Magma Generation PDF

magma generation B. Naveen Kumar M.Sc., Geology  Temperature, measure of hotness or coldness expressed in terms of any of several arbitrary scales and indicating the direction in which heat energy will spontaneously flow—i.e., from a hotter body (one at a higher tempera...

magma generation B. Naveen Kumar M.Sc., Geology  Temperature, measure of hotness or coldness expressed in terms of any of several arbitrary scales and indicating the direction in which heat energy will spontaneously flow—i.e., from a hotter body (one at a higher temperature) to a colder body (one at a lower temperature). Temperature is a measure of the average kinetic energy of the particles in an object. When the temperature increases, the motion of these particles also increases. Temperature is not the equivalent of the energy of a thermodynamic system; e.g., a burning match is at a much higher temperature than an iceberg, but the total heat energy contained in an iceberg is much greater than the energy contained in a match. Temperature, similar to pressure or density, is called an intensive property— one that is independent of the quantity of matter being considered—as distinguished from extensive properties, such as mass or volume. ❑ Kinetic energy is the energy possessed by a body due to its motion. We see a range of kinetic energy in molecules because all molecules don’t move at the same speed. When a substance absorbs heat, the particles move faster, so the average kinetic energy and therefore the temperature increases. ❑ When a substance is heated, some absorbed energy is stored within the particles, while some energy increases the motion of the particles. This is registered as an increase in the temperature of the substance. ❖Solids, liquids, and gases all have a temperature. The particles within a solid don’t move, but they have vibrational motion. ❖The temperature increases when molecules vibrate faster. The melting point of a solid is the temperature at which the vibrational motion overcomes the forces of attraction holding the molecules in a solid formation. Start with one of the two conversion factors between °C and °F. The two conversion formulas are: °C = 5⁄9(°F – 32) and °F = 9⁄5°C + 32 It doesn’t matter which one you use since they are two forms of the same equation. Let’s use the second one. We want to know when °C = °F, so the equation becomes: °C = 9⁄5°C + 32 Solve for °C °C – 9⁄5°C = 32 Factor out the °C (1 – 9⁄5)°C = 32 Solve the fraction (5⁄5 – 9⁄5)°C = 32 –4⁄5°C = 32 -4°C = 160 °C = -40 °C = °F = -40. Temperature We can relate Fahrenheit to Kelvin by converting Fahrenheit to Celsius using the formula: °C = 5⁄9(°F – 32) where °F and °C are the temperatures in Fahrenheit and Celsius respectively. Since these two equations equal the same thing (°C), they equal each other. K – 273.15 = 5⁄9(°F – 32) Now you have the formula to convert between Kelvin and Fahrenheit. We want to know when K = °F, so plug in K for °F in the formula. K – 273.15 = 5⁄9(K – 32) Solve for K. First, get rid of the parenthesis K – 273.15 = 5⁄9 K – 17.78 Subtract 5⁄9 K from both sides K – 5⁄9 K – 273.15 = 17.78 K – 5⁄9 K = 17.78 + 273.15 K – 5⁄9 K = 255.37 9⁄ K – 5⁄ K = 255.37 9 9 4⁄ K = 255.37 9 K = (255.37)9⁄4 K = 574.59 At 574.59 Kelvin, the Fahrenheit temperature is 574.59 °F.  Kelvin Rankine Celsius Fahrenheit Absolute Zero 0 0 -273.15 -459.67 Freezing Point of Water 273.15 491.67 0 32 Room Temperature 298.15 536.67 25 77 Body Temperature 310.15 558.27 37 98.6 Boiling Point of Water 373.15 671.67 100 212 Sources of Heat There are many sources of heat, but the following are the main sources of heat: Sun Chemical Electrical Nuclear Conduction: It is the method in which the transfer of heat takes place between atoms and molecules in direct contact. Convection: It is the method in which the transfer of heat happens by the movement of the heated substance. Radiation: It is the method in which the transfer of heat takes place by electromagnetic waves. If a material is sufficiently transparent or translucent, heat can be transferred by radiation Radiation is the movement of particles/waves, such a slight or the infrared part of the spectrum, through another medium If the material is opaque and rigid, heat must be transferred through conduction. heat must be transferred through conduction. This involves the transfer of kinetic energy (mostly vibrational) from hotter atoms to cooler ones. Heat