Basalts

Basalts - igneous petrology

1. BASALTS SEMINAR ON

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 (Al 2 O 3 ) 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 H 2 O, K 2 O, and Na 2 O whereas alkaline basalts form by partial melting of peridotite richer in CO 2 , TiO 2 , and P 2 O 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 km 2  and a volume of 512,000 km 3   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. 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

25. Reunion hotspot

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

27. 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

28. Lonar lake in Maharashtra

29. 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

30. 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

31. 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

32. 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 .../ deccan .html‎

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