3. INTRODUCTION
Basalt is a common extrusive igneous or volcanic
basaltic
rock formed from the rapid cooling of
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. ENVIRONMENTAND 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
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.
24.
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
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
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
