MAGMA SERIES

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TYPES OF VOLCANISM

1) Interplate volcanism

  • Associated with tectonic activity

  • Mainly occurs during Subduction of plates

  • Region of volcanic arc: High temp but low pressure

  • Region of Subduction zone: High pressure but low temp

  • Source of volcanic material: asthenosphere

  • Composition : Tholeitic, Calc-Alkaline and Alkaline

  • Example : Ocean- Ocean convergence: Andaman Nicobar, Japan, Mariana Trench, Aleutian, Ocean - Continent convergence : Andean, Peru chile trench

2) Intraplate volcanism

  • Associated with hotspot activity(within the plate)

  • Source of volcanic material is D’’- layer

  • Composition: Tholeitic and Alkaline

  • Example: Hawaiian volacano(OIB) , Yellowstone volcanism , Deccan (CFB)

MAGMA SERIES TYPES

Magma series is divided into two types:

1) Alkaline series

  • Found in convergent plate margins and oceanic, continental intraplate settings

  • Restricted to intraplate occurences

  • Higher melt fractions and high pressure (highly incompatible)

  • Melting at high pressure leads to nepheline normatives (alkaline and silica undersaturated)

  • Not found in case of divergence

2) Sub-Alkaline series : (Calc alkaline and Tholeitic)

a) Calc alkaline series

  • Only found in convergent plate margin

  • Only in Subduction zones

b) Tholeitic

  • Generated at MORB(Mid oceanic ridge basalt)

  • Found mostly in divergent plate margin

  • As well as in other plate margins

  • More voluminous

  • High melt fractions and low pressure - favoured by shallow melting.

  • Low pressure partial melt produces silica rich initial melt



INTRAPLATE VOLCANIC ACTIVITY

  • Occurs because of mantle plumes and hotspot activity within plates.

  • Mantle plume: large head followed by narrower tail - upwelling from deep within the mantle

  • Hotspots: Volcanically active surface expressions of mantle plumes, high heat flow

  • ICELAND: Here mantle plume rises from beneath at mid oceanic ridges

OPHIOLITE SEQUENCE

  • fragments of ancient-oceanic crust and upper mantle

  • Podiform chromites and Cyprus type VMS deposits associated with ophiolites

  • Found mainly during subduction

    Ophiolite sequence is given by

    Peridotite - Gabbro - Dike - Pillow basalt - Radiolarian chert


  • HOTSPOT : active surface expression of mantle plumes on the earth’s surface

  • MANTLE PLUME :

    • Nearly vertical columns of hot, upwelling solids ascends from deep within the mantle (D’’ layer)

    • Large head followed by narrower tail

    • shaped like a mushroom with a long conduit (tail)

    • Originated from core-mantle boundary

CONTINENTAL HOTSPOT

  • Afar / Ethiopia

  • Iceland

  • Yellowstone

  • The Iceland Hotspot and Afar Hotspot which are situated at the divergent boundary are exceptions.

OCEANIC HOTSPOT

  • Hawaii : Mouna loa, Mouna kea, Killuae (Shield volcano)-gentle less viscous

  • Reunion : Deccan trap (66 ma)

  • Kerguelen : Rajmahal trap

  • Galapagus

  • Bermuda

  • Martin-vas

  • Discovery

FACTS

  • The Réunion hotspot (produced the Deccan Traps about 66 million years ago) coincides with the Cretaceous–Paleogene extinction event

  • The Kerguelen hotspot produced the Rajmahal trap in upper creataceous and it is older than Deccan trap.

  • Carlsberg ridge : Spreading

  • Ninetyeast ridge : Aseismic

  • Pranhita-Godaveri basin : Rifting

  • Makran coast : Subduction

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