Geology of the Alps:-
The Alps form part of a line of mountain chains, called the Alpide
Belt. This goes through Southern Europe and Asia and all the way to
the Himalayas.
The Alps were formed when the African and the Eurasian Plates
collided, folded and then buckled
The Alps form part of a Tertiary orogenic belt of mountain chains, called
the Alpide belt, that stretches through southern Europe and Asia from
the Atlantic all the way to the Himalayas. This belt of mountain chains
was formed during the Alpine orogeny. A gap in these mountain chains
in central Europe separates the Alps from the Carpathians to the east.
Orogeny took place continuously and tectonic subsidence has produced
the gaps in between.
The Alps arose as a result of the collision of the African and Eurasian
tectonic plates, in which the Alpine Tethys, which was formerly in
between these continents, disappeared. Enormous stress was exerted
on sediments of the Alpine Tethys basin and its Mesozoic and early
Cenozoic strata were pushed against the stable Eurasian landmass by
the northward-moving African landmass. Most of this occurred during
the Oligocene and Miocene epochs. The pressure formed great
recumbent folds, or nappes, that rose out of what had become the
Alpine Tethys and pushed northward, often breaking and sliding one
over the other to form gigantic thrust faults. Crystalline basement rocks,
which are exposed in the higher central regions, are the rocks forming
Mont Blanc, the Matterhorn, and high peaks in the Pennine Alps and
Hohe Tauern (Stampfli & Borel 2004).
The formation of the Mediterranean Sea is a more recent development
and does not mark the northern shore of the African landmass.
Blueschist (with glaucophane)
Lawsonite
Schistes lustres
Coesite
Rocks that crop out in the internal zones of the Alps were once deeply buried. We
know this because the original character of the rocks (e.g. sediments or igneous)
has been altered - metamorphosed. Although the bulk composition of the new
rocks may have remained unchanged from that of the precursor - new minerals
have replaced the old ones. We know the conditions under which the reactions
occur and so can reconstruct the temperature and pressure of the metamorphic
rocks. If we can tell when the minerals grew relative to deformation, then we can
tell under what conditions the deformation happened. Click on the icons to see
some examples of metamorphic rocks from the Alps.
Classic blueschists. The slight blue tinge results from the mineral glaucophane
(an amphibole), which here forms the rather stubby needles. This rock started
life as a volcanic rock of basic composition, part of the old ocean floor of Tethys.
Blueschists are comonly thought to be diagnostic of former subduction zones,
because they imply relatively high pressure conditions relative to the
temperature (compared to normal geothermal gradients).
Schises lusters. These outcrops were once part of the sediment that lay on the
floor of the Tethys ocean. Originally they were slightly calcareous and muddy.
During metamorphism the clay minerals have been changed to mica, here
arranged in dark, elongate clots. The rock is now very shiny - hence it is called
informally "schistes lustrees".
Coescite This is a photomicrograph (a couple of mm across) of a garnet with
inclusions of silica. Most silica at the earth's surface is in the form of quartz. But
under high pressures (equivalent to depths in the earth in excess of 80 km), the
stable form of silica is the mineral coesite. The garnet crystal has acted as a
protective pressure vessel so that pieces of coesite have been preserved to the
earth's surface. The attempted change to quartz has tried to expand the inclusion
- causing radial cracks in the garnet. This classic image (provided by Christian
Chopin) is from the Dora Miara internal basement massif. So this fragment of
continental crust was once over 80 km down in the earth.
http://www.see.leeds.ac.uk/structure/alps/metamorphism/index.htm
their development during pre-historical and historical evolution as both topography and
the distribution of geological resources control human activity. Consequently, some
attention is paid to the geologically interesting cultural heritage of ancient mines as this
is also within the present focus of activity of the United Nations.
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2. Geological and Geophysical Overview
Figure 1. Simplified geological map of Europe showing the main orogenic systems.
A.M. - Armorican massif, B.M. – Bohemian massif, F.Z. – fault zone, M.C. French
Massif Central, O.M.Z. – Oslo-Mjösen Zone, R.G. – Rhine graben, T.B. –
Transylvanian basin
Europe is divided into a number of major tectonic units that represent a sequence of
The
Alps
continental
growth towards the southwest since Late Archaean times (Fig. 1). These
The Alps are the youngest and highest mountain system in Europe.
They stretch across the western and southern part of the continent
of Life
Support
Systems
(EOLSS)
in©Encyclopedia
a broad
arc.
The
mountain
range starts near the
Mediterranean Sea on the border between France and Italy.
Then it curves north- and eastward through northern Italy,
Switzerland Liechtenstein, southern Germany, Austria and Slovenia.