conduction is fairly efficient for metals, in which electrons are free to migrate. If the material is more ductile, and can be moved, heat may be transferred much more efficiently by convection. In the broadest sense, convection is the movement of material due to density differences caused by thermal or compositional variations. Advection is similar to convection, but involves the transfer of heat with rocks that are otherwise in motion. For example, if a hot region at depth is uplifted by tectonism, induced flow, or erosion and isostatic rebound, heat rises physically (although passively) with the rocks. Horizontal movement of fluid. In engineering, physics, and earth sciences, advection refers to the transport of a substance by bulk motions. A vivid example of advection is the transport of pollutants or silt in a river by bulk water flow downstream. During advection, a fluid transports conserved quantity or material via bulk motions.  PRESSURE The force applied perpendicular to the surface of an object per unit area over which that force is distributed. Formula: When a force of ‘F’ Newton is applied perpendicularly to a surface area ‘A’, then the pressure exerted on the surface by the force is equal to the ratio of F to A. The formula for pressure (P) is: P=F/A formula for pressure Units of Pressure The SI unit of pressure is the pascal (Pa). Pressure due to the weight of a liquid of constant density is given by p=ρgh p = ρ g h , where p is the pressure, h is the depth of the liquid, ρ is the density of the liquid, and g is the acceleration due to gravity. Pfluid = P + ρgh where, P = Pressure at the reference point Pfluid = Pressure at a point in a fluid ρ = Density of the fluid g = Acceleration due to gravity (considering earth g = 9.8 m/s) h = Height from the reference point The density of a fluid may be estimated by dividing its mass by the volume of fluid taken into account. ρ = m/V where, m = Mass of the fluid V = Volume of fluid considered If the fluid is subject to atmospheric pressure, then the total pressure on the system is given by Pfluid = Po + ρgh where, Po = Atmospheric pressure The pressure exerted in a ductile or fluid medium results from the weight of the overlying column of the material. ❑ For example, the pressure that a submarine experiences at depth is equal to the weight of the water above it, which is approximated by the equation: P=pgh ❖ where P is pressure, p is the density (in this case, that of water), g is the acceleration caused by gravity at the depth considered,and h is the height of the column of water above the submarine (the depth). Because water is capable of flow, the pressure is equalized, so that it is the same in all directions. The horizontal pressure is thus equal to the vertical pressure (the axis along which the imaginary column of water would exert itself). This equalized pressure is called hydrostatic pressure. Near the surface, rocks behave in a more brittle fashion, so they can support unequal pressures. If the horizontal pressures exceed the vertical ones, rocks may respond by faulting or folding. At depth, however, the rocks also become ductile, and are capable of flow. Just as in water, the pressure then becomes equal in all directions, and are termed lithostatic pressures. Equation (1-1) will also then apply, with p being the density of the overlying rock. The relationship between pressure and depth is complicated because density increases with depth as the rock is compressed. Also, g decreases as the distance to the center of the Earth decreases. Three ways to Generate Magmas The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material (decompression melting) or by intrusion of magma (heat transfer). Lowering the melting temperature can be achieved by adding water or Carbon Dioxide (flux melting). The Mantle is made of garnet peridotite (a rock made up of olivine, pyroxene, and garnet) -- evidence comes from pieces brought up by erupting volcanoes. In the laboratory we can determine the melting behavior of garnet peridotite. Wet and Flux Melting https://www.youtube.com/watch?v=WtSE1svxRm4 Time: 10.37 Decompression Melting / Depressurization melting Decompression melting involves the upward movement of Earth's mostly solid mantle. This hot material rises to an area of lower pressure through the process of convection. Areas of lower pressure always have a lower melting point than areas of high pressure. This reduction in overlying pressure, or decompression, enables the mantle rock to melt and form magma. The melting of mantle material that occurs when the material rises into a region of lower pressure, allowing it to cross from its solidus to its liquidus. If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way. Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys. Thus, generation of magma in these three environments is likely caused by decompression melting. Intrusion of magma (heat transfer). Types of Magma Mafic Magma Mafic Magma or Basaltic Magma, This type of magma is high in iron, magnesium, and calcium, but low in potassium and sodium. It also has a relatively low silica content (45-55%). Basaltic magma is the most common type of magma and is responsible for forming many of the world's largest volcanoes, such as Kilauea and Mauna Loa in Hawaii. Silica content: 45-55% Color: Dark, almost black Temperature: 1000-1200°C (1832-2192°F) Viscosity: Low - 10 - 103 PaS Gas Content: Low Minerals: Olivine, pyroxene, plagioclase feldspar Intermediate Magma Intermediate Magma or Andesitic Magma, This type of magma has an intermediate silica content (55-65%) and mineral composition. It is less viscous than basaltic magma and can produce both explosive and effusive eruptions. Andesitic magma is common in subduction zones, where one tectonic plate is sliding underneath another. Silica content: 55-65% Color: Gray Temperature: 800-1000°C (1472-1832°F) Viscosity: Intermediate 103 - 105 PaS Gas Content: Intermediate Minerals: Plagioclase feldspar, pyroxene, amphibole Felsic Magma Felsic Magma or Rhyolitic Magma, This type of magma is high in silica content (65-75%) and potassium and sodium, but low in iron, magnesium, and calcium. It is also the most viscous type of magma and can produce very explosive eruptions. Rhyolitic magma is common in continental volcanic provinces, such as the Yellowstone Caldera in the United States. Silica content: 65-75% Color: Light gray to white Temperature: 650-800°C (1202-1472°F) Viscosity: High- 105 - 109 PaS Gas Content: High Minerals: Quartz, plagioclase feldspar, potassium feldspar Magmas can also be classified based on their origin. Primary magmas are formed from the partial melting of the Earth's mantle. Secondary magmas are formed from the melting of existing rocks, such as those in the crust or upper mantle. The type of magma that erupts from a volcano has a significant impact on the type of eruption that occurs. Basaltic eruptions are typically effusive, meaning that they produce lava flows that spread out over the landscape. Andesitic and rhyolitic eruptions can be more explosive, producing ash falls and pyroclastic flows. Characteristics of Magma Gases in Magmas At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are: Mostly H2O (water vapor) with some CO2 (carbon dioxide) Minor amounts of Sulfur, Chlorine, and Fluorine gases The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas.  Viscosity The property of a fluid that defines the rate at which deformation takes place when a shear stress is applied is known as viscosity: viscosity shear stress rate of shear strain If a large shear stress applied to a liquid results in slow deformation, the liquid is said to have a high viscosity. Tar and honey are familiar examples of highly viscous liquids, whereas water and gasoline have low viscosities (Table 2.2). Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature. Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma). Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). Thus, mafic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. felsic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it is very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas. Newtonian and non-Newtonian fluids are two main types of fluids that differ in how they behave when subjected to shear stress: Newtonian fluids These fluids have a constant viscosity and obey Newton's law of constant viscosity. They are the simplest mathematical model of fluids that accounts for viscosity. Examples of Newtonian fluids include water, air, oils, and molten metals. Non-Newtonian fluids These fluids do not have a constant viscosity and have a variable relationship with shear stress. Their viscosity can change when subjected to force. Examples of non-Newtonian fluids include ketchup, blood, paint, toothpaste, starch solutions, custard, and shampoo. Thanks for yours’ aTTenTion