SEMINAR ON Properties of magma

Contents Introduction (magma) Types of magma Physical properties of magma

Temperature Pressure Density Viscosity

Chemical properties Chemical composition Volatiles

Conclusion Reference

Introduction Magma is a mixture of molten or semi-molten

rock, volatiles and solids that is found beneath the surface of the Earth.

Magma is a natural fluid rock material beneath the earth’s crust, which may consolidate to form igneous rocks. When it erupted to the surface, it is Lava.

Several interrelated physical properties determine the characteristics of magma, including chemical composition,viscosity,dissolved gases, and temperature.

PROPERTIES OF MAGMA

PHYSICAL Temperature Pressure Density Viscosity

CHEMICAL Chemical composition Volatiles

Temperature The Temperature record for magmas in

nature are normally measured with an optical pyrometer or at depth in lava lakes with a thermocouple .

The Temperature at a 10m depth in the basalt lava lake on Kilauea volcano , Hawaii has been measured as 3000c hotter then the 8600c temperature measured near the surface.

The Temperature of lava fountain was only 50 0 c below the temperature at a 10mts depth.

Visual estimates of temperature can be made form the colour of the hot lava :the same colours are measured with the optical pyrometer.

COLOUR TEMPERATURE( 0C)

No colour <4750c

Dull red 550-6000c

Bright red 7000c

Orange 9000c

Yellow 10900c

White >11500c

Large range in eruption temperatures: Komatiites(ultramafic lava): ~1400-1600°C Basalts: e.g., Hawaiian basalt ~1200°C Rhyolite: < 800°C Carbonatites: down to <500° C Eruption temperatures relate to composition: Mg-rich magma is hot Si-rich magma is cold Eruption temperatures also strongly affect

viscosity

PRESSURE Lithostatic pressure cannot be measured

directly.However, neglecting complicating factors such as overpressure can be calculated simply and reliably as equal to the overlying rocks.

A function of depth, lithostatic pressure amounts to about 265 bars per kilometres of depth in rocks density 2.7 g/cm3 in continental crust, or 2.65 kbrs at 10 km depth.

In the oceanic crust of basalt –gabbro with a density of 3.0 , pressure is about 295 bars per kilometre of depth or 2.95 kbrs at 10 km depth.

In the deep ocean, under about 5 km of water and 5 additional km of rock, the pressure at 10 km is about 1.95 kbrs.

The possibility of additional pressure resulting from tectonic forces, especially at shallow depths of a few kilometres where the rocks are more rigid, has been considered by many works.

Overpressure stresses at the base of the lithosphere and in the oceanic ridge and subduction zones, calculated from earthquake magnitude and mechanisms , apparently amount to only about 50 to 200 bars.

DENSITY(ρ) Density Controls magma buoyancy, crystal

settling rates, etc. The density of magma in the molten state

has not been measured. However, volcanic glass, since it never crystallized, has the structure and density of a liquid.

Densities measured on such magmas are about 10% lower then those of comparable crystalline rocks except for eclogite.

VISCOSITY viscosity is also one of the physical property

of magma, it is resistance to flow of magma. Magmas having low viscosity can flow easily.

viscosity is determined by the composition and temperature of the magma.

As the crystal form, the viscosity of resulting melt- crystal mixture increases as they cools.

Viscosity also depends on volatile content of magma.

Melts near their melting point have structure similar to the mineral that would crystallise from them.

Viscosity of magma depends on silica content of the rock.

Basalt

Rhyolite

High silica

Low silica

Low viscosity

High viscosity

VISCOSITY OF MAGMA AND COMMON SUBSTANCES

Material Viscosity (pa.s) Wt% Sio2 Tem (C0)water 1.002ₓ 10-3 ---- 20ASE 30 motor oil 2ₓ10-1 ---- 20Kimberlite 10-1-1 30 – 35% 1000komatite 10-1-10 40-45% 1400Basalt 10ₓ102 45-52% 1200Peanut Shutter ~ 2.5 ₓ 102 ----- 20Andesite ~ 35 ₓ 103 58 - 62 1200Ketchup ~5 ₓ 10 ---- 20Tonalite(6% H2O) ~ 104 65 950Rhyolite ~ 105 73 - 77 1200Granite(6%H2O) ~ 105 75 800

CHEMICAL COMPOSITION

Composition varies from one extreme minerals that melt only at very high Temperatures, to another minerals that melt at the lowest temperatures.

Lots of materials in geology(rocks, minerals, water ) vary in composition from one extreme to another . The extremes are called end member.

Magma composition, then, has two end members:

1.Magmas composed of minerals that melt at highest temperature.

2.Magmas composed of minerals that melt at the lowest temperatures.

Minerals that at the highest temperatures to be of the independent tetrahedra and single –chain silicate type, and Ca –rich plagioclase feldspars. Remember that the important rock forming minerals composed of independent tetrahedra is olivine and that the important rock forming minerals composed of single chains of tetrahedra is pyroxene.

Minerals that melt at the lowest temperatures tend to be framework and sheet silicates , and Na – rich plagioclase feldspars. Recall that the important rock forming framework silicates are quartz and potassium feldspars , and an important rock forming sheet silicate mica.

VOLATILES H2O most abundant volatile in most magmas. CO2 next most abundant volatile. In general, Basalt magmas are DRY, i.e. H2O < 0.5 wt% MORB = 0.25% H2O Hawaiian Tholeiite = 0.5% H2O Alkali Olivine Basalt = 0.9% H2O

Andesites, Rhyolites, Granites: Higher Water Contents Paricutin Andesite = 2.2% H2O at 1100oC Granites/Rhyolites wide range H2O: 0.5% to 7% H2O by weight. Water lowers viscosity: OH- ions act as Network Modifers, substitute for O2 in tetrahedra. Water lowers solids temperature: Effect greater at higher pressures.

CONCLUSIONS Several interrelated physical and chemical

properties determine the characteristics of magma, and their environment of genesis.

Physical properties includes temperature, pressure, density, viscosity are important to indicate type of eruption or path of genesis.

Chemical properties like composition and volatile content can change their mode of crystallisation, mineral composition, and can influence physical properties of the magma.

REFRENCE Hyndman, 1985, Petrology of igneous and

metamorphic rocks, second edition Pp No: 123 - 131.

John D.Winter,Principles of Igneous and Metamorphic petrology, Second edition Pp No:54 – 56.

Sambhu Nadh, 2013, Encyclopaedia of geology, Pp No: 115 – 125.

http://www.tulane.edu/~sanelson/eens211/magmadiff.ht

http://en.wikipedia.org/wiki/liquid immiscibility