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KULIAH 2 The Earth's Internal Structure Faculty of Agro Industry and Natural Resources
Internal structure How do we obtain information about the parts of earth beneath the surface? Earth’s Internal Structure Density of the earth Material Rheologic Division of the earth Magma What Is Magma How magma forms? How magma of different composition evolve? Igneous activity by plate tectonics FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE
PROPAGATION OF SEISMIC WAVES THROUGH EARTH’S INTERIOR Transverse waves travel through solids only. Longitudinal waves travel through both solids and liquids.
Evidence from seismology tells us that the Earth has a layered structure. Seismic waves generated by earthquakes travel through the Earth with velocities that depend on the type of wave and the physical properties of the material through which the waves travel. Types of Seismic Waves Body Waves - travel in all directions through the body of the Earth. There are two types of body waves: P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. S-Waves- Secondary waves, also called shear waves, travel with a velocity that depends only on the rigidity and density of the material through which they travel: Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE
Seismic waves reflect from and refract through boundaries where there is sudden change in the physical properties of the rock, by tracing the waves we can see different layers in the Earth. Note that we know that density must increase with depth in the Earth because the density of crustal rocks are about 2,700 kg/m3 and the average density of the Earth is about 5,200 kg/m3. If density increases, wave velocity decreases. FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE
Density Of earth material, From the velocity structure, the solid earth Core – 1/3 mass of the earth Mantle – 2/3 mass of the earth Crust - < 0.2 % of the earth Continental (under land) Oceanic (under ocean)
What is Magma Made of ? All magmas contain Si and O Upon cooling, bond together into silicon-oxygen tetrahedrons More silica (i.e. felsic), more viscous (harder to flow, thicker) Also contain varying amounts of other elements like Na, K, Al, Ca, Mg, Fe, etc… Dry magmas – no volatiles Wet Magmas – up to 15% volatiles Volatile content strongly effects the viscosity (ability to flow) More volatiles, less viscous (easier to flow or more fluid)
Formation of Magma Remember that the tectonic plates don’t really float on a liquid asthenosphere, rather the asthenosphere is a ductile solid and is only melted in specific locations. Most magma/lava is not 100% liquid. Magma/Lava is made of many compounds, all of which have different melting temps. Only a few percent of liquid is required to make a melt. Other than a rise in temperature, what causes melting of rock within the Earth? Melting happens because of: Decrease in pressure (decompression) Addition of volatiles (H2O, CO2, etc…) Heat transfer from rising magma
Melting due to Decompression The Earth gets hotter with increasing depth due to primordial heat and radioactive decay of elements near the core. The rate at which temperature increases with depth is called the geothermal gradient, or geotherm Liquids have no organized structure, so to melt a rock, the mineral bonds must be broken (animated gif of atoms) The geotherm of the Earth
Melting due to Decompression At depth, confining pressure prevents atoms from breaking free of crystals Solidus: The temperature when a rock first begins to melt Liquidus: The temperature where the last solid particle melts The asthenosphere cools only slightly as it rises (convection) because it is a good insulator (high specific heat) The solidus and liquidus of peridotite (ultramafic mantle rock)
Melting due to the Addition of Volatiles Volatiles: A substance that can easily change into a gas at relatively low temperatures (H2O, CO2, etc…). The addition of volatiles at depth (mainly H2O) seeps into rocks and helps break bonds (aids in melting). Analogy: Think of putting salt onto ice to lower the melting temperature. Likewise, adding water to rocks changes the melting point of rocks just like adding salt to water.
