The Cretaceous corresponds to the last period of the MesozoicMesozoic (or secondary era). It extends from -145.5 to -65.5 million years ago. The end of the Cretaceous is marked by the biological crisis which saw the disappearance of the dinosaurs.
Usually, the Cretaceous period is subdivided into two series, the Lower Cretaceous (-145.5 to -99.6 million years) and the Upper Cretaceous (-99.6 to -65.5 million years). These two series are themselves divided into several stages: Berriasian, Valanginian, Hauterivian, Barremian, Aptian and Albian for the Lower Cretaceous; then, Cenomanian, Turonian, Coniacian, Santonian, Campanian and Maastrichtian for the Upper Cretaceous.
The term “Cretaceous” refers to the significant deposits of chalk characteristic of this period, which can be observed in particular in France and Great Britain.
The Cretaceous was marked by the end of the fragmentation of the supercontinent Pangea and in particular by the break-up of the GondwanaGondwana, a step that will give rise to the current continents. The configuration of the continental blocks is however still very different from that of today.
Gondwana is thus fragmented into different continents:AntarcticAntarctic, South America, Africa, Australia and India. The South Atlantic Ocean begins to open 120 million years ago, followed by the opening of the Indian Ocean. Conversely, the ocean TethysTethys continues to close. Tectonic and magmatic activity is therefore intense during this period. Several large oceanic plateaus thus date from the Cretaceous, such as the Caribbean plateau or that of Otong-Java.
In a continental context, the end of the Cretaceous is marked by the eruption of Deccan TrapsDeccan Traps, west of present-day India. This major volcanic episode produced gigantic quantities of lavaslavas which came to form an immense plateau more than 2 km thick. Volcanic activity would have extended over a durationduration tens of thousands of years, releasing large amounts of greenhouse gasgreenhouse gas (particularly CO2) in L’atmosphereatmosphere earth, leading to a change in the global climate. The activity of the Deccan traps is suspected to have contributed to the biological crisis which marks the end of the Cretaceous.
Magmatic activity at news level ocean ridgesocean ridgeswhich produce the oceanic crustoceanic crust young oceans in the process of opening, then ensures, for its part, an important contribution in terms of calciumcalcium in sea water, an element that will favor the production ofalgaealgae planktonic with a calcareous skeleton. This explains the abundance of limestone formations of the Cretaceous age.
The Cretaceous begins with a rather cold climate, which warms up quickly under the effect of rising CO levels.2 in the atmosphere, in connection with the intense volcanic activity experienced by the planet during the whole period. The continental blocks not being in the same configuration as today, the oceanic and atmospheric currents are also very different. The temperature difference between theequatorequator and the poles is thus less important than at present. The waters of the oceans are less stirred and therefore less oxygenated. The period then has several episodes of events anoxicanoxicwhich affect ocean life.
The global climate is therefore rather warm and the conditions favorable to the development of terrestrial flora. The Cretaceous sees the diversification of flowering plants and the appearance of leafy trees. This diversification is associated with the appearance of bees in particular, which will help in the dissemination of pollenpollen. This is also an important period for the diversification of insects: butterflies, grasshoppersgrasshoppers, antsants, termites appear. The wildlifewildlife land is largely dominated by dinosaursdinosaurs. Mammals, however, are also well established. In the oceans coexist PiscesPiscessharks, ammonites and reptilesreptiles sailors like the Ichthyosaurs.
Biological crisis of the Cretaceous-Tertiary
This important biodiversitybiodiversity will however begin to decline during the Maastrichtian, certainly under the impact of the volcanismvolcanism Deccan traps. The biological crisis reached its climax at the end of the Cretaceous, after the meteorite impact of Chicxulub which drastically modified the environmental conditions. Due to the large amount of suspended particles in the atmosphere, the amount ofenergyenergy solar reaching the ground is greatly reduced and organisms dependent on the photosynthesisphotosynthesis decline rapidly, causing thecollapsecollapse of the whole food chainfood chain. Terrestrial and marine fauna are impacted and some speciesspecies end up disappearing definitively, like the ammonitesammonites and dinosaurs.
After this mass extinctionmass extinctionbiodiversity will take a long time to regain the richness of the Cretaceous.
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Definition | Cretaceous | Futura Planet
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