Magmatic seam

Minerals in subterranean magma


Magma is a mixture of solid, liquid and gaseous elements that can flow in the subsoil and, during volcanic eruptions, can be released. It is not always destined to see the light of day, however. Some magmas remain captured in the crust (the outer-most layer of the Earth, reaching a depth of 40 kilometres) and solidify slowly there. Others migrate towards the exterior via fractures in the crust, but they halt in an extremely superficial position before they can emerge.


When this happens, the magma remains to solidify in the fracture, slowly but not too slowly, forming a “magmatic seam”.


Some minerals have the time to form crystals that are several millimetres large.


Other mineralscongeal” (at temperatures of many hundreds of degrees) very quickly, forming a base substance that is almost uniform.


This series of photos shows the minerals and the base substance of a rock from a magmatic seam.

Name: Andensite
Type of rock: Filonian magmatic rock, or subvulcanica
Minerals: Plagioclase, pyroxene, magnetite. Chlorite, amphibole, iron oxides, serpentine, calcite, zeolites as alteration minerals
Fossils: absent
Location: Lazise ( N 45° 30′ 39.3″ E 010° 46′ 25.6″)

Formation (deposit):

Formation (pebbles):

pebbles of the glacial deposits of the sarca (morainic amphitheater of the Garda)


Age (deposit):

Age ( pebble):

Pleistocene (less than a million years)


How the pebble was formed:

How the rock was formed:

: on the front of a lowland glacier



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