Melting due to the Addition of Volatiles The addition of H2O into basalt, for example, drastically changes its melting temperature In this case, basalts at 60km depth beneath the continents could begin to melt only if they were volatile rich. Depth (km) The geotherm beneath a continent and the solidus of wet and dry basalt
Melting Due To Heat Transfer Melting can also occur when rising bodies of hot material essentially bake the nearby rock Analogy: Think of injecting hot fudge into ice cream. The hot fudge transfers heat to the ice cream and melts it
Types of Magma - Composition Like rocks, not all magma is made of the same stuff We divide magmas into groups by their composition Felsic (Silicic): 66-76% Silica (SiO2) Most viscous, Least dense (~2.5 gm/cm3), melting point 650-800oC Intermediate: 52-66% SiO2 Mafic: 45-52% SiO2, lots of MgO, FeO, and Fe2O3 Ultramafic: 38-45% SiO2, abundant MgO, FeO, and Fe2O3 Least viscous, Most dense (~3.5 gm/cm3), melting point up to 1300oC Increasing SiO2 Increasing Fe, Mg
Magma Compositions Composition controls density, T, and viscosity. Most important is the content of silica (SiO2). Silica-rich magmas are thick and viscous. Silica-poor magmas and thin and “runny.” These characteristics govern eruptive style.
Why are Magmas so Variable in Composition? Differences in Magma composition occur due to 5 main reasons… Different source rock compositions melt a felsic rock = felsic magma Magma mixing mix felsic magma with mafic magma = intermediate magma Partial melting Assimilation Fractional crystallization
Bowen’s Reaction Series In order to understand the melting and solidifying of magma we need to understand Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, letting them cool, and watching what minerals crystallized This series outlines the order in which minerals form in a cooling melt Also applies in reverse order to rocks that are partially melted
Discontinuous series (different minerals form) and Continuous series (Plagioclase only)
So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.
Partial Melting Most magmas are not 100% liquid Commonly 2-30% melt; called a crystal mush According to Bowen’s reaction series, rocks that are partially melted become more mafic, because the silica-rich felsic minerals are melted first. The melted part of the partial melt is thus more felsic than the remaining rock. The felsic mineral, quartz, is a common cement in many rocks
Assimilation As magma sits in its chamber, it may incorporate minerals from the surrounding wall rock Called assimilation Occurs when wall rocks fall into the magma and melt (stoping) or when the magma partially melts minerals from the wall rock Degree of assimilation depends on composition of wall rock, temp of magma, amount of H20 present, amount fractures in and strength of the wall rock, and residence time
Stoping & Xenoliths Stoping: The process of incorporating chunks of wall rock into a magma body Xenolith: A non-melted chunk of wall rock incorporated into a magma body May have a very different composition than the magma
Xenolith A xenolith in granite in the Mojave desert Usually recognized because they may have a different texture (grain size) and composition than the rest of the rock
Fractional Crystallization Not all minerals crystallize at the same temperature – This is fractional crystallization As magmas cool, they become more felsic. Mafic minerals crystallize first and are more dense than the melt, so they sink to the bottom Bowen’s reaction series is an example of fractional crystallization
Magma Movement If magma did not move, no extrusive/volcanic rocks would ever have formed Magma rises because: hotter and less dense than the surrounding rock and therefore buoyantly rises. the weight of the overlying rock (lithostatic pressure) literally squeezes the magma out. Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle Viscosity affects a magma or lava’s ability to flow Controlled by: Temperature (high temp - low viscosity) Volatile content (more volatiles – less viscous) Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow (more silica – more viscous)
Extrusive Igneous Rock Environments Explosive eruptions generally occur when source magma is: High in silica (felsic-intermediate) Low temp High in volatiles These volcanoes form Lava domes Ash clouds and ash flows
Effusive eruptions generally occur when source magma is:
Intrusive Igneous Rock Environments Magma rises by percolating between grains and/or by forcing open cracks in the subsurface The magma that doesn’t reach the surface of the Earth cools into intrusive igneous rocks Country rock or wall rock: The pre-existing rock that magma intrudes into Intrusive contact: The boundary between the igneous intrusion and the wall rock Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like) Non-tabular intrusions: Pluton, Batholith, Stock Mt. Rushmore is carved out of a granitic igneous intrusion