Download UNIT 2 Metamorphism and tectonic plates: Metamorphism is a

UNIT 2
Metamorphism and tectonic plates:
Metamorphism is a process where the type or distribution of the minerals in rocks changes because of
high pressure or very high temperatures. This process is called isochemical because the global
chemical composition of the rock essentially remains unchanged.
Metamorphism can sometimes take place in faults or at the sites of meteor impacts. However, it is
most commonly found in areas with convergent plate boundaries, such as subduction and continental
collision zones.
Next to a trench (collision of both boundaries): the main factor acting on the rocks is pressure.
In a volcanic arc (produced by rising magma): it is a high temperature belt.
Continental collision zone(collision between the continents) caused by the collision and the
heat generated by friction and the rising magma.
OROGENS: orogens are formed where convergent boundaries or subduction zones are.
Orogens associated with subduction: in subduction zones the oceanic plate is destroyed and
the continental plate is built up from the sediments in an accretionary prism and magma from
volcanic arcs. This produces an
orogen on the continent. (Ex.
Andes). Eventually, the continental
plate will reach the trench and join
the oceanic plate.
Volcanoes created by this process
have very acidic and viscous lava
(they are created from molten
crust, which is rich is silica, quartz
and feldspar [filsic]. The melting
point of the quartz is very low (700
Cº).
This forms volcanoes with very steep slope because it doesn’t flow fluently.
Hot spots: the lava comes from the
lower mantle, so they have different
composition to the previous orogen (ultramafic
composition: magnesium and iron), so the lava
is very fluid. Their melting point is 1700 ºC. We
have this type of lava in rifts.
Orogens associated with continental
collision: these orogens are produced by the collision
between two continents (ex. Himalayas). They are
called Alpine-type orogens. On a geological scale it is
a much faster process than the formation of Andean-type orogens. They hardly have
volcanoes. The seismic activity covers a wide area and the deformation and metamorphism of
rocks are greater. There is only erosion, no subduction (obduction)
Accretion orogens: small ridges in the oceanic plate
(island arcs, small pieces of continental lithosphere)
collide when they reach the trench.They don’t go
down the trench, they join the continent. (Ex. Rocky
Mountains). They have different origin (ridges and
volcanoes are from ultramafic origin and the
continents of felsic origin).
Orogens are created in orogenic belts, and were created in geological
periodinthe past when the crust was folded and cordilleras were
created, called orogenies.
ISOSTASY: is the state of equilibrium between the lithosphere and the asthenosphere: if the
weight of the lithosphere increases it usually sinks into the mantle; if it decreases, it usually rises.
These movements are very slow, and the process only takes place when the weight is considerable.
SUBSIDENCE ()
In: zones where sediments accumulate or in
continents that are covered by ice during
glaciation.
ISOSTASY
After the erosion of a mountain range, or when
is no ice after glaciations.