GEOLOGICAL GLOSSARY

ALLUVIAL (deposit): Sediment originated by the slow down of a water flow, and composed by pebbles, gravel and send or silt. It forms, frequently, terraces or filling of basin.

ALTERATION: change produced by chemical or physical actions in minerals and rocks, on the earth's surface, changes occur due to atmospheric agents.

BASALT: magmatic effusive rock, rich of iron and magnesium and they cover the oceanic floor. They are formed by partial melting of mantle. Basalts are typically found on the continental crust in region of crustal up warping and rifting and “contaminated” by the rocks passed through.

CALC-SCHISTS: metamorphic rock of sedimentary origin formed in variable proportions by carbonate, quartz and mica in association with other minerals depending on the place of origin and the metamorphism grade undergone.

DEGLACIATION: age, in our Alps dates back 15,000 – 12,000 years ago, in which occurred the retreat of the the big snouts glacier that were settling the valleys.

DEBRIS FLAP: deposits of lithic fragments and fine derivatives accumulated.

EARTH'S CRUST: The thin outermost solid layer of the Earth. Il can be oceanic or continental.

EFFUSIVE ROCK: volcanic rock formed by a nonexplosive outpouring of lava in molten or plastic form

EVAPORITE: sedimentary rocks which have formed by the precipitation of salt, rocks rich of: chlorides, potassium sulphates, sodium, magnesium, calcium. In the evaporitic sequences we can find, from the bottom: quartzite, lime, dolomite, carniole, anhydride, gypsum, halite.Typical Triassic depositional units (-250, -200 million of years ago).

FAULT: plane surface of fracture in a rock body caused by brittle failure and along which observable relative displacement has occurred between adjacent blocks. The movement may happen along a slightly or very inclined plane, in this case the detachment may show up as a smooth fault plane. Faults are considered to be active if there has been movement observed or evidenced during the last ten thousand years.

GABBRO: intrusive igneous rocks chemically equivalent to basalt.

GARNET: group of silicate minerals with thick, and compact structure.

INTRUSIVE ROCK: rocks which have been forces, while in a plastic or melted state, into the cavities or between the cracks or layers of other rocks.

LITHOSPHERE: the upper layer of the solid Earth, comprising all crustal rocks and the brittle part of the uppermost mantle. It comprises numerous oceanic and continental blocks, known as tectonic plates.

MAGMA: from Greek language, a hot silicate, carbonate or sulphide melt containing dissolved volatiles (mainly water vapour and carbon dioxide) and suspended crystal, which is generated by partial melting of the Earth's crust or mantle and is the raw material for all igneous processes. Generally less dense than the rock in which it is found (at least in the mantle), it tends to rise towards the surface. If it reaches it, it erupts (lava) from volcanic ducts or from underwater or sub-aerial cracks, otherwise it crystallizes in magmatic chambers or fissures in the rocky mass. In the first case it produces flows or dikes of effusive magmatic rocks (basalts, rhyolites ...), in the second case it produces batolites (or plutons) of intrusive magmatic rocks (gabbros, granites ...). When it starts to crystallize it is called igneous rock

MAGNETITE: an important rock-forming iron oxide. It is an essential component of serpentinite.

MANTLE: zone lying between the Earth's crust and core, approximately 2300 km thick, similar in composition to garnet peridotite, rock with silicate of iron, magnesium, and calcium.

METABASITE: metamorphic rock rich in iron and magnesium, formed as a result of the metamorphism of basic igneous rocks.

MICAS: group of phyllosilicates rich of potassium that crystallize in shining sheets. They are divided in ferromagnesian black micas (e.g. biotite), and in white aluminous (e.g. muscovite).

MINERAL: usually inorganic substance which occurs naturally, and typically has a crystalline structure. A mineral is defined by it's chemical and crystallographic properties. Generally, rocks are an aggregate of minerals. In a rock, minerals may be: essential, accessory or accidental.

OCEANIC PLATE: layer of lithosphere in which happen with major intensity the dissipation of the thermic energy generated inside the Earth. This occurs on alignments of linear, elevated volcanic structures that make the plate spreading. Characterized by a thin crust (8 km on average) with basaltic composition, it is progressively covered by sediment of open sea, ending sooner or later in subduction at its margins. It has a geological short life. (170 millions of years maximum).

OCEANIC BASIN-FLOOR: abyssal plain corresponding at the surface of a oceanic plate

OROGENESIS: the processes that lead to mountain building especially when a belt of the Earth's crust is compressed by the lateral forces to form a chain of mountains.

PERIDOTITE: a coarse-grained, ultrabasic, igneous rock consisting of essential magnesium-rich olivine accompanied by lesser amount of other ferromagnesian minerals such as pyroxene and chromite, with or without pyrope garnet, and spinel. Metamorphosed peridotites in a water-rich environment generates serpentinites, while partial melting of peridotites generates basaltic magmas.

PYROXENE: group of inosilicates that is widely distributed in both igneous (peridotite, gabbros, and basalt) rocks and metamorphic rocks (granulites, and eclogites). In the Western Alps, sodium pyroxenes characterize subducted rocks (very high pressure and low temperature).

PLATE: a segment of the lithosphere bounded by almost continuous belt of earthquakes and, in most cases, by volcanic activity and young subsea mountain chains. There are major and smaller plates that move away or colliding with respect to each other, they can be oceanic or continental.

REGIONAL METAMORPHISM: mineralogical transformations at the solid state that affect pre-existing rocks in response to simultaneous changes of temperature, lithostatic pressure, and in many cases shear stress, occurring in orogenic belts where lithostatic plates are converging. The speed and efficiency of the process depend on temperature, pressure, volatile content and on higher or minor presence of fluids. In nature specific associations of minerals recurring in metamorphic equilibrium with each other at various depths. Metamorphic rocks or metamorphic mineral relicts allow the reconstruction of the vertical movement of the rocky mass.

ROCK: rocks are the constituent material of the Earth's crust, aggregates of minerals which may be of one type or of many types. The aggregate of mineral can form by accretion or precipitation of grains during Earth surface processed, to give sedimentary rocks; by crystallization of magma to give igneous (intrusive and effusive) rocks, and solid-state recrystallization in response to changes in external conditions ti give metamorphic rocks. OUTCROP: that part of a rock formation which is exposed at the Earth's surface.

SERPENTINE: group of minerals belonging to the group of phyllosilicates, hydrated magnesium silicate, in the Western Alps, in massive form (antigorite) or in fibrous form (chrysolite).

SERPENTINITE: altered rock produced by the hydrous alteration of ultrabasic rocks at low temperature, is essentially composed of serpentine and magnetite.

SILICATE: mineral in which the basic building block is the silica tetrahedron: one silica atom in the center, and four oxygen atoms in the vertices. There are several families of silicate, from those “dark and heavy” (olivine, garnets...) in isolated tetrahedral, to those “clear, and light” (feldspar , and quartz) with all the shared vertices. The more the oxygen vertices are shared between the tetrahedra, the lower the crystallization temperature is.

SUBDUCTION: the process of consumption of a lithopheric plate at convergent plate margins. It results in one plate, generally the oceanic one, being drawn down by another, sinking in the mantel, where it is slowly assimilated. Subduction means a significant doubling of the crustal thickness, and then a decreasing of the heat coming from inner Earth. In this way, the possibility of crystallizing minerals at high pressure is expanded without the rock melting due to the heat. The rock of Ayas Valley have risen to the surface without completing the subduction.

ZOISITE: silicate of the epidote group, rich of calcium, that resists at high pressures. It form prismatic crystals, and bacillary aggregates of clear colour.