This document summarizes a seminar on the origin and evolution of magma and its relationship to plate tectonics. It discusses how magma originates from partial melting of the solid earth due to increased temperature and pressure. Magma composition evolves through processes like fractional crystallization, assimilation, and magma mixing. Magmatism is related to plate tectonics through mechanisms like decompression melting at mid-ocean ridges and continental rifts, flux melting near subduction zones, and intraplate volcanism from mantle plumes not associated with plate boundaries. Examples discussed include the Deccan Traps basalts formed by a mantle plume in India.

1. SEMINAR ON GEOLOGY
GUIDED BY:-
JAY SINGH SARASWAT
TOPIC:- ORIGIN & EVOLUTION OF MAGMA, MAGMATISM
RELATED TO PLATE TECTONICS.
SUBMITTED BY:-
DEVASHISH SAHU
(M.Sc.:- 2nd sem.)

2. CONTENT
•Origin of magma .
•Evolution of magma .
•Magmatism related to plate
tectonics.

3. ORIGIN OF MAGMA
•As we have seen the only part of the earth that is liquid is
the outer core. But the core is not likely to be the source of
magmas because it does not have the right chemical
composition.
•The outer core is mostly Iron, but magmas are silicate
liquids.Thus magmas DO NOT COME
FROM THE MOLTEN OUTER CORE OF THE EARTH.
•Thus, since the rest of the earth is solid, in order
for magmas to form, some part of the earth must
get hot enough to melt the rocks present.

4. •We know that temperature increases with depth in the
earth along the geothermal gradient. The earth is hot
inside due to heat left over from the original accretion
process, due to heat released by sinking of materials to
form the core, and due to heat released
by the decay of radioactive elements in the earth.
PARTIAL MELTING :- Melting of solid rocks to form magma is
controlled by three physical parameters : its temperature,
pressure, & composition.
•Individual minerals in source rock melt at different
Temperature.
•Magma is enriched in many elements, especially SiO2.

5. •For a pure dry (no H2O or CO2 present) mineral,
the melting temperate increases with increasing
pressure.
•As pressure increases in the Earth, the melting
temperature changes as well. For pure minerals,
there are two general cases.

6. •For a mineral with H2O or CO2 present, the melting
temperature first decreases with increasing pressure. Since
rocks mixtures of minerals, they behave somewhat differently.

7. •Melting of dry rocks is similar to melting of dry minerals,
melting temperatures increase with
increasing pressure, except there is a range of
temperature over which there exists a partial melt. The
degree of partial melting can range from 0 to 100%.

8. •Melting of rocks containing water or carbon dioxide is
similar to melting of wet minerals, melting temperatures
initially decrease with increasing
pressure, except there is a range of temperature over
which there exists a partial melt.

9. •Norman Bowen did laboratory experiments to determine
the order of crystallization of minerals growing in a magma
as it cools, and made chart showing the order.
•This is called BOWEN’S REACTION SERIES.
•Minerals crystallize in a systematic fashion based on their
melting point.
•During crystallization, the composition of the liquid portion of
the magma continually changes.
•Because early formed igneous rock might contain one set of
minerals, but a later forming igneous rock, forming from the
same magma, but changed, will have a different mineralogy.

10. BOWEN’S REACTION SERIES

11. PRIMARY MELTS:-
•Primary melts have not any differentiation & represent the
starting composition of a magma.
•In a nature it is rare to find primary.
•The leucosomes of migmatites are the examples of
primary melts.
•Primary melts derived from the mantle are especially
important, and are known as primitive melts or primitive
magmas.
•By finding the primitive magma composition of a magma
series it is possible to model the composition of the mantle
from which a melt was formed, which is important in
understanding evolution of the mantle.
EVOLUTI0NOF MAGMA

12. PARENTAL MELTS:-
•Where it is impossible to find the primitive or
primary magma composition, it is often useful to
attempt to identify a parental magma.
•A parental melt is a magma composition from
which the observed range of magma chemistries
has been derived by the processes of igneous
differentiation, it need not be a primitive melt.
•A composition from which they could reasonably be
produced by fractional crystallization is termed a
parental melts.
•At high degrees of partial melting of mantle,
Komatite and Picrite are produced.

13. Processes responsible for changing a magma’s
composition-
MAGMATIC DIFFERENTIATION:- Separation of a melt from
earlier formed crystals.
ASSIMILATION:- Changing a magma’s compositions by
incorporating surrounding rock bodies into a magma.
MIXING OF MAGMA:- Two chemically distinct magmas may
produce a composition quite different from either original
magma.

14. Fig. show properties due to change’s magma composition.

15. MAGMATISM RELATED TO PLATE TECTONICS
DECOMPRESSION MELTING:- The process of
decompression melting involves the upward movement
of the earth’s mantle to an area of approximately the
same temperature but the pressure is at reduced
Eg.- oceanic ridge, continental rift valley etc.

16. The process of flux melting is a rock is close to its melting
point and some water (a flux that promotes melting) is
added to the rock, the melting temperature is reduced
(solid line versus dotted line), and partial melting starts.
FLUX MELTING:-

17. A third tectonic setting where volcanism occurs is
called intraplate- or hot-spot-volcanism, which describes
volcanic activity that occurs within tectonic plates and is
generally NOT related to plate boundaries and plate
movements.
INTRAPLATE VOLCANISM
Eg.- intraplate island etc.

18. A mantle plume is an upwelling of abnormally
hot rock within the Earth's mantle.
MANTLE PLUME
•Mantle plumes :- flood basalt( Columbia river basalt).

19. •Divergent plate boundaries : sea floor mechanism basaltic
magma.
•Convergent plate boundaries :- subduction produces partial
melting of magma.
•The volcanic basalt beds of the Deccan were laid down in the
massive Deccan Traps eruption, which occurred towards the
end of the Cretaceous period between 67 and 66 million years
ago.
•Typically the Deccan Plateau is made up of basalt extending up
to Bhor Ghat near Karjat. This is an extrusive igneous rock

20. THANK YOU
“ NATURE IS PLEASED WITH SIMLPICITY.”