Plate Tectonics and Metamorphism
All of the important processes of metamorphism that are directly or indirectly related to
geological processes caused by plate tectonics. The relationships between plate tectonics and
metamorphism are summarized in following Figures below
Figure 1. Environments of metamorphism in the context of plate tectonics: (a) regional metamorphism related to mountain building at a
continent-continent convergent boundary, (b) regional metamorphism of oceanic crust in the area on either side of a spreading ridge, (c)
regional metamorphism of oceanic crustal rocks within a Subduction zone, (d) contact metamorphism adjacent to a magma body at a high
level in the crust, and (e) regional metamorphism related to mountain building at a convergent boundary.
Most regional metamorphism takes place within continental crust. While rocks can be
metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of
mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to
great depths, as depicted in Figure .2 An example would be the Himalayan Range. At this continent-
continent convergent boundary, sedimentary rocks have been both thrust up to great heights
(nearly 9,000 m above sea level) and also buried to great depths. Considering that the normal
geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre,
rock buried to 9 km below sea level in this situation could be close to 18 km below the surface of the
ground, and it is reasonable to expect temperatures up to 500°C. Metamorphic rocks formed there
are likely to be foliated because of the strong directional pressure of converging plates.
Figure 2. a: Regional metamorphism beneath a mountain range related to continent-continent collision (typical geothermal
gradient). (Example: Himalayan Range)
At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving
away from the plate boundary Fig.3. Water within the crust is forced to rise in the area close to the
source of volcanic heat, and this draws more water in from farther out, which eventually creates a
convective system where cold seawater is drawn into the crust and then out again onto the sea floor
near the ridge. The passage of this water through the oceanic crust at 200° to 300°C promotes
metamorphic reactions that change the original pyroxene in the rock to chlorite and serpentine.
Because this metamorphism takes place at temperatures well below the temperature at which the
rock originally formed (~1200°C), it is known as retrograde metamorphism. The rock that forms in
this way is known as greenstone if it isn’t foliated, or green schist if it is foliated. Chlorite and
serpentine are both “hydrated minerals”.
Figure 4 b: Regional metamorphism of oceanic crustal rock on either side of a spreading ridge. (Example: Juan de Fuca
spreading ridge) [SE]
Fig 3. Hydrothermal metamorphic condition at divergent plate margin
At a subduction zone, oceanic crust is forced down into the hot mantle. But because the
oceanic crust is now relatively cool, especially along its sea-floor upper surface, it does not heat up
quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding
mantle (Figure 5). A special type of metamorphism takes place under these very high-pressure but
relatively low-temperature conditions, producing an amphibole mineral known
as glaucophane (Na2(Mg3Al2)Si8O22(OH)2), which is blue in colour, and is a major component of a rock
known as blueschist. If the subducted slab re subducted more than 40km depth to form Eclogite.
blueschist rock exposed north of San Francisco. The blue colour of rock is due to the presence of the
amphibole mineral glaucophane.
Figure 5 c: Regional metamorphism of oceanic crust at a subduction zone. (Example: Cascadia subduction zone. Rock of
this type is exposed in the San Francisco area.)
Magma is produced at convergent boundaries and rises toward the surface, where it can form
magma bodies in the upper part of the crust. Such magma bodies, at temperatures of around
1000°C, heat up the surrounding rock, leading to contact metamorphism fig.7. Because this happens
at relatively shallow depths, in the absence of directed pressure, the resulting rock does not
normally develop foliation. The zone of contact metamorphism around an intrusion is very small
(typically metres to tens of metres) compared with the extent of regional metamorphism in other
settings (tens of thousands of square kilometres).
Figure 7 d: Contact metamorphism around a high-level crustal magma chamber (Example: the magma chamber beneath Mt.
St. Helens.) e: Regional metamorphism in a volcanic-arc related mountain range (volcanic-region temperature gradient)
Regional metamorphism also takes place within volcanic-arc mountain ranges, and because of the
extra heat associated with the volcanism, the geothermal gradient is typically a little steeper in these
settings (somewhere between 40° and 50°C/km). As a result higher grades of metamorphism can
take place.
The various types of metamorphism described above are represented in Figure 8.
Figure 8. Types of metamorphism shown in the context of depth and temperature under different conditions. The
metamorphic rocks formed from mud rock under regional metamorphosis with a typical geothermal gradient are listed.
Metamorphism in plate interiors:
In relatively thick depositional basins, the deeply buried sedimentary and volcanic material
can be subjected to temperatures of 200-300ºC, sufficient high to cause recrystallization. The classic
terrane is a thick stack (> 10 km) of volcanoclastic rocks, like in southernmost New Zealand
(Mesozoic). Although of regional extent, burial metamorphism has little or no associated penetrative
ductile deformation so that relict depositional fabrics are usually preserved in rocks now composed
of low-T minerals such as zeolites. Super-heated aqueous fluids are oozes up from the deeper
portion are also involved. Thick piles of sedimentary and volcanic rocks, accumulated along passive
continental margins, such as along the eastern and Gulf of Mexico coasts, that experience burial
metamorphism, may subsequently undergo deformation as changes in plate motion occur. In this
polymetamorphic, the earlier simple burial effects may be completely obscured ( unclear).
Figure: Schematic cross section through a distensive intraplate setting. At the upper crust level,
this is the setting of burial metamorphism of the zeolite-facies series at low T and P. At the deeper
crust, amphibolite to granulite facies with migmatites develops. Burial metamorphism occurs in
areas that have not experienced significant deformation.
