GEOLOGY Subject : GEOLOGY (For under graduate student.) Paper No. : Paper 02 Introduction to Geology 02 Topic No. & Title : 37 Magma Bowen Series (Part 01) Academic Script What is Igneous Petrology? Igneous petrology is the field of geology in which, we can study the process involved during the formation of igneous rocks. Megascopic (Hand), Microscopic (thin section) identification and field examination of igneous rocks are basic techniques used in petrology and even more we can predict through petrography studies. Modern igneous petrology also relies on experimental investigation of melting. After development of the concept of plate tectonics in the 1960s, a more convenient way to account for the location of most volcanic activities on Earth, because major igneous activities occur near the plate boundaries, like plate divergence (mid oceanic ridges) or Convergence (e.g. the Andes). Magmas and Igneous Rocks The question is why? Where does magma come from? Why there are so many types of volcanic rocks? Why do volcanoes erupt? Earth Scientists so far studied different types of volcanoes and on the basis of their observation a wide variety of igneous rocks are known. First and main important observations are about composition. Composition and Appearance 1
Volcanic, Basalt, Andesite, Dacite, Rhyolite Plutonic Gabbro Diorite Granodiorite Granite MineralsOl, px, plpx, amph, plamph, pl,qtz, pl, mica Description mafic intermediate Silicic Silicic (high silica) Viscosity low intermediate High very high Color darker dark Light lighter SiO2 <53% 53-63 63-70 >70 Note: names based on SiO2 content and not mineralogy. Magma its composition and origin Molten mass comprising most abundant elements in earth Si, Al, Fe, Ca, Mg, K, H & O. the SiO2 is most abundant amongst all. Definition: Magma is molten rock located beneath the surface of earth. It can be defined as a complex high temperature (between 650o & 1200oC) silicate solution that is primitive to all igneous rock type. It is capable of intrusion or extrusion into crustal rocks. The viscosity of magma dependent on the percentage (% of Si) of silica, volatiles etc. Magma when extruded on the earth surface is termed as lava. The salient features of magmas are as follows: a)extensive variations in igneous rocks with regard to minerals composition, textures, variable magma composition and rate of cooling etc. b)magmas are usually made of silicate minerals or silicate in composition and usually begin to crystallize at high temperature. c)magma has a range of crystallization temperature like ultramafic magma crystallizes at higher temperature as compare to granite. d)they have sufficient amount of fluidity and contain variable amounts of volatile materials. Studies by Alfred Harker and N.L. Bowen mark the turning point in petrology from petrographic approach to petrogenetic approach at this stage attention was focused on understanding more critically than before. The magmas are indicated to be derived through various degrees of melting of the upper mantle and their possible compositions can be deduced from experimental data obtained through high pressure experiments. 2
Melting of solid rock is controlled by three physical parameter i.e. 1. Temperature which is controlled by geothermal gradient which average is 25oC/Km 2. Pressure 3. Composition Magma is generally of four types: a)primitive magma b)primary magma c)parental magma d)derivative magma a)primitive magma: Magmas that are not very evolved are called primitive. b)primary magma: Primary magma is that magma, which formed at depth directly by partial melting of some source and has no characteristics that reflect the effects of subsequent differentiation, fractionation (FX) or Assimilation. c)parental magma: The parental magma is the most primitive one found in an area, and thus the one from which we suppose the others are derived. Any primitive or primary magma may be parental magma. d)derivative magma: Those magmas that experienced some form of chemical differentiation are referred to as evolved or derivative magmas. Composition of magma There are three basic types of magma: 1.Mafic (basaltic) i.e. rich in Fe, Mg minerals 2.Andesitic i.e. intermediate in composition and 3.Felsic (rhyolitic) i.e. rich in light colour minerals and Silica Chemically the magma is composed of silica, alkalis (Na, K, Ca, Mg etc) and iron. The more mafic is magma the gentle will be the eruption because low level of silica cause less volatiles to be build up. The composition of magma will change depending on the make up of the rocks that it melts as it penetrates the Earth crust to erupt in the form of lava. One more type of magma is ultramafic type (picritic type) Observations concerning Volcanic Eruptions Viscosity -- the resistance to flow -- determines the kind of eruption and volcanic deposit. 3
Viscosity is dependent on two things: 1) Temperature -- the higher the temperature the less viscous the lava. This is because as the temperature of solids become higher the bonds become weaker. 2) Composition-In a melt, silicate tetrahedra become polymerized, that is they become joined together by sharing oxygen s. The more polymerization, or the more shared oxygen s, the more viscous the magma. Increased silica ions increase the degree of polymerization and thus the viscosity of the melt. Gas content - nearly all magmas contain gas that evolves at lower pressures as the magma reaches the surface. The same thing happens to CO2 in soda when the cap is removed and pressure is released. The viscosity of the magma controls how easily this gas can be removed from the magma. In fluid magmas, the gas can be released easily. Gas bubbles caught in a frozen magma (a volcanic rock) are called vesicles. In a viscous magma (like a rhyolite) the gas cannot easily escape. The expanding gas sometimes tears apart the sticky, viscous magma and causes an explosive eruption where much ash and tephra are erupted. More about these later. Origin: The magma is produced by melting of rock mass below the earth surface under high temperature and pressure condition i.e. it result from the melting of the crustal rock by pre-existing magma whose temperature is greater, that it melts the rocks which comes in contact. As magma is less dense than its source rock it is propelled up by the buoyancy that its lower density creates. Tectonic environment of magma generation: A combination of high pressure near surface environment is most conductive to melting because of pressure reduction. Magma can also be formed by the addition of volatile to heated rock. Magmatic differentiation and assimilation Magmatic differentiation is defined as any process by which magma is able to diversify and produce a magma or rock of different composition. Differentiation (and partial melting) involves two essential processes. 1.Due to intensive change in variable, such as pressure, temperature or composition, there is a composition difference between one or more phases as elements partition themselves in response to change in variables. 4
2.Fractionation is the physical process by which different portions (usually distinct phases) are mechanically separated. Process of develop of heterogeneity out of homogenous body. It is manly due to a)liquid immiscibility: due to difference in density, composition and volatiles magma do not mix with each other and separate out. b)liquid fractionation: only crystal formed separate out c)movement of volatiles: due to the volatiles entrapped in magma the magma of two different composition mixes and hence separate out. d)fractional crystallization: due to gravity settling of crystal formed, selective nucleation, gas streaming, flow differentiation etc Assimilation or contamination: As magma melts it reacts with or incorporate material from wall rocks its final product is a rock differing in composition from original magma. Assimilation may be capable of significantly altering the composition of magma. Evidence of partial assimilation can be found in variously altered and resorbed contacts or xenoliths suspended in igneous rocks. Two types of contamination are: 1 Assimilation by melting 2 Assimilation without melting 5