The Alps are about 1,000 km long, the broadest section over 260
km wide. The highest peak, Mont Blanc, situated on the border
between France, Italy and Switzerland, rises 4807 meters above
sea level . Other famous peaks are the Monte Rosa, the
Matterhorn, the Großglockner and the Zugspitze.
The whole mountain range is divided into three sections:
• The western Alps lie west of the Great St. Bernard Pass and
include the highest mountains.
• The central Alps lie between the Great St. Bernard and Lake
Constance.
• The eastern Alps stretch east of Lake Constance into Austria,
northern Italy, southern Germany and Slovenia. They are the
lowest section of the mountain range .
How the Alps were formed
Millions of years ago the area of today’s Alps was covered by a
large sea that separated Europe and Africa. The southern land
mass started moving northwards. This movement folded rock
layers at the bottom of the sea. Heat and pressure transformed
the rock and pushed the material upwards . Today these regions
are the highest parts of the Alps. Most of the newly formed rock is
granite and gneiss, but many ranges consist of limestone
which also formed on the seabed.
During the Ice Age, which started about a million years ago, the
Alps were covered with a thick blanket of snow. Glaciers moved
down valleys and made them wider and deeper. As they moved
they took rock and other material with them, creating moraines.
When glaciers started to melt water filled up behind these natural
dams and created the alpine lakes we know today.
The largest of these glaciers is the Aletsch in Switzerland which
reaches a length of about 25 km. The longest glacier of the
eastern Alps is the 8 km long Pasterze, at the foot of the
Großglockner.
The ice and snow of the alpine regions helped create the large
rivers of today : the Rhine, Rhone, Danube and the Po.
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The Aletsch Glacier in Switzerland
176
Figure 1 Tectonic map of Alps - (1) Europe-vergent collisional belt: i) Western (WA) and Eastern (EA) Austroalpine; ii) Penninic domain:
continental and ophiolitic (o) nappes in western Alpine arc (P) and tectonic windows (otw: Ossola-Ticino, ew: Engadine, tw: Tauern, rw:
Rechnitz); Prealpine klippen (Pk); iii) Helvetic-Dauphinois (H-D) domain; iv) Molasse foredeep (M); v) Jura belt (J).
(2) Southern Alps (SA), bounded to the north by the Periadriatic lineament (pl). Pannonian basin (PB), European (EF) and Po ValleyAdriatic (PA) forelands, Dinaric (DI) and Apenninic (AP) thrust-and-fold belts.
within the Adriatic upper plate. If we integrate surface geology with
interpretation of seismic images, the Europe-vergent belt is a mantlefree crustal wedge which tapers to the north, floats on top the European lower plate and is indented, to the south, by the present Adriatic
(Southern Alps) lithosphere (Figure 2). Both continental plate margins originally extended way into the Penninic-Helvetic and Austroalpine domains presently incorporated into the collisional belt.
This wedge groups the Austroalpine, Penninic and Helvetic units,
and may be subdivided into two diachronous parts: i) the internal,
older part (Austroalpine-Penninic), which forms now the axial zone
of the Alps, is a fossil subduction complex which includes the
Adria/Europe collisional zone; it is marked by one or more ophiolitic
units (in different areas) and displays polyphase metamorphism
evolving from blueschist or eclogite facies imprint (CretaceousEocene subduction), locally coesite-bearing, to a Barrovian overprint (mature collision, slab break-off) of Late Eocene-Early Oligocene age (Frey et al., 1999); ii) the outer, younger part (Helvetic) is
made up of shallower basement thrust-sheets and largely detached
cover units derived from the proximal European margin, which
escaped the low-T subduction regime and, from the Oligocene, were
accreted in front of the exhumed Austroalpine-Penninic wedge.
In the following, we outline the essential features of the
Europe-vergent Austroalpine, Penninic and Helvetic tectonic
domains and the antithetic Southern Alps.
The Austroalpine thrust units
The Austroalpine is subdivided into two sectors (western and
eastern), based on contrasting distribution, structural position, and
main deformation age.
The western Austroalpine consists of the Sesia-Lanzo zone and
numerous more external thrust units traditionally grouped as
Argand’s Dent Blanche nappe. These units override and are partly
tectonically interleaved with the structurally composite ophiolitic
Piedmont zone, the major remnant of the Mesozoic ocean. Two
groups of Austroalpine units are identified: i) the upper outliers
(Dent Blanche-Mt. Mary-Pillonet) and the Sesia-Lanzo inlier occur
on top of the collisional nappe stack; they overlie the entire ophiolitic Piedmont zone and display a blueschist to eclogite facies metamorphism of Late Cretaceous age; ii) the Mt. Emilius and other
lower outliers are interleaved with the Piedmont zone, along the tectonic contact between the upper (Combin) and lower (Zermatt-Saas)
ophiolitic nappes, and display an eclogitic imprint of Eocene age.
