The trapps ( staircase in Swedish) are very vast continental plates consisted of thick deposits of basaltic rocks . It is supposed that their installation was done by épanchement on the surface of a magmatic increase mantellic and basic, giving place to a Volcan tholeiitic ism of the type, through cracks or of specific mouths.

The most famous trapps are:

• the Higher Lake (the United States), 1100 to 1200 million years
• the Trapps of Siberia (Russia), 248 to 216 million years
• the Parana (Brazil), 140 to 110 million years
• the Trapps of Deccan (India), 65 to 42 million years
• the Trapps of Ethiopia, 40 to 15 million years
• the submarine plateau of the islands Kerguelen (TAAF, France) with the island Heard (Australia ) in the South of the Indian Ocean, since 35 million years (always in activity).

The surface occupied by one épanchement varies between values of 200.000 km ² (Karoo) to 1.500.000 km ² (Siberia), their thickness oscillating between 2.000 m (Deccan) and 12.000 m (lake Supérieur). However, it should be considered that erosion reduced the volume of castings.

Their origin can be placed between 100 and 400 km of depth, in the higher asthenosphere. But to obtain a fusion partial as important as that of the trapps, épanchant of enormous quantities of lava, it is necessary to bring a great quantity of heat. Such a fusion could be done near hot spots, it would result from them a mixture of the abyssal magmas from hot spots and surface magmas produced by the heat of the plumes.

Petrography

The rocks of the trapps are basic rocks, in the majority of the Basalte S, having compositions of Tholéiite S with Olivine (according to the classification of Yoder and Tilley). The composition of basalts of Parana is rather characteristic of the trapps: one finds phenocrysts there occupying approximately 25% of volume of the rock, taken in volcanic glass. These phenocrysts observed are pyroxenes (augite and pigeonite), Plagioclase S as well as the opaque ones such as the Titanomagnétite or the Ilménite, and some time of olivines. One finds also sometimes volcanic products more differentiated like Andésite S or Dacite S and Rhyodacite S, but in small quantity, at the top of the tanks.

Structures

Air castings of trapps can be of two types:

• on smooth or twisted surface: very compact casting, low in cells and bubbles of gas; degasification was easy (maintained magma at higher temperature and more fluid in an enough broad room not to impose a too strong pressure before its expulsion). Lava flows which can form bored underground rivers of " soupapes" of degasification can support the formation of trapps very wide on the surface.
• on chaotic surface: run rich in gas bubbles, on scoriaceous and not very compact surface; degasification at difficult summer (expelled less fluid magma of a fault without possibility of progressive expansion in a hot room, degasification occurs more close to surface where casting forms a crust which fissures under the pressure of gases inside casting itself, and cooling is more brutal).

One finds in the Massif Central, in Auvergne, an good example of run of chaotic type, consisted castings of the Puys of the Cow and Lassolas.

In-depth, castings can crystallize more slowly, and form vertical joints organized out of more or less regular prisms, sometimes hexagonal, sometimes pentagonal, named basaltic Orgues.

Geochemistry

The geochemical major oxide analysis indicates a composition near at the same time to basalts of the médio-oceanic wrinkles (MORB) and basalts of the volcanic islands (OIB). It is indeed tholéiites, having a percentage of Silice near to 50%.

One can distinguish two types of basalts of trapps:

• those low in PO and TiO, named LPT
• those rich in PO and TiO, named HPT

Isotopic reports/ratios 87 Sr 86 Sr and 206 Pb 204 Pb being different from those of basalts in the broad sense, the magma at the origin of the trapps at summer contaminated at the time of its crossing of the continental crust. It is this contamination which explains the differentiation observed between basalt deus standard: the LPT grew rich in elements by the crust like K and Sr.

The contents of incompatible of basalts of trapps are weaker than those of the OIB, but stronger than those of the MORB. The phenomenon of contamination implies an enrichment in certain elements, which means that they come from the impoverished coat, with a depth close to that at the origin of the MORB.

Another planets

On the planet Venus, basaltic flows even more important will be perhaps the source of a better comprehension of the mechanisms which can be with work in these major geological events.

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