The Cretaceous Period

Cretaceous Climate

Note the opening of the Atlantic Ocean. By the end of the Mesozoic the arrangement of the continents is looking more like the present.


Oxygen Isotope Evidence for Climate


The End of the Cretaceous and the

Meek Inherit the Earth

--A major extinction of land and marine organisms occurred abruptly at the rock boundary between the Cretaceous Period and the Cenozoic Era.

--Many forms of life that had played major ecologic roles for tens of millions of years disappeared.

--The most prominent to us is the disappearance of the dinosaurs However, flowering plants which now dominate the terrestrial environment suffered extremely in the extinction event.

--In the ocean all the large reptilian swimmers (mosasaurs, plesiosaurs and giant turtles) all disappeared.

--The base of the food chain (the small marine plants in the ocean) declined to such an extent that most of the larger organisms also collapsed---a chain reaction.

In the early 1980s scientists from the University of California at Berkeley discovered a layer of sediment at the boundary between the Cretaceous and the Cenozoic that contained 30% more Iridium than occurred in rocks previously or after the boundary.

The scientists argued that since iridium is so rare on Earth the enriched layer at the boundary must have come from space. They advanced the hypothesis that the extinction of dinosaurs and other organisms occurred abruptly when a large meteorite hit the Earth and spewed vast amounts of dust and water vapor into the atmosphere. The size of this meteorite was estimated to be 10km in diameter (about 6 miles).

The dust from such an explosion would have been spread all over the Earth. Indeed, the iridium layer has now been found in places all over the world.

Other scientists argued against the hypothesis, suggesting that the iridium came from volcanic explosions. It is true that iridium is found in higher concentrations within the molten interior of the Earth. We know that there was a massive eruption of lava near the end of the Cretaceous in India.

However, two lines of evidence have convinced scientists that the extinctions and the iridium anomaly were caused by a meteorite impact.


The Cretaceous Period came to a dramatic end with the impact of a large extraterrestrial bolide!



glass spheres formed from molten rock

shocked qtz formed from impact by a meteorite





Some of the expected impacts of the meteorite impact on the Earth's environment

1. Perpetual Night. dust particles would blow high into the atmosphere, spread around the world and block out sunlight. Much of the dust would have remained in the atmosphere for months and kept plants from photosynthesizing.

2. Months of global refrigeration. By darkening Earth, the atmospheric dust would plunge the Earth into a winter for several months.

3. Delayed greenhouse warming. A bolide that landed in the ocean would send into the atmosphere not only dust but also vast amount of water vapor which would remain in the atmosphere long after the dust settled out. Water vapor trapped in the atmosphere would intensify the solar heat absorbed in the atmosphere leading to greenhouse-like warming. This would bring the cold conditions to an end and plunge the Earth into a period of extreme warmth.

4. Acid Rain. The energy release by the impact would cause oxygen and nitrogen to combine in the atmosphere to form oxides nitrogen. When these oxides of nitrogen contact the water vapor in the atmosphere it would make nitric acid. This would rain out as acid rain.

5. Wildfires. Particles of soot have been found at the boundary horizon where the iridium occurs. This may be the remains of great fires that spread across the globe when the hot meteorite hit the Earth.

Cooling seems to be the biggest problem at the boundary. The extinctions seem to have hit hardest in those organisms and groups of organisms that were adapted to warm conditions. Plants in the higher southern latitudes seem to have suffered less than those in the northern higher latitudes.

The location of the impact is uncertain. But, there is a large crater in the Yucatan Peninsula that may be large enough and of the right age to be the location where the impact occurred. This is supported by the occurrence of larger glass spheres (see photo above) near the crater. The larger sphere would have fallen out of the atmosphere sooner, near the source while the smaller ones would have been transported farther from the impact site.

An interesting outcome of the winter scenario is the concept of "nuclear winter". Scientists now contemplate the effect that nuclear explosions would have on the environment. These explosions would, like the Cretaceous impact, cause huge amount of dust to be put into the atmosphere which would act to block sunlight and cool the Earth.


Finally, the meek inherited the Earth.