Lecture 1 Introduction and the Earth's Interior. Wednesday, January 26th, 2005.
IGNEOUS and. METAMORPHIC. PETROLOGY. GEO - 321 ...
Lecture 1 Introduction and the Earth’s Interior
Wednesday, January 26th, 2005
IGNEOUS and METAMORPHIC PETROLOGY GEO - 321
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PETROLOGY – comes from petros for rock – hence the study of rocks Sedimentary – deposition of material from water or air
Igneous – formed through the solidification of molten material
Metamorphic – formed from a previously existing rock (usually at high temperatures and pressures)
PETROLOGY encompasses:1. Description of rocks 2. Their classification 3. Generation and interpretation of data 4. Theories on how these rocks formed Tools of the trade include:1. Field relationships 2. Hammer and hand lens 3. Thin sections and petrological microscope 4. Mineralogy and electron microprobe 5. Major element data 6. Trace element data 7. Isotopic data 8. High pressure and temperature experiments
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The Earth’s Interior Crust: Oceanic crust Thin: 5-10 km Relatively uniform stratigraphy = ophiolite suite: Sediments pillow basalt sheeted dikes more massive gabbro ultramafic (mantle)
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Continental Crust Thicker: 20-90 km average ~35 km Highly variable composition Average ~ granodiorite
The Earth’s Interior Mantle: Peridotite (ultramafic) Upper to 410 km (olivine → spinel) Low Velocity Layer 60-220 km Transition Zone as velocity increases ~ rapidly 660 spinel → perovskite-type )
SiIV → SiVI
Lower Mantle has more gradual velocity increase Figure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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The Earth’s Interior Core: Fe-Ni metallic alloy Outer Core is liquid
No S-waves
Inner Core is solid
Figure 1-2. Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Figure 1-5. Relative atomic abundances of the seven most common elements that comprise 97% of the Earth's mass. An Introduction to Igneous and Metamorphic Petrology, by John Winter , Prentice Hall.
The Pressure Gradient z z
P increases = ρgh Nearly linear through mantle
Some useful pressure conversions: 1 bar = 102 * 103 Pa 1 kbar = 102 * 106 Pa = 102 MPa = 0.1 Gpa 10 kbar = 1.02 * 109 Pa = 1 GPa Approximate pressure gradients in the crust and mantle Crust:
30 MPa/km or 0.29 kb/km
Mantle:
35 MPa/km or 0.35 Kb/km
What will the gradients be in GPa?
Calculate pressure at depth Pressure = Density x Acceleration due to gravity x Depth Garnet peridotite is thought to start melting at a depth of about 130 km in the mantle to produce Hawaiian basalts. Assuming the density of mantle peridotite is 3.3 gm/cc, Calculate the pressure of melting IMPORTANT for Pa, units need to be in kg and m P (Pa) = 3300 kg x 9.8 m x 130,000 m m3 s2 = 4.2 x 109 = 4.2 GPa
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Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth
still slowly reaching surface
Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth
still slowly reaching surface
2. Heat released by the radioactive breakdown of unstable nuclides
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The Geothermal Gradient
Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.
Plate Tectonic - Igneous Genesis 1. Mid-ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins
