Kuliah geokimia 2

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  • 1. KULIAH 2
    The Earth's Internal Structure
    Faculty of Agro Industry and Natural Resources
  • 2. 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
  • 3. PROPAGATION OF SEISMIC WAVES THROUGH EARTH’S INTERIOR
    Transverse waves travel through solids only.
    Longitudinal waves travel through both solids and liquids.
  • 4.
  • 5. 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
  • 6. 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
  • 7. 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)
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  • 17. 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)
  • 18. 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
  • 19. 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
  • 20. 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)
  • 21. 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.
  • 22. 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
  • 23. 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
  • 24. 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
  • 25. 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.
  • 26. 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
  • 27. 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)
    • 28. 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
  • 29. 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
  • 30. 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
  • 31. 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
  • 32. 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
  • 33. 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)
  • 34. 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:
    • 35. Low in silica (mafic)
    • 36. High temp
    • 37. Low in volatiles
    • 38. These volcanoes form
    • 39. Fluid lava flows
    • 40. Fire fountains, lava tubes
    Hawaii
    Cascades
    NW USA
  • 41. 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
  • 42. Crystalline Igneous Rocks