaurs.
e) Mammals out-competed the dinosaurs.
The second most severe of the Earth's five great periods of extinction440 million years ago
Ultraviolet radiation from the sun after gamma rays destroyed the Earth's ozone layer, when a supernova exploded within 10,000 light years of the Earth, destroying the chemistry of the atmosphere and allowing the sun's ultraviolet rays to cook fragile, unprotected life forms.
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