Basalts - igneous petrology

2. CONTENTS
 Introduction
 Environment and occurrence
 Mineralogy
 Types of basalts
 Origin
 Deccan traps
 Conclusion
 References

3. INTRODUCTION
 Basalt is a common extrusive igneous or volcanic
rock formed from the rapid cooling of basaltic
lava exposed at or very near the surface.
 It is also known as a dark volcanic rock
 The term basalt is at times applied to
shallow intrusive rocks with a composition typical of
basalt, but rocks of this composition with a coarse
groundmass and are generally referred to as gabbro.

4. ENVIRONMENT AND OCCURENCE
 Basalts commonly occur as lava
flows because of their low volatile
content
 When rising basaltic magma
encounters groundwater the magma
may vesiculate to form flow
breccias or erupt to form cinder cones
 It covers all the ocean floor except where they
themselves are covered by continental-margin
sediments.

5. CONT……
 It is the most abundant igneous rock at or near the
earth’s surface
 The most voluminous are also called MORB
 The great flood of basalts that have been extruded
onto the continents, form extensive nearly horizontal
flows erupted from fissure swarms
 Most basalts are erupted from tensional or rift
environment except for the arc volcanic rocks or hot-
spot basalts.

6. Mineralogy
 Composed of plagioclase feldspar and pyroxene in
the ratio of 50:50.and magnetite and may contain
olivine.
 Alkaline basalts may contain a little nepheline or
alkali feldspar
 On the basis of mineralogy there are three different
types of basalts viz., tholeiitic, transitional and alkali
basalts

7.  Basalt is usually grey to black in colour, but
rapidly weathers to brown or rust-red due to
oxidation of its mafic (iron-rich) minerals
into rust
 Alteration minerals in ocean floor basalts
include green to light brown palagonite,
chlorite, epidote, celadonite, goethite,
hematite, calcite and zeolite

8. vesicular basalt with many small
vesicles
Basalt showing aphinitic
texture

9. Pillow basalts on the south Outcrop of a pillow basalt
Pacific seafloor

10. Columnar basalt in Turkey Thin section

11. TYPES OF BASALTS
 Tholeiitic basalt: relatively rich in silica and poor
in sodium. Basalts of the ocean floor and
continental flood basalts comes under this.
 MORB (Mid Ocean Ridge Basalt): characteristically
low in incompatible elements. Commonly erupted
only at ocean ridges.
 High alumina basalt :may be silica-undersaturated or
oversaturated. Alumina (Al2O3) content is more than
17% and intermediate in composition i.e.between
tholeiite and alkali basalt

12. CONT….
 Alkali basalt:relatively poor in silica and rich in
sodium. It is silica-undersaturated and may
contain feldspathoids, alkali feldspar and phlogopite
 Boninite: is a high-magnesium form of basalt that is
erupted generally in back-arc basins, distinguished by
its low titanium content and trace element
composition.

13. ORIGIN
 The origin of basaltic magma is universally accepted
as involving melting within the earth’s mantle
 Evolve by fractional crystallization as separate series
along different paths. Each is chemically distinct
 Tholeiites are generated at mid-ocean ridges, oceanic
islands, subduction zones
 Alkaline basalts generated at oceanic islands and at
subduction zones

14. Cont..
 The melting behavior of basalts indicates that it is
the partial melting products of a more primitive
rock (e.g. garnet peridotite).
 In the region of magma generation (below 60 km)
the parental material, presumed to be garnet
peridotite, yields an eclogitic magma and its
fractionation depends on the garnet and omphacite
of the eclogite, not on plagioclase and
clinopyroxene of a basaltic magma.

15. Cont…
 Increase of the garnet constituents in the magma at
high pressure by effective removal of omphacite or
shift of the garnet-omphacite boundary surface will
give rise to a tholeiite-type magma at low pressure.
 Increase of the omphacite constituents in the magma at
high pressure by physical or physicochemical means
will give rise to an alkali basalt-type magma at low
pressure.
 In general, alkali basalt-type magmas are to be
expected to be generated at greater depths than
tholeiite-type magmas from the same primary source
rock.