Therefore, these groups of nappes originated from different structural domains, were diachronously subducted to various depths, and
finally juxtaposed during their later exhumation.
In the central Alps, east of the Ossola-Tessin window, the western Austroalpine may be correlated to the Margna nappe (Staub’s
interpretation), which is thrust over the Malenco-Avers ophiolite and
overlain by the Platta ophiolite, both being potential homologues of
the Piedmont zone. The Platta nappe is in turn the tectonic substratum of the eastern Austroalpine system. This means that the western
Austroalpine and Margna nappes are presently located at a structural
level lower than that of the capping eastern Austroalpine.
The eastern Austroalpine is a thick pile of cover and basement
nappes which extends from the Swiss/Austrian border to the Pannonian basin (Figure 1). Its allochthony with respect to the Penninic
zone is documented by Mesozoic and ophiolitic units exposed in the
Engadine, Tauern and Rechnitz windows. To the north, the Austroalpine overrides the outer-Penninic Rheno-Danubian flysch belt;
to the south, it is juxtaposed to the Southalpine basement along the
Periadriatic fault system. Part of the Austroalpine displays an
eclogitic to Barrovian metamorphism dated as early-mid Cretaceous
(Eoalpine; Frey et al., 1999). In addition, thrust surfaces are sealed
by Gosau beds (Coniacian-Eocene intramontane basins), testifying
that the principal tectono-metamorphic history of the eastern AusSeptember 2003
Composition
Composition of igneous rocks is properly identified by determination of the
rock's chemical composition. This, however, requires chemical equipment and
apparatus that is unavailable in this lab. Fortunately determination of the exact
chemical composition is not necessary. Color is often an indicator of the
composition of a rock or mineral and can be effectively used to identify the
composition of most igneous rocks. Light colors, including white, light gray, tan
and pink, indicate a felsic composition. Felsic compositions are rich in silica
(SiO2). Dark colors, such as black and dark brown, indicate a mafic or ultramafic
composition. Mafic compositions are poor in silica, but rich in iron (Fe) and
magnesium (Mg). Intermediate compositions have an intermediate color, often
gray or consisting of equal parts of dark and light mineral . Beware that even
though an igneous rock may have a felsic composition (light color), the rock can
contain dark colored minerals. Mafic rocks may contain light colored minerals as
well. As mentioned above, the composition of most igneous rocks can be
identified using this system, formally known as the Color Index. However, there
are exceptions. The two most notable are obsidian and dunite. Obsidian is
volcanic glass which erupts as a lava flow. Most obsidian is felsic in composition,
yet typically it will have a very dark color (dark brown to black). Dunite has an
ultramafic composition yet is apple green to yellowish green in color. Dunite is
composed almost entirely of the mineral olivine which usually contains both iron
and magnesium
Texture
The texture of an igneous rock does not refer to the roughness or smoothness of
the surface. Textures are based primarily on crystal size. Pegmatitic texture is
composed of very large crystals (larger than 2-3 cm). Phaneritic texture is
composed of crystals which are large enough to see but smaller than pegmatitic
texture, and the entire rock is composed of crystals. Aphanitic texture is a fine
grained texture but the crystals are too small to see. Porphyritic texture is
composed of crystals of two different sizes. Typically the large crystals
(phenocrysts) are visible while the smaller crystal are not (referred to as
groundmass). Glassy texture is the most readily recognized. The rock is
composed entirely of glass. Few, if any, crystals will be visible. Vesicular texture
is formed when lava solidifies before gases are able to escape. The result is a
"bubbly" appearance. Lastly, pyroclastic texture is composed of volcanic
fragments. These fragments or clasts can be very fine (ash) or coarse (lapilli) or
very coarse (bombs and blocks).
Erosion
Glaciers erode predominantly by three different processes:
abrasion/scouring, plucking, and ice thrusting. In an abrasion process,
debris in the basal ice scrapes along the bed, polishing and gouging the
underlying rocks, similar to sandpaper on wood. Glaciers can also
cause pieces of bedrock to crack off in the process of plucking. In ice
thrusting, the glacier freezes to its bed, then as it surges forward, it
moves large sheets of frozen sediment at the base along with the
glacier. This method produced some of the many thousands of lake
basins that dot the edge of the Canadian Shield. These processes,
combined with erosion and transport by the water network beneath the
glacier, leave moraines, drumlins, ground moraine (till), kames, kame
deltas, moulins, and glacial erratics in their wake, typically at the
terminus or during glacier retreat.[citation needed]