16. Theories
Origin 1: Mechanism involving partial melting under
under different condition
-higher pressure or lower temperature partial melting
of mantle material produce alkaline basalt whereas
lower pressure or higher temperature partial melting
produce tholeiitic magma
2: Mechanism involving stage of melting or degree of
melting
-early stage of partial melting of garnet peridotite
produce tholeiitic basalt, whereas an intermediate
stage give rise to alkaline olivine basalt

17. Cont….
3. Mechanisms involving partial melting of a mantle
source of different composition
- MOR basalts contain less radiogenic Pb and Sr, and
more radiogenic Nd, and depleted light REEs
compared with continental tholeiites and are
probably derived from mantle of different
compositions
- Tholeiitic basalts form by partial melting of peridotite
containing H2O, K2O, and Na2O whereas alkaline
basalts form by partial melting of peridotite richer in
CO2, TiO2, and P2O 5

18. Cont…
4. Mechanisms involving differentiation or fractional
crystallisation
- Higher pressure fractionation of basalt formed by
partial melting in the mantle could give alkaline
basalt, whereas lower-pressure fractionation of the
same basalt could give tholeiitic magma
- Partial melting at a depth of about 60km could give
alkali basalt. Partial crystallisation at a depth of about
40km could produce transitional basalt
- Partial melting of peridotite and leaching of

19. Cont…
Wall rocks during ascent of the magma could form
alkali basalt
- limited partial melting of garnet peridotite could
produce alkali basalt and more extensive melting
give tholeiitic basalt
- Incipient melting of heated mantle wall rocks
produced early alkalic melts; later melting could
produce tholeiitic melts; stagnation as the volcano
moved off the hot spot and a decrease in melting of
the wall rocks would form the latest alkalic basalts

20. Cont…
-Separation of high Mg olivine and pyroxene at a depth
of 15 to 35 km could form high Al basalt
5. Mechanisms involving a particular tectonic
environment
-Basaltic melt from a deep mantle plume accumulates
at the base of the lithosphere.
Magma at the base of the lithospheric plate finds access
to the surface along zones of crustal weakness

21. Cont….
- Alkali basalts volcanism may be associated with the
lateral edge of a subducting lithosphere plate.
- Tholeiitic and transitional basalts such as those
formed at a mid-oceanic ridge could originate by
partial melting at modest pressures below about 8 to
10 kbars or depths of 30 to 35 km.

22. Deccan Traps
 The Deccan Traps are a large igneous province
located on the Deccan Plateau of west-central India
and one of the largest volcanic features on Earth
 They consist of multiple layers of solidified flood
basalt that together are more than 2,000m thick and
cover an area of 500,000 km2 and a volume of
512,000 km3
 The basalt flows are generally massive, compact
and coarse grained in central part but become fine
grained near top and bottom parts

23. Cont…
 Within the Deccan Traps at least 95% of the lavas
are tholeiitic basalts, however other rock types like
Alkali basalts, Nephelinites, Lamprophyre,
Carbonatites also occur.
 Mantle xenoliths have been described
from Kutch (northwestern India) and elsewhere in
the western Deccan.

25. Cont…
 The Deccan Traps eruption was associated with a
deep mantle plume.
 The area of long-term eruption known as
the Reunioun hotspot, is suspected of both causing
the Deccan Traps eruption and opening the rift that
once separated the Seychelles plateau from India.
 The basalt deposits in the Seychelles are from
the Deccan Traps eruption, which occurred in the
central part of the Indian sub-continent 65 million
years ago, at the end of the Cretaceous period

26. Reunion hotspot

27. Pillow lava Columnar jointing at
St.Mary’s island

28. The Deccan Traps at Mahabaleshwar.These comprise multiple layers of flood
basalt, over a mile deep, the result of massive volcanic activity at the K/T
boundary. Some regard this as the cause of the extinction event

29. Lonar lake in Maharashtra

30. Economic importance
 Flood basalts are known to host important
deposits of native copper and platinoids
 Bauxite cappings over Deccan Trap as in
Belgaum are useful as aluminium ore
 Natural zeolites filling the cavities in volcanics
are useful as gemstones and have industrial and
agricultural applications
 used in construction as building blocks or in the
groundwork, making cobblestones (from
columnar basalt) and in making statues. Heating
and extruding basalt yields stone wool, an
excellent thermal insulator

31. Conclusion
 Basalt is a common extrusive igneous rock formed
from the rapid cooling of basaltic lava exposed at or
very near the surface.
 It is an aphanitic igneous rock
 tholeiitic, transitional and alkali basalts are the three
important types of basalts

32. Cont….
 It is formed by fractional crystallisation along
different paths
 Seafloor spreading at the boundary between
the Indian and African Plates subsequently
pushed India towards north, which now lies
under Reunion island in the Indian Ocean,
southwest of India
 Important texture present in basalt is called
amygdaloidal texture

33. Reference
 Petrology of igneous and metamorphic rocks, Donald
W.Hyndman, Mcgraw-Hill Book company, Pp 171-193
 Deccan volcanism and related basalt province in other parts of
the world, K.V Subbarao and R.N Sukheshwala
 2010,Geology of India, M.Ramakrishnan and R.Vaidyanadan,
Geological society of India, pp733-776
 http://en.wikipedia.org/wiki/Basalt
 en.wikipedia.org/wiki/Deccan_Traps
 volcano.oregonstate.edu/vwdocs/volc_images/europe.../decca
n.html